The Fireman’s Guide to Main Street: 5 Buildings to Know, Part 3

Alexis Shady & Chris Tobin

There is a quote that I believe represents the vast majority of the fire service concerning buildings, it reads;

“Many an object is not seen, though it falls within the range of our visual ray, because it does not come within the range of our intellectual ray.”

Thoreau, Henry

Simply put, we all see buildings but few understand what they’re actually looking at. That’s a problem, a really big problem and for two important reasons; which are a building is the one thing that directly or indirectly effects everything we do on the fireground, and the only thing we can do about a compromised building is avoid it entirely. We show up with no solution to sagging roofs, crumbling walls, or missing floors other than staying away. We can mitigate smoke, fire, and rescue trapped victims but we can do nothing about the leaning wall. It’s this stark reality that many forget and have paid the price. You can know all there is about fire behavior, your tools and strategies, none of which hold any value if you’re unfamiliar with the space in which they are relied upon. Some may say all fires are the same, which is true until you put one in a building. Behind every door are an infinite amount of variables, some known, some unknown, and some unexpected. This is why nothing’s routine till it’s over and why knowing your buildings on a visceral level is paramount. If you want to be able to forward think you must understand the data you’re receiving.

This will be a five part series exclusively examining five different types of legacy construction, each with its own article as it pertains to firefighting. The types of buildings were selected based on their prominence in today’s main streets and historic districts. These specific types of buildings exist in small towns from coast to coast but more commonly found East of the Mississippi River where our national building stock originated before moving Westward. 

The five buildings are the old house, the taxpayer, the old mill, the vacant theater, and the bowling alley. Each of these will be examined along with inherent hazards and a play book for handling fires specific to each occupancy. Additionally since many of these buildings are found in small towns with departments that may not have the adequate resources, there will be a section based on short staffed responses for each. The objective of this series is to present the most useful amount of information in the least amount of space. Each of these buildings are worthy of their own book in themselves, this series is meant to be concise and simple information for any level of firefighter. As with any article on architecture, regional vernacular and Departmental jargon may vary. Nothing in this piece is the final say, only the individual reader and their streets can make that claim. 

Part 3

The Building

The old mill: What would a town be without the mill? Nonexistent, that’s what. Unless you’re in a planned bedroom community platted in the 21st century then it’s highly likely your town grew around industry during the American Industrial Revolution. Such times gave us terms like “mill town”, “company town,” “factory house,” and “mill village.” The first mills were built in the countryside along powerful waterways and were wheel driven. The advent of steam power did away with this type of construction, and lead to the hulking heavy timbered factories that would dominate the landscape for the next century.

It was due to both the prevalence and their inherent construction features that so many of these buildings still exist in towns all over America. Textile mills were more common in New England, but as with everything else that expanded westward, so did industry and its buildings. Whether you lived in the corn belt, rust belt, or the cotton belt didn’t matter. Every town needed jobs, and those jobs were in one of many large factories that were surrounded by neighborhoods of their workers.

The term “mill type construction” originally in reference to the large New England textile mills is now used interchangeably with any such type four heavy timber factory. These buildings are exclusively legacy or heritage era construction with rough cut sawn timbers and joinery. The exterior walls are noncombustible masonry that are a thicker wythe at the bottom. Generally 24 to 36 inches thick at the ground level and tapering up as the building rose in height to typically no more than six stories. All four walls will be load bearing. Type lV “heavy timber” can be numerous types of occupancies such as churches, gymnasiums, and even residential. Mill type is a subtype of this construction, specifically referring to commercial use, such as factories or warehouses with masonry walls and specific dimensions on components such as floors and roof decking. It all comes down to what building code was used, and when it was published. It’s important the reader understand that Type lV construction can exist in numerous types of occupancies such as places of worship, lofts or restaurants, not just old mills; even though the construction features are the same. This article will be focusing solely on Type lV heavy timber mill type buildings.

The definition of Type IV heavy timber is commonly confused with Type V heavy timber wood framed buildings such as heritage framed construction or old barns. While the heavy timber framing may exist, Type V is still a wood framed building, thus not mill type construction due to the lack of masonry walls. There can be some variance amongst building codes, however all adhere to the minimum dimensions for heavy timber mill construction which is defined by the NFPA as follows:

– Noncombustible exterior load bearing walls.

– Main framing members being no less than eight inches by eight inches.

– Wood columns supporting solely roof members shall be no less than six by eight inches.

– Beams and girders supporting floor loads shall be no less than six by ten inches.

– Beams and girders supporting roof members shall be no less than four by six inches.

The idea behind heavy timber is its inherent fire protection based on mass. Simply put the beams will char, while structural integrity isn’t effected as severely as smaller diameter wood. While heavy timber is typically slower to ignite, once ignited it conversely has the potential to become unstoppable. This was no secret and why Mills were designed in quarters or thirds separated by true fire walls with fusible linked rated metal clad fire doors. The first automatic fire sprinkler systems were also installed in these buildings as early as 1874. These buildings were designed around the idea of passive fire protection in a time when fires were devastating the textile industry. These passive construction features will be a major part of any modern attack strategy, the key is knowing how to use them effectively during operations.

The Hazards

The connections between columns and girders are far more susceptible to collapse than the beams themselves. The connections were often made of cast iron, which we already know is unreliable and fails at variable temperatures, unlike steel which has uniform failure points. Additionally, cast iron fails completely and catastrophically without warning. This is due to how the fire effects the variance in the casting process, not cold water hitting hot metal. In some cases there were cast iron floor columns as well. Depending on the load requirements for the building some floors would have cast iron columns and others heavy timber. It just depended on what the manufacturer wanted.

The floors of these buildings also pose a special hazard over time. Layers of floor, typically layered tongue and groove (also known as planks) made up the flooring system. Years of chemical spills, and oil from machinery add risk. Cold storage facilities typically had a coating applied to help preserve the wood and keep the floors from being slippery add yet another layer of risk. These both can make floors a fire hazard as they speed up the combustion process which obviously then increases collapse risk. You could also expect to see floor joists in these buildings having fire cuts. Fire cuts are angled cuts at the ends of beams along the exterior masonry walls. The purpose of these cuts was to maintain structural integrity of the exterior walls, having the interior floors collapse inward, onto themselves instead of forcing the walls out. Great for exterior firefighters, not so great for ones inside. Keep in mind fire cuts were designed in an era before interior firefighting with the idea of keeping the exterior operations safe

Lack of windows, whether from original construction or bricked in later significantly reduce the ability to vent these fires. Few entrances also make safe operations a concern. Another hazard created when one of these buildings becomes vacant are the presence of unmarked open shafts. When workers are present or the buildings routinely preplanned, elevator and freight lifts are known and marked, or railed for the workers safety. These things become absent when an old mill falls into disrepair. Railings are scrapped, lifts are removed, and what’s left is an unmarked, unknown large opening spanning multiple floors. The obvious fall hazard, combined with a now unprotected shaft acting as an interior chimney, should be an immediate red flag for anyone arriving on scene of an old vacant mill. These unprotected openings also allow for rapid vertical fire spread, so checking for extension on upper floors is key.

