Wood Frame Construction: Firefighter Size-Up and Tactical Training
Common methods of wood framing construction include Balloon and Platform
In balloon style, the wall studs ran the entire height of the building, from foundation to attic spaces. There was no fire blocking in the stud cavities. Additionally, the floor and ceiling joists were attached to the studs by resting on a ledger (ribbon) board which was cut-in and, or nailed to the studs. This led to the void spaces between the floor and ceiling joists being open to the void spaces in the stud cavities, resulting in the void spaces in the entire building being totally interconnected. Hence, fire entering any void space would quickly spread throughout the entire structure. If fire does enter the stud cavities in the walls, often the quickest and most effective method of preventing extension further into the structure, as well as overhauling is to open the exterior siding of the building. This results in less damage to the living spaces, and requires less time to rebuild the structure once the fire has been extinguished. The occupants can even continue to occupy the structure while it is being rebuilt in some cases.
There are certain characteristics of balloon-frame construction that are a giveaway, a tattle-tale if you will, from the outside that should alert us to the presence of balloon-frame construction. Things such as window and door openings from first to second floor, all lining up vertically. Tall wall heights for two-story structures are fairly common. Also what looks to be a very old appearance with clapboard siding and usually roof rafters that are in some cases exposed and appear to be much smaller dimensional material than later conventional construction. Whereas present codes may call for a 2x6 or a 2x8 rafter, in the balloon-frame construction era, the rafters were frequently 2” x 4”. This , and the age of the building, frequently lead to the roof sagging at the ridge pole. Due to the age of the building, it has usually been re-roofed one or more times and several different materials may be visible. Frequently, the old roofing was not removed, causing added loading to the roof, further aggravating the sagging appearance. Any significant attic fire in a building that has been re-roofed, without the old roofing having been removed, is a candidate for early collapse, especially in older structures with undersized rafters.
This style of construction is no longer being built due to the cost of twenty to thirty foot long two-by-four’s being prohibitive. This led to the platform style of construction, in which each floor of the building is built as a separate unit from floors above and below it. In a two-story home, the foundation is laid, on top of that they will put a sole plate for the first floor and then floor joists, and then the sub-floor. The wall studs go up to the second-floor level to a top plate. On top of that top plate, the process is repeated for the second floor, floor joists, sub-floor, wall studs etc. and if its a two-story building, from there on up will be the structural elements for the roof assembly, either rafters or trusses, depending on the nature of the construction. A top plate on top of each level of wall, creates an inherent fire blocking at the top of each floor. So if a wall fire develops in the first floor, it could only go as high as the top plate and then it would be stopped by the top plate before it could extend easily into the second floor and up into the attic space.
With the advent of the new, lightweight wood frame types of construction, we are essentially seeing the 1990's version of balloon-frame construction because now all of those wall cavities and void spaces are now open and continuous throughout the structure. So a small fire in a void space in a wall in one portion of the structure on the first floor will rapidly extend into a number of void spaces in the second-floor floor system and into the second-floor walls and attic. Hence, we should consider the same type of tactics if the fire has extended into the stud cavities of the walls that we employ in balloon frame construction, namely, open the exterior siding.
Other potential hazards for firefighting crews in large residential occupancies, apartment complexes, condominiums, town homes, that sort of thing, is the occurrence of what are called “double-studded” walls. These basically are walls built with a set of studs, and then several inches away, another set of studs on the other side of the wall, which act as a sound barrier, greatly reducing the sound transmission from one unit or occupancy into the next. The problem with this is while there is fire blocking in between the studs of each wall, preventing vertical extension that way, the space in between those two walls is wide open. This is frequently where pipe chases, and utilities are run from one floor to the next. As a result, it provides a means of extension from one floor to the next. And once that extension has taken place, the fire is free to extend throughout the inside of the wall because there is quite literally no fire blocking between these walls, which would prevent it from simply moving in between these two walls. If there is reason to believe that the fire has gotten into the void spaces in the walls or in other portions of the building, figure out what paths of extension the fire could travel through and open those areas up and make sure to go as far as the fire went in those void spaces. Make sure to reach the end of where it was burning. Make sure those areas get overhauled properly.
