ROOF OPERATIONS: LIGHTWEIGHT TRUSSES AND VARIATIONS
In “Ventilating Flat Roofs” (September 1996), I discussed basic roof operations for a first-due truck company in buildings primarily of ordinary construction with flat roofs, skylights, and scuttle covers. Different construction components have a direct impact on our operations. The first personnel on the roof must recognize unusual roof features and relay such information to the incident commander. The IC in turn will determine the feasibility of roof operations and committing additional resources to this position.
LIGHTWEIGHT STEEL BAR JOIST TRUSS
One of the most common types of roofs you may encounter is a steel bar joist truss assembly covered with a corrugated metal roof, commonly known as “Q” decking. The metal bar joists can be used to span distances greater than 50 feet and are usually found spaced four to six feet apart (larger spacing is possible, depending on how the entire roof was designed). These roof assemblies` lack of fire resistance provides little or no protection for operating forces. The roof`s rating comes from the type of roof covering installed and the type of ceiling tile installed under the roof. Do not rely on lightweight steel trusses to withstand any amount of fire.
The covering you usually will find on this roof is a metal decking covered with a combination of hot asphalt, which is covered with tar paper; a rigid insulation approximately one to two inches thick; and a built-up roof of paper, tar, and gravel. One hazard associated with the metal decking is that the sheets are installed in a staggered pattern. The joints and seams do not necessarily end at supporting beams; most times, they do not. This leaves the possibility of an unsupported “flap” left after you`ve cut a ventilation hole.
Another type of roof decking found with trusses is gypsum decking. There are two main types of gypsum roofs. One is installed using gypsum planks usually two inches thick and two feet wide by eight feet long. The other is poured in place on form boards with reinforcing wire added for strength. Both types are covered with a waterproof roofing material.
With its tar and gravel covering, this type of roof may be hard to identify. A telltale sign of gypsum roof decking is that once you start to cut it with a power saw, a cloud of dust will be thrown from the saw and a powdery residue will be left. Because both forms of gypsum are very heavy and supported by a truss roof assembly, anticipate early collapse and do not attempt operations under heavy fire conditions.1
The trusses` underside may be covered or left open. In office buildings or shopping malls, ceiling tiles will normally be installed in all business and sales areas. Some areas, such as storage, utility, and staff locker rooms, may not have tiles. In large commercial buildings, such as light manufacturing occupancies, garages, and storage facilities, only the front reception area might have tiles. In most cases, the tiles are suspended on a grid of lightweight metal framework, which is hung from the trusses with wire. Entire grids have been known to fail during a fire, dropping down in large sections, ensnaring anyone below.
You must perform roof operations on buildings with this type of roof at the earliest possible moment, if at all. The first-due truck officer, or the IC if he is first to arrive, must try to determine the fire`s location and severity. If smoke or heat conditions indicate an advanced fire condition, operations over the main body of fire are not safe. These trusses will start to fail after only a few minutes of exposure to high heat conditions. If only light smoke is showing or if there is a light fire condition, you may perform roof operations. Again, this is a judgment call based on life hazards–both firefighters and trapped occupants–and fire conditions.
If the IC decides to send firefighters to the roof, they should perform operations quickly and then withdraw to an area of safety. In large buildings, this may involve their moving to another section or simply backing up so many feet.
As with buildings of ordinary construction, first look for any natural openings that you can use for rapid ventilation. In large buildings, you may find skylights. Luckily for us, skylights are made of plastic and will self-vent if they are near the main body of fire. If there are no outward indications of fire, look for smoke pushing near these skylights or smoke inside the skylights. Remember that randomly removing the skylights may spread the fire.
Look for any ventilators or ductwork that may terminate at roof level. They may offer clues as to where the main body of fire is located. Remove turbine-style ventilators at serious fires, even if they are remote from the main body of fire. The way they are designed to alleviate heat from the cockloft or attic areas will also rapidly spread the fire: The more heat that builds up, the faster the turbine spins and the more air it moves.