Even after the fires out one often often over looked hazard is the post fire collapse zone. These buildings will burn and smolder for days and without constant supervision the collapse zone becomes less enforced with time. Crews from different shifts may arrive days later and meander into an area with deadly consequences. The walls of these buildings will stay free standing up to heights of many stories and fall without warning. Keep any collapse zone set up at the time of the fire clearly marked with tape even after the fires out.

The Playbook

The interior ops margin for mill type construction is narrow. These fires are either catastrophic block long conflagrations or mundane rubbish burning from vagrants or industrial processes. The play here is for the few fires that are still vent limited and deep seated, requiring interior lines stretched, searches for both fire or victims, and interior exposure protection.

Confinement is key, use the inherent construction features to your advantage. The closing of the fire doors should be an early task done simultaneously with line advancement. The firewalls in these buildings were constructed of non load bearing isolated masonry that goes from the basement up through the roof. There will be fire doors on each level typically directly above one another in the floor plan. The worst case scenario is a well off fire of unknown location. This will be even further complicated if it’s discovered sub grade in a basement. These buildings lack basement windows and stair wells are very small; making the idea of laboriously advancing a large diameter line into zero visibility vent limited conditions seem suicidal.

Tag lines while good in theory, will be of no practical use with stock or palletized goods to maneuver around. Members not operating on a hose line must have a TIC. Becoming lost and low on air is a high probability in these buildings. Accountability and forward thinking of committing members inside must be at a higher level of scrutiny due to this risk. Multiple RITs should be assigned due to the size and reflex/travel time requirements. Too much manpower is just as much of a problem as too little. Use your crews accordingly and diligently, not wasting manpower on things such as utilities or lesser important tasks at hand. Searches must be limited to reflect good intel and not comprehensive like on typical residential fires. You can not send crews searching into oblivion, these buildings demand a conservative search strategy. If you have an employee onsite, it’s imperative a role call is done to account for the workers.

Once the life safety objectives have been addressed, the fire must be located and the fire doors closed. If the fire can’t be located but the doors can be shut, then you’ll soon find out what section you’ve written off. If the fire can’t be located and doors not shut, then you will lose the building and every piece of property in it.

Once the fires been located, it’s a battle against time and GPM. Keep in mind a small fire in a big building is actually a big fire anywhere else. Don’t let the optics play tricks on you. What seems like a one line fire may in reality take multiple lines and even master streams to extinguish. If conditions have not improved after 10-15 minutes, start considering a shift to confinement over extinguishment. It’s better to save 3/4s of a building than none at all. Once the fire has extended up through multiple floors it’s no longer extinguishable from the inside. Lapping will soon follow once the fire reaches the windows. Now you’re confronted with horizontal interior fire spread and exterior vertical fire spread by combustible window frames. You can see how narrow the margin is for successful interior operations. You will only have one shot with an interior attack.

The decision to transfer to interior confinement over extinguishment can be based on some benchmarks.

– Interior streams showing no improvement in conditions.

– Fire showing on more than one floor upon arrival.

– Unable to locate the fire for an extended period of time under deteriorating conditions.

– Hazardous material making a direct attack unsafe.

– Localized structural integrity problems such as a portion under construction, floors missing, etc. making entry into the involved section impractical.

Confinement is a last chance strategy to save the remaining unaffected parts of the building and its property. This strategy should only be attempted when there are no indicators of compromised structural integrity, and the fire hasn’t spread to multiple floors for an extended period of time. If the building is sectioned off by firewalls, the decision needs to be made on where to hold the fire. Depending on conditions this may be the next closest fire wall or two sections away if it’s a fast moving fire or resources are slow to deploy. The objective is to hold the fire doors containing the spread of fire to the original section of origin as much as possible. These buildings may carry large quantities of stored products so as with any property conservation strategy, the risk should be taken accordingly. Interior streams and personnel should be positioned on the protected sides of firewalls only long enough to set up portable ground monitors. Once the interior streams are in place aerial master streams should be positioned at the upwind side of the fire wall and directed onto anything burning through the roof. If the fire doors are missing or the fire walls have been compromised this tactic shouldn’t be attempted and operations should transfer into a defensive posture using only exterior master streams. Unfortunately, due the robustness of heavy timber roof construction, fire will spread horizontally unchecked from one section to another instead of quickly burning through like in other types of construction. This makes exterior master streams completely ineffective when trying to stay ahead of the fire. Roof ops are limited to existing openings such as skylights, roof top stairwells, and HVAC vents. Keep roof crews on the protected sides of firewalls if they’re monitoring conditions. Don’t waste the resources on heavy timber roofs, the fire will likely outpace you with tradition vertical ventilation tactics. Once exterior operations are warranted, position aerial master streams at corners and junctures of firewalls in long runs of masonry walls. After resources are positioned it’s then a war of water, from here GPM wins the day.

The Short Staffed Response

Manpower! Manpower! Manpower! Followed by water and even more water. The bigger the structure, the more people you will need. It’s important for Officers to have a clear understanding of their departments resources, and even more so, their limitations. Remember, fires in these structures can be deceiving. Like we discussed above, what appears to be a small fire in these buildings can be rather large, purely due to the size of the structure, and you need to be prepared. These fires are definitely what one would consider a high risk, low frequency event, so extra command staff may prove to be beneficial. Accountability and continuous 360’s will be important for safety on these fires. Extra eyes on the outside can also help identify fire spread on additional floors, as it may look contained to an interior crew. Depending on the size and location of the fire, you may want to consider assigning division or operations chiefs, as well as water supply management responsibilities to additional officers. Doing so allows command to focus on the bigger picture.