A very common practice in the construction trades when they are working lightweight construction is to simply run the utility chases, plumbing runs, etc., through the floor systems. When they need to go through a plywood “I” beam, if they have a 12 inch deep web (the central portion of the truss), and a 10 inch diameter duct, they’ll simply cut an 11 inch diameter hole through that web member and run the ventilation duct right through that web member. This is very common. It creates a perfect means of extension from the void space in-between two plywood “I” beams into the void spaces for all the neighboring plywood “I” beams around it.
Bathroom Hazards in Wood Frame Structures
Another common location for structural compromise is the bathroom, due to extensive plumbing runs and utility chases within the walls. In wood frame buildings, this is the equivalent of “poke-through” construction seen in concrete and steel structures.
Hidden Dangers of Plywood “I” Beams and Finger Joints
Bathrooms often contain heavy materials like cast iron tubs, porcelain sinks, toilets, tiled floors, and tile-covered walls. As a result, the same degree of fire damage that might be survivable under lighter areas of the structure can cause rapid localized collapse in bathrooms due to higher floor loading.
During overhaul, always evaluate the structural integrity of the floor before entering a fire-damaged bathroom. Many firefighters have unexpectedly fallen from the second floor to the first because the weakened flooring gave way beneath them. Worse, splintered wood can trap the leg, making escape difficult and increasing risk of burns from fire still burning below.
A close look reveals the completely open nature of the void spaces in-between plywood “I” beams. It also shows the glued finger joints. They allow the creation of plywood “I” beams of any length simply by gluing it to one end of another plywood “I” beam. By doing so, they’ve created a combustible joint. The glue they use is combustible, and just as the glue in plywood is combustible, it will burn. It will increase the rate of burning and that’s one of the reasons that plywood is as flammable as it is and it tends to loose its structural integrity as quickly as it does in fire conditions. By the adhesive burning, which hastens the rate of flame spread, which in turn causes the plywood to de-laminate, losing its structural integrity, exposing more surface area, thereby increasing the rate of burning and so on. It is an element that contributes to very fast rates of flame spread in the types of construction where it is used heavily.
Lightweight Hangers: Fire VulnerabilitiesAppearances Can Be Deceiving
Another common practice to expedite construction and to reduce costs is the use of preformed lightweight hangers for structural elements, ceiling and floor joists, roof rafters, etc. The lightweight galvanized sheet metal style hangers you see in the picture on the left basically qualifies as unprotected structural steel, and as a result, under extreme fire exposure, just as with any unprotected structural steel, will fail quickly when exposed to fire.
Unfortunately, the building trades are somewhat deceptive. They do not want to give perspective buyers the appearance that they have built these structures out of the cheapest possible materials and in the cheapest possible fashion, so they will quite literally do everything they can to fool that perspective purchaser, and unfortunately us as well, by dressing up the portions of the structural elements that we can see once its completed, in such manner that the building looks to be built much more substantially than it really is.
There are a number of examples of buildings that appear to have 4” x 6” or 4” x 8” rafter material, 24 inch on center and solid 1” x 4” roof sheathing. Upon arrival if one of these structures were burning, you would think you had a fairly well built structure to work on and that you had approximately 20 minutes on the roof before you could expect the structural elements to weaken to the point where serious consideration should be given to withdrawing personnel from the roof. The problem is, if you look in the attic space in these structures, you will find that the entire attic space is made out of metal gusset plate trusses. It is open from one end to the other. And in fact, those gusset plate trusses are made with 2” x 4”s, not 4” x 6”s or 4” x 8”s. The 4” x 6” material that you see in the form of rafter tails under the eaves is simply window dressing. It was put there to lead any perspective buyer into feeling that the building is well built.
