Know Your ROOF
BUILDING CONSTRUCTION
Photo by Frank Brannigan
Most fire departments are very concerned about the safety of their aerial ladders, ropes, and aerial platforms. Because firefighters depend on this equipment for physical support, its failure can cost lives. Specifications are tightly written, and acceptance and inservice tests are carefully monitored.
There is one working platform that supports firefighters, which is not designed specifically for their use, is not monitored by anyone, and has cost many firefighters their lives.
It is the roof.
During training exercises, the roof is always assumed to be stable. The fire service offers a few warnings, but they are often too little and too late. The only instruction provided in one pre-fire plan lists architectural roof types-Flat-BUtterfly-Lantern-Gable-Hip-Gambrel-Shed-Mansard-Dome. While such information is useful, it is by no means complete. We must know the roof’s structure as well as its appearance.
At one fire, where three firefighters died in a roof collapse, a chief officer said that he had learned nothing and would use the same tactics again. We simply cannot accept that kind of attitude in the fire service today.
It is time to look at the subject systematically. We cannot assume that the roof is usable until we have immediate positive evidence that it is about to collapse.
The roof’s chief purpose is to protect the building’s insides from the effects of weather. In some types of buildings, such as tilt slab concrete construction, the roof is vital to the stability of the walls. The designer never conceived of the roof as a firefighter’s working platform.
How safe and how useful is the roof as a working platform? What are its assets and liabilities?
The force of gravity is the eternal enemy of the building. The designer provides a system to resist the pull of gravity. In most buildings there is no protection from fire attack on the gravity resistance system, since they are legally classified as “non-fire resistive.”
The principal characteristic of fire-resistive design is resistance (within limits) to collapse because of fire. (Only after disastrous fire extension in “fireproof” buildings were requirements for fire containment added.) Non-fire-resistive buildings, therefore, are not legally required to resist collapse in a fire. They may have some inherent fire resistant qualities, but this is not mandatory.
Sometimes, when I show pictures of current construction, which demands changes in tactics, I am asked “Why are ‘they’ permitted to do this to us? What is wrong with our codes?” Nothing. A combustible building is permitted, and even expected, to collapse in a fire. As long as the structure can carry its normal load, it does not matter to the code officials that one type will collapse in a fire faster than another. We must look out for ourselves. Firefighter safety is not one of code authority’s problems.
JOISTED ROOFS
In general, senior officers have learned to estimate the remaining inherent fire resistance of a structure under attack from working on buildings with solid sawn wood joisted roofs.
Consider the extruded steel I beam. The top and bottom flanges are heavy, but are separated by a relatively thin web. In your mind, draw this I shape on the end of a sawn beam. The excess wood is “fat”—wood that is not necessary to the structure—along the sides. As long as this wood is all that is being consumed, the beam loses little strength and can support firefighters for a period of time.
In the American Society for Testing and Materials (ASTM) El 19 test, solid sawn beams lasted about five minutes per inch of thickness. (This is not to be used as a rule of thumb.) As the beam loses substance it softens and sometimes (but by no means consistently) gives warning of impending failure.
While the amount of time to failure cannot be accurately predicted, the amount of time burning should have a prominent place in the fireground commander’s thinking. Once the structure itself is involved in the fire, it is deteriorating in an unknown manner and rate.
In one fatal collapse, a mutual aid ladder company that arrived about 45 minutes after the first alarm was assigned to ventilate the roof. In another case, firefighters were delayed for some time while electrical service was cut. Then they charged into an immediate collapse. This problem can be especially serious when a senior officer arrives some time into the fire and orders tactics that might be suitable only on arrival.
When working on roofs supported on wooden I beams, light wood trusses, heavy wood trusses, steel bar joist trusses, and noncombustible but not fire-resistive concrete, do not use the same tactics as you would for sawn joist or rafter roofs. This can only lead to disaster.
Even “good old reliable” wood joist roofs may have inherent defects. I once examined a 90-year old building in Orlando, FL, that was being renovated. The joists were exposed. I noted that the joists in the center section between girders were short. They did not rest on the girders. They were nailed to joists that overhung the girders. This use of overhanging and “drop-in” beams is not uncommon. The nailed connections could fail very early in the fire.
Excess live or dead loads on top of or hung from joisted roofs can accelerate collapse. I have read press reports of a fire in which three firefighters died in a roof collapse where there was heavy ice on the roof. This should have been a clear warning that the roof had already expended much of its normal resistance to collapse.
In another case, a grocery market was converted to a Japanese restaurant where the guests sit around a grill and watch the chef cook. A restaurant of this type requires a heavy steel exhaust hood for each table. The hoods in this one were suspended from the roof. This created an additional undesigned dead load, and the roof is by no means the same working platform for firefighters that it was when its only purpose was to keep the rain off the Wheaties.
photos by Frank Brannigan
In yet another instance, three firefighters died in New York as they overhauled a ceiling. They literally pulled a roof mounted air conditioner down on their heads.