Accept the fact that complete interior searches will be next to impossible to complete safely due to the size and complexity of these structures without adequate manpower and prior training on large area search. Even more so than the other fires we’ve discussed, preplanning is key. Command needs to know how long it will take mutual aid or additional staffing to arrive on scene. Preset mutual aid assignments can potentially help with these response times. Prior knowledge of the building such as location of utilities and the interior layout for the location of open elevator shafts, fire doors, and all available exits will help ensure the safety of your crews. Having contact information for maintenance staff or the owner readily available can help you as well if you don’t have floor plans on hand. You’ll also want to consider additional water sources as the use of aerials have the potential to quickly overpower the nearest hydrants depending on pressure available.

Unlike taxpayer fires, where we suggested the use of the smaller diameter hose due to low manpower and maneuverability; selecting the larger diameter hose may be the key to these fires. The likelihood of deploying more than one attack line is high as well, and closing the fire doors as mentioned above should be a priority. Two more reasons manpower is important. Because there are few walls throughout the large open space, fire spreads rapidly, and your stream may need to be able to reach from the stairwell to the far corner before conditions allow you to advance, yet another reason to choose the 2.5”. One benefit to the open space, is that there are few concealed spaces allowing hidden fire spread, which in theory should make finding the seat of the fire easier.

While fires in these buildings are considered difficult to start due to the low surface-to-mass ratio, they expand quickly. It is important that you acknowledge your department’s limits, and understand that these fires can quickly overwhelm even a well staffed urban department’s resources. That fact, combined with response times, manpower, and collapse risks, may make defensive operations your only option.

Remember, a small fire in a big building is a big fire anywhere else. Manage your resources wisely and don’t be afraid to change tactics if you’re not seeing changes. Utilize the inherent construction features to your advantage. In theory, with a quick response and enough manpower, if you can get to the fire, you can put it out. That is, if you have enough water.

Citations

Due to the nature of how common trade knowledge, jargon, terminology and methods are passed down amongst the fire service much of the articles information can not be cited as a proprietary source to one particular piece of work, individual, group or otherwise.

Dunn V. Collapse of Burning Buildings: a Guide to Fireground Safety: 2nd Ed. 2nd. Ed. Tulsa, OK: Pennwell; 2010.

Dunn, V. (2007). The strategy of firefighting. Tulsa, OK: PenWell.

Hill HJ. Failure Point: How to Determine Burning Building Stability. PennWell Publishing Company; 2012.

NFPA 220: Standard on Types of Building Construction. (06.2018).

The Fireman’s Guide to Main Street: 5 Buildings to Know, Part 2

Alexis Shady & Chris Tobin

There is a quote that I believe represents the vast majority of the fire service concerning buildings, it reads;

“Many an object is not seen, though it falls within the range of our visual ray, because it does not come within the range of our intellectual ray.”

Thoreau, Henry

Simply put, we all see buildings but few understand what they’re actually looking at. That’s a problem, a really big problem and for two important reasons; which are a building is the one thing that directly or indirectly effects everything we do on the fire ground, and the only thing we can do about a compromised building is avoid it entirely. We show up with no solution to sagging roofs, crumbling walls, or missing floors other than staying away. We can mitigate smoke, fire, and rescue trapped victims but we can do nothing about the leaning wall. It’s this stark reality that many forget and have paid the price. You can know all there is about fire behavior, your tools and strategies, none of which hold any value if you’re unfamiliar with the space in which they are relied upon. Some may say all fires are the same, which is true until you put one in a building. Behind every door are an infinite amount of variables, some known, some unknown and some unexpected. This is why nothing’s routine till it’s over and why knowing your buildings on a visceral level is paramount. If you want to be able to forward think you must understand the data you’re receiving.

This will be a five part series exclusively examining five different types of legacy construction, each with its own article as it pertains to firefighting. The types of buildings were selected based on their prominence in today’s main streets and historic districts. These specific types of buildings exist in small towns from coast to coast but more commonly found East of the Mississippi River where our national building stock originated before moving Westward.

The five buildings are the old house, the taxpayer, the old mill, the vacant theater, and the bowling alley. Each of these will be examined along with inherent hazards and a play book for handling fires specific to each occupancy. Additionally since many of these buildings are found in small towns with departments that may not have the adequate resources, there will be a section based on short staffed responses for each. The objective of this series is to present the most useful amount of information in the least amount of space. Each of these buildings are worthy of their own book in themselves, this series is meant to be concise and simple information for any level of firefighter. As with any article on architecture, regional vernacular and Departmental jargon may vary. Nothing in this piece is the final say, only the individual reader and their streets can make that claim.

Part 2

The Building

The Type 3 Taxpayer: AKA “Main-street USA”. These are the quintessential multi-story brick buildings that exist in some version in every North American town, big or small. Ordinary Type 3 construction is by far the most common type of buildings that make up what most consider Main Street. The notion that brick buildings are an “urban thing” is no further from the truth. Take a drive through any rural community and you’ll likely see a row or what’s left of an old general store, post office, or bank. These may be the only brick buildings for miles, but they are still the center of town and no less a fire hazard. For some rural fire departments that only go on a few dozen runs per year, a fire in a two or three story brick building built in 1909 will be an unfamiliar playing field.

The term taxpayer originates from a time period during which buildings were cheaply constructed in densely populated places so that property managers could recoup their property taxes. This manifested itself into a multi story brick building, with commercial occupancy on the bottom floor and residential on the upper floors. These buildings came into popularity after the Great Depression with the intent to be redeveloped, many of which were not and still exist. This definition has evolved with modern times to include type 2 non-combustible strip malls. For the sake of this article, we’ll be specifically speaking of the traditional definition of a taxpayer type building; a type 3 multistory, mixed use occupancy with commercial on the first floor and residential above.

Main streets in general exist in three types of configurations. The aforementioned, as an actual street fronted with legacy construction, a Shelbyville Square, or four sided town square built around a courthouse or common area, and lastly as a historic district with multiple blocks of taxpayer type buildings. This matters when preplanning incident action plans since a row of connected taxpayers is more hazardous than single buildings separated by gangways. Also, if you have a town square or multiple blocks of historic districts you may have roundabouts that will affect apparatus positioning differently than a traditional linear Main Street, as well as collapse zones for aerial master streams.