Attic Fire Inspection: A Critical Step
Whenever you enter any kind of wood frame type construction that’s on fire, a good general practice, if you suspect an attic fire or a fire overhead, as soon as you enter that occupancy, take a pike pole or an axe and make a small inspection hole in the ceiling and make sure you do not, in fact, have fire overhead already. If that area is clear, then move on. There are a number of incidences where fire loss has been considerably increased because crews were concerned about creating unnecessary damage by performing this technique, and in the time it took for them to find the attic scuttle hatch access, they frequently ended up sustaining much greater structural loss to the building. It’s much cheaper to replace drywall or lath and plaster than it is to replace the structural elements in the floor or roof system. Unfortunately, there are also cases where firefighters were burned because they were too concerned with not causing added damage, and wasted time looking for the scuttle, while they had a well involved attic fire overhead, which eventually collapsed into the living/working spaces of the occupancy, trapping and burning them.Whenever entering a wood frame occupancy with suspected attic involvement, make an immediate inspection hole in the ceiling using a pike pole or axe. Don’t wait to find the scuttle hatch—valuable minutes can be lost, allowing fire to spread. Countless fires have extended unnecessarily due to delays caused by concerns over cosmetic damage. Remember: it’s much cheaper to repair drywall than to replace a collapsed roof or lose a life.
Common Attics and Shared Spaces in Multi-Unit Buildings
We also need to be alert for exterior fire extension. A fire on a ground floor on a multiple-floor occupancy blows out through some sort of opening, runs the outside of the building to large, overhanging eaves, finds its way in through an attic vent or a soffit vent and extends into the unit or attic space above. So if you knock down the main body of the fire in that ground-floor unit, and you’ve witnessed severe exposure on the outside of the building where the fire occurred, you should take a look overhead to see if there is any way that the fire could have gotten into the attic and is still burning up there. Also, with the extension up the exterior of the wall, did it lap over any windows that may have led to extension into that second-floor unit directly above the main body of the fire on the first floor?
Large residential complexes, apartments, garden apartments, town homes, condominium units, will frequently have a common attic over a number of units. There may be very little in the way of firewalls, division walls, required by code, and if they’re not required, they’re not put in, simply to keep the cost down. When a unit experiences a large fire involvement, which extends into the attic space directly over it, there isn’t anything to prevent it from running the length of the attic. This is why the exposures on each side of the affected unit that was the original seat of the fire, need to be accessed quickly. Open the ceiling below the attic as soon as possible to make sure its not making a run over the top of that unit. If so, you need to get hose lines into that location as quickly as possible to slow or stop that extension.
Burn Time: Lightweight vs. Conventional ConstructionTraditional wood framing typically offers 15 to 20 minutes of burn time before structural collapse becomes likely. With the advent of lightweight (engineered) types of structural components replacing conventional joists and rafters, the burn time is greatly reduced. Depending on the type of lightweight assembly, failure can occur in as little as 5 to 7 minutes (plywood “I” beams). Examples of lightweight structural components are plywood “I” beams, metal gusset plate (M. G. P.) trusses, bar joist trusses, and open web trusses in both floor and roof systems. This reduced burn time is critical, as it eliminates the time we normally have to enter the structure and begin extinguishing the fire or searching for victims. As a result we should expect to see more firefighters injured, trapped or killed in the line of duty as lightweight construction becomes more prevalent, and we begin to experience greater numbers of fires in this type of construction. These components lose integrity rapidly, increasing danger to both occupants and firefighters. Fire attack strategies must account for the reduced survivability time of modern structural assemblies.
The only defense we presently have, is to be aware of the lightweight structures currently in existence, which requires thorough inspection practices. This means we need to examine the floor and roof systems for the presence of lightweight components. It also means we must observe new construction to find any use of lightweight components.