Ventilation tactics can accelerate collapse. We are aware that the impact of an axe may be the “straw that breaks the camel’s back.” Many think that impact is avoided by using saws. Not so!
There is no such thing as no impact. Even if a firefighter steps onto a roof so lightly that an egg would not be broken, he is still imposing a load on the roof that is at least twice its normal weight.
BEAM-SUPPORTED ROOFS
Apparently, roofs that are supported on heavy wood beams or laminated arches can be much less reliable than they appear. Long wooden beams are scarce. What appear to be long beams are often several beams spliced together with metal connectors, which can heat up and burn out of the wood.
A jai-lai fronton burned in a Florida city. The owners of the laminated timber arch building attempted to sue the suppliers of the foamed plastic roof for the early collapse of the “heavy timber” building. However, pictures taken by a visiting chief showed that the arches had simply fallen apart at the steel splices. The “slow burning” (another myth) laminated timber arches just fell apart.
Except for fire-resistive structures, the roofs of buildings being built today have little or no inherent resistance to early failure in a fire. This includes the roofs of code classified “non-combustible” buildings.
The typical non-fire-resistive roof built today is usually supported on wooden I beams, wood trusses, steel trusses (bar joists), concrete T beams, or, in some cases, cored concrete slabs.
As we have seen, wooden I beams follow the pattern of the well known steel I beams. The top and bottom flanges, which handle the compressive and tensile stresses respectively, are wide. They are separated by the “web,” which is only strong enough to keep the top and bottom flanges apart. Plywood delaminates as it burns, exposing the successive layers. As soon as the fire attacks the plywood, the structural strength is destroyed because there is no “fat.” It has been eliminated by a design that substitutes shape for mass. Because the plywood web is punctured for utilities of all types, the fire is on both sides of the plywood at once.
HAZARDS OF TRUSSES
Wood trusses
Let’s consider a plywood, or sawn board, roof on wood gusset plate trusses or wood and tubular steel trusses. Trusses substitute geometry for mass. All parts, all connections, of a truss are vital to its stability. There is no redundancy— nothing extra. (Some wood trusses are shipped with labels warning the carpenter not to cut any part of the truss.) The failure of any part of the truss entitles the truss to fail.
When fire hits such a roof it spreads rapidly over the surface of the plywood, delaminating and weakening it. This increases the chances that a firefighter will fall through, as happened in a southwestern city.
Photo by Frank Brannigan
The lightweight wood members burn on all surfaces. They have a high surface-to-mass ratio and little or no “fat.” The metal gusset plates, or gang nails, heat up and destroy the compressed wood fibers, which hold the teeth, by pyroletic decomposition. The fire moves so fast over the plywood that there may be little evidence of the tar bubbling or softening on the surface. Opening a vent hole increases the airflow and accelerates the burning of the structure near the hole. Such roofs should be provided with automatic vents.
Photos by Frank Brannlgan
If a flimsy roof, like the one described above, is involved in fire you shouldn’t be on it.
“But we must vent the roof; forget the risk!” says Chief “Muchomacho,” who is is extremely proud of his direct descent from Lord Raglan who planned, but wisely did not lead, the Charge of the Light Brigade. In other words, this fictitious chief is saying “You must use a working platform that is demonstrably unsafe.” In a precedent setting case in Chicago, the officers of a chemical company, who required employees to work under known dangerous conditions, were found guilty of homicide.
A roof collapse also is a severe hazard to those inside. An officer of an upstate New York fire department told me about a fire in a typical wood truss fast food type structure. There was “fire showing” on arrival. Luckily for the firefighter, there was a problem with the hydrant, which delayed their attack. As they got water, the entire roof collapsed.
The building boom after World War II gave us the peaked roof supported on gusset plate trusses. When we moved into a garden apartment in New York in 1949, I went into the attic to set up our television aerial. I saw the forest of two by four’s. I didn’t know anything about the hazard of trusses. I didn’t know what a truss was, but I did recognize a change for the worse in roof construction. I made some vain efforts to alert those responsible. In 1971, a firefighter died when the roof of the identical unit next door collapsed as he was trying to vent it.
The triangular trussed attic space is useless for occupancy. At one time, we could be confident that if we saw attic windows the roof was constructed of sawn joists. This is not so any longer. Parallel chord trusses are being used for peaked roofs. This creates a huge void space between the ceiling and the roof, through which the fire can rage unimpeded.
Before you go on a peaked roof, open a hole low down, just above the eaves. If smoke flows from the opening, it is very probable that the fire has extended into the void.