The buildings of taxpayer construction come in all shapes and sizes. They can be type 2, 3, and 4. Some have metal truss roof supports, cast iron columns, concrete floors, and heavy timber void-less layouts. The most common by a wide margin is the type 3 Ordinary building. This taxpayer will be masonry non-combustible walls with wooden combustible roof, floors, and interior walls. Foundations are typically stone with variably sized basements for storage. Keep in mind these may be connected to adjacent buildings or even across the street. Even if basements are not common in your areas residential building stock, they may be present in taxpayer buildings. Basements were cooler and a way for businesses to keep produce and stock before refrigeration.

Taxpayers are full of unique features that can make fighting a fire in these buildings a challenge. Typically found in rows of connected buildings their inherent exposure problems are an obvious concern. Occupancies may also share what was originally designed to be two separate buildings separated by a party wall. Renovations can make these layouts impossible to anticipate without preplanning. The masonry brick walls will be of triple wythe, tapering up as the building gets taller. This is important to understand when sizing up structural integrity. A crack down low doesn’t mean the same as a crack up high. Load bearing walls will be the shortest length of roof span, typically the sides with few exceptions. These buildings were long, narrow, and a perfect fit for densely populated lots. Roof designs can be either pitched or flat; however, a built up combustible flat roof is far more common on Taxpayers than a shingled pitched roof. The built-up flat roof consists of 1×6-1×8 planks across dimensional rafters on 16” centers with a slight slope for watershed. Expect multiple layers of tar applied over the decades, in some cases a newer membrane roof covering may have been put in place. Taxpayers with flat roof construction will have a cockloft. This is a common void between the top floor ceiling and roof that extends front to back getting smaller with the roof slope. Cockloft vents near the roof line in the front or side walls are an indication of its exact depth and presence in relation to the roof which may have side parapet walls. Taxpayers have three types of common facade features to beware of; cornices, parapets, and awnings. More will be discussed about these and their hazards later on. Additionally, billboard framework and water towers weren’t uncommon loads to find on top of these buildings. Cast iron is yet another facade design feature common for Taxpayers. These were often brightly painted ornate pillars, columns, and lintel work set into the brick, or supporting the bottom corner entrance of the business. The first floor also had large windows for viewing products by pedestrian traffic.

The residential aspect of these buildings was more of an afterthought during construction; their main function was to sell something, not be a home. The upper floors were accessible from either a front set of stairs off the main street or a rear alley. In a row of connected taxpayers it can be hard to tell which door accesses which upper floor apartment. Depending on the occupancy there may be a set of open interior stairs making any first floor fire even more detrimental to those above. The typical layout was bedrooms and common areas up front with kitchens in the rear. In shotgun style layouts the stairs come into the middle room. In wider buildings a side hallway was common. It is important to consider this if your stairs are in the rear and you’re stretching a line to a 2nd floor fire on the A side.

The Hazards

Taxpayers are infamously known to firefighters for their voids. There are two kinds of voids in Type 3 buildings: inherent and acquired. Inherent voids are due to the buildings original design and acquired voids are due to additional construction and alterations. The latter being much more hazardous to firefighters since additions were commonly done without permits or with any regard to fire safety or building codes.

Inherent voids of type 3 Taxpayer construction:

Cornice work, mansard parapets, wood framed canopies and awnings

Window frame voids for sash weights

Decorative trim and crown molding

Floor voids

Ceiling voids

Stud bays behind plaster & lath

Cocklofts

Pipe chases

HVAC plenums

Cant stripping along parapet walls

Acquired voids commonly seen in Taxpayer construction:

Dropped ceiling on the bottom floor creating a double void under tin ceiling

Dropped ceiling on top floor creating a double void under a cockloft

Transom windows concealed with framed canopies or dropped ceilings

Hallways framed off into closets

Larger original rooms being framed into smaller rooms

Rear additions, enclosed porches into living space

Rain roofs added on top of original flat roof

These are just the commonly occurring voids in type 3 ordinary construction, there can be many more. The main takeaway is knowing where to expect the fire before it gets there. A good firefighter will build the skill of foresight in legacy construction. The benefit being a more effective and efficient operation. Preplanning and studying the construction of your districts are the first steps to accomplishing this. These buildings will often have legacy construction features making overhaul much more laborious. These include embellished trim work, crown moulding, tin ceilings and wainscoting.

The voids in these buildings also contribute to another well known hazard; the hostile fire event. Whether a backdraft or smoke explosion, either can be just as catastrophic. These hazards are two-fold, the initial hostile fire event and the ensuing collapse of structural members. This can set up a synergistic effect of calamity on the fireground.

The inherent/acquired voids and design of Taxpayers make them a high risk for both floor and exterior wall collapse. Fire cuts are angled cuts on the end of a wood beam, where it rests against a masonry wall. As the beam burns away, the fire cut allows it to pull away from the wall. The purpose of these cuts is to help save the exterior wall from collapse. The disadvantage is the increased potential for interior floor collapse. This was a construction feature from a time when firefighting was an exterior operation, keeping firemen safe. Terrazzo floors, polished floor covering made of chips of marble, quartz, glass, etc. poured into cement are another significant collapse hazard. Terrazzo floors add significant weight to the floor beams, and hides heat and weakness in the beams from the firefighters above. Twelve firefighters were killed at the 23rd Street Fire in New York City in 1966 from a Terrazzo floor collapse. To evaluate the floor’s structural integrity, forcefully strike the floor with your tool. Wooden floors make a hollow sound, and your tool with bounce significantly. Masonry floors will make a loud clanking sound with a lot of vibration across the floor. Finding Terrazzo floors needs to be communicated to Command immediately.

There are three ways a masonry wall can collapse: 90 degree angle, curtain fail, and inward/outward. Of the three, 90 degree is most common. Once interior floors collapse, the pile of debris creates a lateral force on the remaining exterior walls. This extra force on the walls can cause cracks at the top, or separation at the corners. The walls then begin to lean outward, starting at the top, and will fall at a 90 degree angle. This collapse area will be at minimum, the height of the building. The top of the wall falls forward, striking the ground at the height of the building. As always collapse zones need to be at least 1.5x the height of the building to account for pieces of brick and metal that bounce. Parapet walls with decorations, lights, signs, etc. increase the collapse risk of the wall.

Photo Credit: Brianna Mason

Curtain fall collapses occur when the wall crumbles and falls down, straight to the base of the wall. This is most common with veneer walls where the plywood backing is burnt away by fire. If there is an interior collapse and the exterior wall has windows whose lintels are made of brick arches, the masonry walls may start to lean out. If the lintels begin to crumble and fail, the wall will fall downward rather than out.