Roof systems usually found in wood frame construction normally consist of flat, gable or hip roof styles.
Asphalt Siding Asphalt impregnated felt and paper with a printed or pained surface imitation of brick or stone
Attic The space between the uppermost ceiling and the roof of a building (a. k. a. Cockloft).
Balloon Frame A wood framing method in which studs are two or more stories high.
Beam A structural member subjected to loads perpendicular in its length.
Bearing Wall A wall that supports all, or a portion, of a superimposed load such as a floor or roof.
Brick Veneer A single thickness of brick wall facing placed over frame construction or masonry other than brick.
Chase A channel or groove, as in a wall, for pipes or wires.
Chord Main members of trusses as distinguished from diagonals.
Cockloft The space between the ceiling and roof of a structure, distinguished from an attic by its low height, (a. k. a. Attic).
Cornice A horizontal projection that crowns or finishes the eaves of a building.
Delaminate To come apart in sheets or layers.
Drywall A system of interior wall finish using sheets of gypsum or button board and taped joints.
Duct A channel, usually for ventilating, heating or air conditioning.
Fascia A flat vertical board located at the outer face of a cornice.
Footing That part of the building that rests on the bearing soil and is wider than the foundation wall.
Furring Wood strips fastened to a wall, floor, or ceiling for the purpose of attaching covering material, a.k.a. lath.
Gusset Plate A plate that is used to connect the members of a wood or metal truss.
Header The beam spanning over a door or window opening.
Hip The junction of two sloping roof surfaces forming an exterior angle.
Joist A horizontal beam used to support a floor or ceiling.
Lamination Several layers of lumber making up a laminated beam.
Lath Narrow, rough strips of wood, or wire mesh, used to support plaster or stucco, a. k. a. Furring Strips
Ledger Board A board nailed to studs or bolted to concrete wall slabs to support joists or rafters, a. k. a. Ribbon Board.
Non-bearing Wall A wall that bears no load other than that of its own weight.
Parallel Chord
Truss A truss whose upper and lower chords are parallel.
Party Wall A bearing wall separating and supporting two adjacent buildings.
Penthouse A room or building built on the roof, usually to cover stairways, house elevator machinery, contain water tanks and/or heating and cooling equipment.
Pier A supporting section of wall between two openings. Also a short masonry column used as a footing in constructing a foundation.
Pitch The slope of a roof expressed as a ratio of rise to span.
Plate (Frame construction) The top or bottom horizontal structural member of a frame wall or partition e.g., top or sole plate.
Pokethrough An opening made in a wall, floor, or ceiling to accommodate utility services.
Rafter A beam that supports a roof.
Ribbon (Frame construction) A narrow strip of board cut to fit into the edge of studding to help support joists, a. k. a. Ledger Board.
Sheathing The covering applied to the framing of a building to which siding or roofing is applied.
Sill Frame construction: The bottom rough structural member that rests on the foundation
Soffit The underside of the fascia of a building; also, false spaces above cabinets, etc.
Stucco A material made of cement, sand and plaster and applied as siding.
Surface-to-
Mass Ratio The relationship between the surface area and the mass of structural members.
Stud Vertical structural uprights which make up the walls and partitions in a frame building.
Tie Masonry veneer: A metal strip used to tie masonry wall to the wood sheathing.
Truss A framed structure consisting of a group of triangles arranged in a single plane so that loads applied at points of intersection of the members will cause only direct tension or compression stresses in the members.
Void An empty space occurring between members or elements of a structure.
Web The wide vertical part of a beam between the flanges.
Web Member Secondary members of a truss contained between chords.
Wired Glass Glass reinforced with wire mesh.
Information adapted by LBFD from Brannigan, Frank. Building Construction for the Fire Service, Third Edition; Dunn, Vincent. Collapse of Burning Buildings; and IFSTA's Building Construction and the Fire Service.
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