Caution: This indicator does not work in reverse. If no smoke comes out, the fire still may be in the void but venting at another point. Also be aware of any firestopping that is installed in the building, which may prevent smoke from moving freely. Thus, there may be more smoke and fire inside the structure than is actually indicated through the inspection hole.
Some make a distinction between heavy and light trusses. I do not. To me, the distinction is irrelevant. A truss is a truss. The early collapse of a heavy wood truss killed six firefighters in New York.
A Florida country club has a wood paneled dining room. The roof is supported on big deep timber trusses. The trusses rest on an unprotected steel beam, incorporated into the wooden wall. Fire heated steel will attempt to elongate. If it is restrained at the ends and cannot, it must twist (rotate on its axis). The trusses will be dumped and the roof will collapse. Hopefully, the responding units will go into a defensive mode before a fatal collapse occurs.
Steel trusses
Unprotected steel trusses are particularly dangerous. Steel is such a strong material that units of very small cross sections can be assembled into trusses, which can provide long (clear) spans. The steel bowstring truss is typical.
In Wichita, KS, fire units turned out for an alarm at an automobile dealership that was housed in a bowstring truss building. A second alarm was ordered as they turned out of the fire station. Despite the heavy fire and the vulnerable structure, a hand line was advanced into the building. Five firefighters died in the collapse of the roof.
Such structures have no inherent fire resistance at all. Because trusses are inherently unstable, they are tied together to resist overturning. The ties transmit undesigned torsional (twisting) loads from one truss to another, resulting in multiple truss failure.
Collapse may extend far beyond the immediate fire area. A prime fire suppression tactic is to cool the steel, operating from a safe location. It doesn’t matter if the steel cannot be seen due to smoke. It is heating up. If it heats sufficiently, it will fail. Cool it.
The most common steel truss is the light parallel chord truss, known as the bar joist. These are sometimes used as part of listed fire-resistive floor and ceiling assemblies, which present serious problems.
When bar joists were tested by the ASTM El 19 test for fire resistance they failed within 7 minutes, compared to 2-inch thick beams, which lasted 10 minutes. In a basement fire in a building with unprotected bar joists, the floor was failing and opening up before first-floor occupants could escape. In tests at Underwriters Laboratories, bar joists that were 30 feet above a fire in light combustibles reached 1,540°F in 5+ minutes.
Convected heat moving laterally under the roof can heat up trusses that are far from the fire seat. At about 1,000°F, elongating steel is exerting its greatest thrust because it starts to fail at higher temperatures. Elongating steel trusses of all types, but particularly bar joists, have pushed down walls that are often far from the location of the fire. Personnel on the roof, a good distance away from the fire area, have been caught in the collapse.
Study a building under construction. Almost without exception, the roof trusses are spaced further apart than the floor trusses. A 6-foot spacing is not unusual. Firefighters cutting a vent opening may find themselves standing (for a very short time) on the cantilevered end of a flimsy sheet of corrugated steel.
When cutting such a roof with a power saw, it is easy to cut the top chord of a truss. The top chord is under compression and the cut sections will push together. Under fire conditions, such a precise cut is impossible. The structural elements are moving constantly as stresses increase.
ROOF AND WALL STABILITY
The roof is essential to the stability of many structures. This is particularly true of so-called “tilt slab” buildings. Precast concrete sections are tilted up and joined together to form the wall. Note that the contractor braces these walls with “tormentors” (adjustable diagonal braces) until the roof is in place and takes over the function of stabilizing the walls. The roof may be of wood, steel, or concrete T beams.
If you are losing the roof, the walls are in danger of collapse. I have been told by fire officers that such buildings are “designed to collapse inwardly.” This statement cannot be documented. I have pictures that show walls collapsed both inwardly and outwardly.
Large area one-story buildings are usually not required to be fire resistive. Concrete is inherently noncombustible. It can be made to be fire resistive if you are willing to pay the extra cost.
The concrete T beams in a typical huge warehouse are not fire resistive. The concrete below the tendons in the bottom of the beam can fall off early in the fire. The exposed cold drawn tendons, which provide the tensile strength that is necessary to a beam and which concrete lacks, totally fail at 800°F. Reinforced concrete is a composite material. If the connection between the concrete and the steel fails, the structure fails.
Watch carefully the reconstruction or alteration of existing buildings. The new roof may not be the same as the current roof.
SUMMARY
Few roofs built today are safe working platforms for firefighters. A dependable three-section articulating boom apparatus would provide firefighters with a safe working platform from which they could perform roof tasks.
“Roof” is not normally a threatening word. Quite the opposite. It evokes feelings of safety and hospitality. Unfortunately, to the firefighter “roof” must represent an ever present enemy.