Photo Credit: Brianna Mason

Inward/outward collapse occurs when the top of the wall falls one direction, forcing the bottom of the wall in the other. Interior floors collapsing due to fire damage combined with the weight of the water being applied to the building; in turn, a massive burst of force is applied on the outward walls, which causes them to lean outward until they reach the point of failure and fall. An inward/outward collapse can also occur if the top portion of the wall begins to lean in. Just because the wall leans in, doesn’t mean it will collapse that way. The top portion could fall in and kick the bottom portion outward, or the top portion could begin to fall in, and then slide down, with the bottom of the wall going first.

Photo Credit: Brianna Mason

Evaluating walls for collapse risk needs to be done continuously throughout the entirety of the fire scene. Interior floor collapse increases the risk of the exterior walls failing. The force of a master stream directly on these walls can also cause the wall to collapse. To identify whether the wall is brick or veneer, look for quoin work in the corners or for what is referred to as the header course. Brick quoin work acts as a decoration and as structural support, tying the two walls together. If there is separation in these walls, it indicates weakness in the support systems, and collapse is imminent. Header course appears approximately every 7th layer; bricks will be laid end facing to act as a layer of support.

The Playbook

Size up is key in Type 3 Taxpayer buildings. Before any action can be taken, the structural integrity of the building must be assessed. Brick and joist construction has a high collapse potential, and compromised structural integrity is the one problem with no solution other than avoidance. Keep an eye on cracks above windows and doors; openings are inherent weak points in any wall, so they’re early indicators of wall collapse. If cornice work or a mansard brow is heavily involved, avoid the front entrance if possible. Cornices come down like a guillotine blocking entrances, burying lines and personnel masking up before entry. Buildings connected to one another in a row will have limited access and egress from the front and rear only. Keep this in mind if the front entrance is unsuitable, you’ll have to stretch the line through an adjacent building to the rear if the alley isn’t an option for apparatus placement. If the fire is confined to the cockloft, keep an eye on the parapet wall. Failing roof rafters can push out or pull in parapets. A parapet wall falling inward is just as destructive for those inside under a roof as those in the street when one falls outward. Floors are the next structural assessment, particularly the first floor, since basements commonly have open joist construction unprotected by plaster, making basement fires particularly hazardous. Beware of terrazzo and tiled hard flooring surfaces. These will mask any structural deficiency under them while adding weight to the potentially weakened decking. These surfaces are found in bottom floor business vestibules, stairwell landings, kitchens and bathrooms. Sounding with a tool will give you little indication of the actual wood decking so tread lightly and keep your eyes down for floor separation at the bottom of walls.

Once structural integrity has been assessed and an offensive strategy decided, the next key decision is your weapon of choice. Taxpayers present a unique challenge of having commercial fire loads with residential components, so many choose a larger diameter attack line for bottom floor fires. Keep in mind, every action has an equal reaction. What you gain in GPM for the bottom floor you’ll lose in maneuverability on the upper floors. This poses a challenging conundrum of sorts for incident commanders. The optimal choice of attack lines will vary based on conditions, resources, and training. Regardless of your line choice, plan for a long stretch. These fires are not 200ft victories. This is often figured out after the fact, causing valuable time to be wasted extending line under subpar conditions. A fire on an upper floor or basement especially will be a longer than normal hose line length due to stairs and corners. Once these arrival considerations are addressed the objective remains the same; to locate, confine, and extinguish the fire while protecting the upper floor living spaces.

For basement fires a line should be stretched to an exterior entrance if possible; this may be in the front on the sidewalk down bifold doors, but typically it will be in a rear walk down or bulkhead entrance. For interior entrances care must be taken in assessing floor integrity while understanding the greater load of merchandise on top of open joist construction underneath. A second line must be stretched to the first floor to secure egress on interior entrances and to cut off extension. Basements in Taxpayers are usually cramped with stock in storage lockers made of lightweight lumber and wire mesh. Taking this into consideration a smaller diameter, more maneuverable hand line may be more effective. Keep in mind the wire mesh will diminish stream quality if not removed. Basement windows are also commonly secured with burglar bars, so be ready for egress issues that need to be immediately addressed by the first arriving company assigned to truck duties.

First floor fires will be of commercial fire loads so be prepared for commercial fire flows. Forward progress will be slow going; these fires are tough and are almost never seated easily near the front. Rows of merchandise inhibit movement, drop ceilings collapse from HVAC ducts, stocks falls over on hoselines, and high heat zero visibility vent limited conditions are common.

Some places opt for a large diameter hose line as a default while others quickly deploy two smaller diameter hand lines in tandem. One large diameter line may fully extinguish the fire quicker but redeployment to the second or third floor for extension will be greatly diminished compared to smaller more maneuverable hand-lines that occupy more real estate in the same amount of time. One option is leading off with a large diameter line for the first floor fire and stretching a second small diameter line for the upper floors. This gives you the benefit of greater GPM with enhanced maneuverability. The deck gun isn’t typically a good option due to subpar stream angles. Save that card for upper floors.

Fires on the upper floors have two key considerations. First is the likelihood of living space and the second is fire extension into the cockloft. Any fire in a taxpayer on an upper floor has an implied life threat so operations should be conducted in a rescue mode with emphasis on search. If conditions permit, Truck companies should search ahead and close doors confining any rooms of fire while lines are being stretched. VES is a an effective option as well as ladder based entry to upper floors keeping the stairs clear for the Engine company deploying lines. Fires on 3rd floors or higher should be dry stretched one floor below for speed. If overwhelming fire conditions meet you on arrival, don’t hesitate to deploy the deck gun. Make sure to sweep any cornice work above the windows before zeroing in on the rooms of fire. This will slow any lapping into the cockloft and cool the combustible cornice work. Roof operations are implied on any top floor fire in taxpayer construction, even more so in a row of connected type 3 buildings with a common cockloft. This needs to be proactive and happen early in the operations. A report of cockloft conditions needs to be transmitted as soon as possible by roof teams. Assign multiple roof crews downwind to monitor for extension, as well as the top floor of adjacent buildings.

If the fire has taken hold of a cockloft, your options are resource dependent. If you choose offensive roof ops, you have three options; use existing scuttles, skylights and vents, dice a heat hole or holes where needed, or a trench cut. Built up flat roof ventilation is a laborious task requiring numerous saws and more manpower than a residential op. If that’s your play, then position additional crews downwind first and work the party walls if the buildings are connected. The point here is to balance the need:time ratio. If you don’t have the manpower for roof ops, then the work must get done from below by pulling ceilings and extinguishing fire without the assistance of vertical ventilation. This will be a tough endeavor, but it’s possible and may take multiple advances into and out of the fire area by crews pushed back by deteriorating conditions. In this situation it’s best to position crews inside the downwind exposure to pull ceilings along the party wall and hold the fire to the original building.

The last possibility and the most hazardous circumstance on arrival is an impending backdraft situation. If you’re facing high heat vent limited conditions forcefully pushing from the first floor, then you have a few options.

-Flanking at an angle with a large diameter lines

-Remote water application via piercing nozzles, Bresnan distributors

-Remote water application via piercing nozzles, Bresnan distributors

-Vertical ventilation while flowing into the superheated gases before advancing

The inherent voids in Taxpayers makes these conditions a common occurrence. Once again conditions and resources will determine your play. The simplest is by flanking two large diameter hose lines on superheated conditions to cool the environment from outside. Crews are positioned safely at offset angles in case of a backdraft or smoke explosion. Never congregate in front of openings in case of a hostile fire event unexpectedly occurs. Vertical ventilation is the most effective, but obviously a first floor or basement fire in a multi story building limits that option to an extent. One option is opening the display bump-outs inside the front windows. Remote cooling or water application requires some special equipment and training. You may also need to breach a wall or floor which is a time consuming operation. In the end, be cognizant of ventilation limited indicators before haphazardly opening doors and windows leading to a hostile fire event.

The Short Staffed Response

The best thing you can do for these structures is to pre-plan. Short staffed departments do not have the luxury of resources making time all that more important upon arrival on scene. Knowing the inherent risks of each individual building can help you quickly make initial assignments upon arrival. Get your crews out into these buildings- regularly. There are many things to look for during walk throughs, just a few of them are:

– Air vents in the floors indicating basements

– Exterior/interior basement access

– High ceilings in one building, low ceilings in the next indicate it is probably a drop ceiling (void above)

– Parapet walls, marquees, canopies, and cornices as they increase the risk for structural collapse.

– Construction remodeling- legacy vs. lightweight construction

– Location of stairs

– Terrazzo Floors

Modern codes require fire walls between occupancies; however, sometimes these codes are not enforced in existing structures or in older historic districts. When fire gets in a cockloft without fire walls, it can rapidly spread horizontally to the exposure structures on either side. If this occurs, you will not win against these fires with 4-6 firefighters. A “simple” fire in a taxpayer can overwhelm your resources quickly due to the excessive amount of voids we’ve already spoken of. Additional manpower isn’t a want, but a necessity. Refusing to call for more help can result in the loss of an entire block.

As always, situation dictates response. Like every fire, Taxpayer fires need an attack crew for the seat of the fire, protecting stairs/egresses, search, ventilation, and more. With the complexity and variability of commercial and residential properties, successfully fighting these fires is not something you can do with minimum manpower. Big fire, big building, means lots of water. As discussed above, typically the initial attack line we think to pull is a large diameter line. However, with minimum manpower, when advancing through structures such as these; you risk a slow advance on the fire, and you can quickly wear even the most advanced firemen. Short staffed departments may want to automatically opt for the two smaller handlines, if for no other reason than not wearing out your crew. Once additional crews arrive, if you need more GPM on the first floor, you could choose to also advance the large diameter line. Yet another reason you’ll need the additional manpower.

Knowing which structures are occupied vs. vacant is extremely important for departments with short staffing. As initial response is small, you’ll want to know which structures are more likely to be occupied- thus being your primary focus for initial search. Once additional crews arrive, search can be extended to the other structures if not already completed.

As discussed previously, roof ops may not be an option for short staffed departments. In this case, going interior may not be possible. Without the lift of heat and smoke from vertical ventilation interior conditions can become unbearable. You may have to utilize deck guns, flanking the angles with large diameter lines, or an aerial if available to fight the fire from the exterior.

Many have mistaken sound decisions in unsound buildings. All things considered, collapse and unseen fire spread should be the two red flags in the back of your mind on arrival on a Taxpayer fire. These fires aren’t won at 3am; they’re won the previous day during a preplanned walkthrough. Use every available opportunity to become intimately familiar with these buildings, whether it’s an EMS call or just getting out of the house on a nice day. The devil is in the details, and the minutiae matters.

Citations

Due to the nature of how common trade knowledge, jargon, terminology and methods are passed down amongst the fire service much of the articles information can not be cited as a proprietary source to one particular piece of work, individual, group or otherwise.

Dunn V. Collapse of Burning Buildings: a Guide to Fireground Safety: 2nd Ed. 2nd. Ed. Tulsa, OK: Pennwell; 2010.

Hill HJ. Failure Point: How to Determine Burning Building Stability. PennWell Publishing Company; 2012.

The Fireman’s Guide to Main Street: 5 Buildings to Know

Alexis Shady & Chris Tobin

There is a quote that we believe represents the vast majority of the fire service concerning buildings; it reads,

“Many an object is not seen, though it falls within the range of our visual ray, because it does not come within the range of our intellectual ray.” Thoreau, Henry

Simply put, we all see buildings, but few understand what they’re actually looking at. That’s a problem, a really big problem for two important reasons: a building is the one thing that directly or indirectly affects everything we do on the fire ground and the only thing we can do about a compromised building is avoid it entirely. We show up with no solution to sagging roofs, crumbling walls, or missing floors other than staying away. We can mitigate smoke, fire, and rescue trapped victims, but we can do nothing about the leaning wall. It’s this stark reality that many forget and have paid the price. You can know all there is about fire behavior, your tools and strategies, none of which hold any value if you’re unfamiliar with the space in which they are relied upon. Some may say all fires are the same, which is true until you put one in a building. Behind every door are an infinite amount of variables, some known, some unknown and some unexpected. This is why nothing’s routine till it’s over and why knowing your buildings on a visceral level is paramount. If you want to be able to forward think, you must understand the data you’re receiving.

This will be a five part series exclusively examining five different types of legacy construction, each with its own article as it pertains to firefighting. The types of buildings were selected based on their prominence in today’s main streets and historic districts. These specific types of buildings exist in small towns from coast to coast but are more commonly found East of the Mississippi River where our national building stock originated before moving Westward.

The five buildings are the old house, the taxpayer, the old mill, the vacant theater, and the bowling alley. Each of these will be examined along with inherent hazards and a play book for handling fires specific to each occupancy. Additionally since many of these buildings are found in small towns with departments that may not have the adequate resources, there will be a section based on short staffed responses for each. The objective of this series is to present the most useful amount of information in the least amount of space. Each of these buildings are worthy of their own book themselves; this series is meant to be concise and simple information for any level of firefighter. As with any article on architecture, regional vernacular and departmental jargon may vary. Nothing in this piece is the final say, only the individual reader and their streets can make that claim.

Part 1

The Building

The old house: The older single family homes in historic districts and around Main streets come in a countless styles and forms, from large ornate Victorians that line avenues owned by early prominent landowners to small shotgun bungalows within walking distance to factories facilitating the local workforce. The majority of what we see today are legacy framed or masonry brick buildings dating from the 1850s-1930s. Due to their abundance and because the other buildings in this series are all commonly of masonry construction, this section will address specifically wood framed houses. Access to railroads for Eastern timber played a pivotal role in how early or late your areas legacy building stock will be. A towns layout also determined what type of structure would be built. Long narrow urban lots would be best suited for gable front or shotgun style houses while wider ones would accommodate “I-Houses,” two rooms wide and one room deep. Most legacy frame houses were multi-story with half-storied gable windows or dormers of some sort. These buildings are easily identified from the exterior by stone foundations, center chimneys, wood siding, transoms, long narrow windows, metal roofs, and upper floor window AC units are common due to sub par centralized air.

The Hazards

The inherent architectural hazards in old legacy homes starts in the framing. With access to timber and saw mills these “mass planned” homes went up fast with balloon framing techniques taking the place of rough cut post and beam. The fire spread in balloon framing is nothing new to firefighters. The continuous stud bay existing from basement to attic creates a direct avenue of fire extension that can blow past floors of unsuspecting occupants and the unprepared fireman. Balloon framing ended in the 1940s and was replaced with today’s platform framing. The classic indicators of balloon framing are, long stacked windows sharing the same stud bay, stone foundation and center chimney placement. Beware of modern additions to the exterior of these houses such as garages or sun rooms. This will create an interior balloon framed wall that was originally an outside one.

In addition to the framing voids created, legacy homes contained older lumber. Go into an old attic and rub your finger on a ceiling joist, you’ll find what appears to be charcoal like dust. The structural members in these places are subject to what’s called pyrophoric carbonization. The wood is slowly oxidizing, losing moisture and thus burns more intensely due to lower ignition temperatures than expected. Many a fire has been started by a lightbulb being hung too close to a century’s old piece of lumber.

The layouts of these homes present some challenges to firemen. Two sets of stairs would be common, one main set near the entrance and smaller set in the rear off the kitchen called “servants stairs.” A common roof style is the “ saltbox or catslide” depending on your region, which poses a significant upper floor fire spread potential via the roof soffit. In addition some lesser known hazards are pocket doors, window transoms, laundry chutes, tin ceilings, metal roofs and combustible varnished wall coverings. These are all things to be cognizant of during firefighting operations.

The Playbook

Three words, hold the stairs! These buildings are almost always multiple stories with the bedrooms up stairs. The stairs and egress paths must be your strategic priority, fire heat and smoke will naturally be drawn up the stairs and into the sleeping areas above. If there are servants stairs this task will be doubled but no less important. Keep an eye out for a side entrance, these were common and offered access to the basement and second floor stairs from one location. The first line off must address this without delay. Any time paused outside is more time the upper floors are filling with deadly byproducts of combustion. Confinement is key in legacy wood framed houses, search teams need to be aggressively diligent in getting above and ahead of hose lines if conditions permit in order to close doors and start overhaul concurrently with fire attack. Keep in mind if you VES and close the door for confinement there may be a failed transom window above, a high scan with a TIC should address this.

Ventilation in these buildings should play off the compartmented nature of their design. This means hydraulic ventilation by the hose team will be much more effective than the wide area floor plans seen in modern homes. This also allows extinguishment, overhaul and ventilation to be done simultaneously with one or two crews in the area of origin. Careful with PPV early on, as the many voids will give way to some very undesirable conditions. Fans should be used only in conjunction with overhaul well after the fires under control. Due to the prevalence of hip and gabled roofs, vertical ventilation is a common tactic for top floor fires containing knee walls. Understand you’re venting voids not living space.

Since these buildings are compartmentalized by design a single 1.75 hose line flowing 150gpm will do considerable knockdown to multiple rooms or even multiple floors of fire. Even so, as a regular precaution a second line should be put advanced to the upper floors for cutting off extension. This line can be dry stretched initially to get it in place quicker to upper floors.

Maneuverability wins the day in these buildings. If conditions require a large amount of water on arrival, choose the deck gun before the 2.5 if possible. Trying to advance a large diameter line to upper floors inside a building with numerous small rooms creating corners will be futile. A well off fire will out pace your advance regardless of your available GPM. Save the 2.5 for defensive operations or exposure protection.

Once the main body of fire is knocked, overhaul needs to start in two places as simultaneously as possible. Once in the room of origin, and the bathroom or kitchen closest to the fire. Plaster and lath walls and ceilings are common and need to be overhauled appropriately by working the connections along a stud, rafter or joist. Kitchens and bathrooms are important to immediately check due to the presence of “wet walls,” or the wall that contains the plumbing and vents creating a pipe chase. These walls are even more important when they’re directly above the fire floor or any fire in the basement. A general rule of thumb for overhauling in legacy construction is fire in the floor, open the wall. Fire in the wall, open the ceiling. Fire in the ceiling, open the baseboards on the floor above. These are fires you must stay ahead of by understanding where they’re going before it gets there. This forward thinking is the difference between evacuation tones and a quickly extinguished fire.

The Short Staffed Response

The initial response to an “old house” fire for a short staffed department includes the same actions as that of an urban department. The initial attack line gets stretched, water is pumped, and command is established. It has the potential to get complicated when your initial response consists of anywhere from 2-6 personnel. If your initial response does not have adequate manpower to complete tasks safely, your attack choices are made for you until additional personnel arrive. The only exception to this rule is for rescues, with confirmed or suspected entrapment departments must search.

As we talked about before, protecting the stairs can make or break how quickly you beat the fire. With low man power you may have to make an educated guess as to where the fire is, and make a choice as to which set of stairs to protect. Another line must be stretched to the second set of stairs as soon as manpower allows. The wrong choice can have dire consequences, so understanding fire behavior and fire spread in your old homes is extremely important.

Then there are the “other” tasks that must be completed at every fire. Manpower and the scene will dictate the tasks that are prioritized. Officers must be capable of reading fire behavior and be able to quickly and correctly prioritize tasks across their members. Understanding the construction of the homes in your district will help you with these decisions. It is imperative that officers know their crews strengths and weaknesses and assigns tasks appropriately.

Without the luxury of having specific companies your members will need to be cross trained and capable of completing all of the tasks a fire requires. Frequently, members will be responsible for more than one task. An example of this could be your initial attack team also completing the primary search. Nozzle man focuses on the fire, and the second man quickly searches rooms and closes doors as they advance. Engineers must be comfortable pumping tank water to the first line, and then establishing their own water supply. Communication and speed are crucial.

Pre-planned mutual aid agreements may be beneficial. Having mutual aid departments automatically dispatched to your fires allows the officer to focus on the fire in front of them instead of worrying about what resources he or she will need to request from mutual aid – help is already coming. Recall for members and mutual aid from surrounding departments can take anywhere from 5-20 minutes on a good day, which is great for manpower on longer scenes, but doesn’t help for initial response tactics. You’ll still need them though, as you’ll need to provide your members with a chance to rest.

Old house fires also come with the potential for it to be a rural fire which adds an entirely new set of complications. Rapid response and establishing adequate water supply early are the keys to success for these scenes. This is where pre-planning of your district is extremely important. The officer and driver must be aware of where the closest hydrant or dry hydrant is and be able to rapidly establish a tanker shuttle if necessary. Tank water can get you a long way, but with delayed response due to travel times, it may not be enough. Don’t be afraid to call for help. It’s better to have too many on scene and send them home, than not enough. You can’t put out a fire if you run out of water.

Regardless of department size or response, legacy wood framed buildings require strategic foresee-ability on arrival. A wood framed building and its seven sides of fire spread have been the thorn in many a Chiefs side. The old houses that exist in every small town, in every state, demand a certain level of respect that has been lost to modern lightweight construction in the name of a penny. These homes were built to last and will test every skill set a firefighter claims to have. Never forget the reality is, no matter how much you know, where you work, or how good you think you, are the simple fact remains; the building does not care.

Saltbox Construction

Building construction styles vary across the country, states, and even in your own city. It’s important to understand your district, so you can understand how a fire may spread in a structure, and what the inherent fire spread/collapse risks of that structure may be.

As I started to study the construction styles in my city, I noticed several unique design styles. I honestly wasn’t sure what I was looking at, so I started to do some research. I assumed that with unique styles comes changes in fire spread and firefighting tactics. One such style of construction in my district is the “saltbox”.

A saltbox house is designed with a Colonial style of architecture originating in New England, first seen around 1650. The rumor is that saltboxes gained popularity during colonial times due to the Queen Ann taxation of houses taller than one story. By having one roof line, with the rear being single-story, they could avoid the tax. In reality, during that time, multiple families would live in the same home. So most likely, the style came from needing more space, not needing to avoid a tax.

Note the central chimney.

Saltboxes are frame houses with two stories in front and one in back, having a continuous pitched roof. The original homes were designed around a central chimney with a family room and kitchen on the first floor and the bedrooms upstairs. When the families expanded and they needed more room, a shed was added to the back of the home and the roof extended. The kitchen was then moved to this room. In the early 17th century original homes began to be constructed in this style. The name comes from wooden boxes used at that time to store salt. The boxes and homes both shared the same gable style roof shape. (Gable roofs are the most common roof types in the midwest- two roof sections sloping in opposite directions, the highest edge meeting to form the roof ridge.) Saltbox gable roofs have asymmetrical sides, one section being tall, the other short.

It’s important to note there will be additional void spaces in these types of structures.

Not every structure that has a rear addition with a pitched roof is considered saltbox. An example would be the images below. The important difference here is that there is no continuous roof line. The soffit will act as a sort of “fire stop” preventing the trench effect spread of fire from the first floor.

Notice the soffit provides a break in the roofline.

So why does knowing this construction style matter? Typically first floor fires spread to the upper floors in various ways such as extending out the windows to the second story windows, traveling up a stairwell, burning through the floor, etc. With saltbox style construction, fire can spread from the first floor to the attic space by following the voids in the roof line all the way to the top. This can happen fairly quickly, without being seen from the outside. Think similar principles to a basement fire traveling up the walls of a balloon frame house.

In a saltbox style home, this vertical fire spread occurs through the trench effect, such as the Kings Cross Fire in London in 1987. Once the fire gets into the roof line, there is nothing to stop it from going to the ridge and spreading across the rest of the roof. As with any peaked roof structure, there is collapse risk from fire exposure due to the structural framing, roof decking, etc. What makes these structures different, is the potential for rapid fire spread to the stories above. When fighting these fires you’ll want to be sure to send a second crew to the upper floors to check for extension.

Citations

Chimney- and Trench Effect, MSB [Video file]. (2015, February 16). Retrieved from https://www.youtube.com/watch?v=PSmeg-vn6ho

Dunn, V. (2010). Collapse of Burning Buildings, 2nd Edition: A Guide to Fireground Safety. PennWell Books.

Framing Styles for Timber Frames and Post & Beam Barns. (2018, September 10). Retrieved from https://www.vermonttimberworks.com/learn/post-beam-framing-styles-bents/

Gable roof. (2009, November 25). Retrieved from https://en.wikipedia.org/wiki/Gable_roof

History of Saltbox Style Homes. (n.d.). Retrieved from https://houseplansandmore.com/resource_center/history-of-saltbox-homes.aspx

The King’s Cross fire, 1987 ? fires that changed history. (n.d.). Retrieved from https://www.london-fire.gov.uk/museum/history-and-stories/historical-fires-and-incidents/the-kings-cross-fire-1987/

London Fire Journal. (2005, July 13). KINGS CROSS FIRE – 1987. Retrieved from https://londonfirejournal.blogspot.com/2005/07/kings-cross-fire-1987.html

Saltbox. (n.d.). Retrieved from https://www.britannica.com/technology/saltbox

Trench effect. (2004, June 27). Retrieved from https://en.wikipedia.org/wiki/Trench_effect