If there are no skylights or if they are not in an advantageous position, you may have to cut the roof itself. You can cut the roof decking with a carbide-tip blade, but a metal cutting blade works best. The first cut will normally not go deep enough to cut the entire metal decking, but it will make it possible to remove the roof covering. You will need a second cut inside the original cut, by three to four inches, so that the guard will not get hung up on the built-up roof.
Because a rectangular cut may leave the saw operator standing on an unsupported flap of metal decking, some companies are experimenting with large triangular cuts on metal roofs. Although the size of the opening is reduced, you get the benefit of a quicker operation (total cutting of 16 feet for a four-foot-square hole vs. 12 feet for a four-foot by four-foot by four-foot triangle) and the added stability of the unsupported roof sections.
Depending on the size of the roof and the extent of the fire, it may be necessary to leave the roof as soon as you complete your cuts. On a large roof, it may be possible to retreat and start another hole. Take care, as the fire may be spreading rapidly below your feet. Fires in buildings with this type of roof have been known to spread in the valleys of the corrugated metal roofing. The heat of the fire turns the hot asphalt covering back to liquid, which produces flammable gases that then ignite. The fire spreads along the valleys of the “Q” decking, heating more asphalt as it goes. This may occur a fair distance from the original fire area. There are two ways to fight these deck fires: from above by cutting the roof (potentially with a trench) or from below by using hose streams to cool the underside of the deck, stopping the gas production.
Remember, these roofs will rapidly fail under advanced fire conditions. If there are any signs to indicate that it is a fast-moving fire or that the building`s integrity is in doubt, do not commit personnel to a building with these trusses.
The wood products industry has also introduced several products that greatly impact the fire service. Lightweight wooden trusses are used both as roof and floor assemblies. Once a building is enclosed, it is almost impossible to determine what it was constructed of. That is why you must keep apprised of what is going on in your district.
The lightweight wood trusses usually are comprised of 2 2 4 lumber for both the roof and floor assemblies. These are held together with gang nails or gusset plates. Although the gang nail should penetrate to a depth of about one-half-inch, variations exist, including those that go down to as little as one-quarter inch. These trusses normally are used on a 24-inch center spacing. Roofs then can be covered with plywood or flake board. Depending on the thickness of the roof decking, personnel who begin working on the roof might find that it feels slightly spongy.
As with fire operations in buildings with lightweight steel trusses, you must perform operations on these roofs early in the fire. The IC must anticipate the need for vertical ventilation and assign personnel early. If the IC waits until the fire has extended into the attic area, it may be too late to assemble the needed firefighters, raise ladders, and begin ventilation cuts–the fire will already be attacking the truss components. Under heavy fire conditions, anticipate early truss failure. Dead load storage in the attic area, gang nails that may have loosened in transit or installation, and the added weight of clay tile or second or third roofs can all contribute to early truss failure.
When the trusses are used as floor assemblies, the collapse potential is just as great. An additional problem is the fact that this truss void is protected by gypsum board below and covered by plywood above. Both act as insulators and may hide the fact that the fire has entered this loft area. The fire now has the run of this entire area. The only vertical obstacles to horizontal fire spread are draftstopping if it has been installed properly or the four outside walls. Fire can burn unchecked through the void area, reach the outside wall, and spread to the upper levels–all the time being shielded from firefighters by the walls. That is why even at relatively minor room and contents fires in drywall buildings it is important to check for extension.
Firefighters` lives are held in the balance by drywall. Properly installed, it will give us a fair amount of fire resistance. This is without any improperly covered openings. Ductwork, electrical services, and plumbing runs are just some of the services that may penetrate the drywall. If the openings are not properly sealed, fire and smoke have a path to follow. From the building`s exterior, the IC must observe the area between floors of the fire building. Smoke pushing from this area may signify fire in the void.
Wooden I-beams are also gaining in popularity. They are used to span large distances, up to 60 feet, and can be found in all types of structures. In live fire tests of an unprotected floor assembly, they performed very poorly, failing in around five minutes.2 So again, we will be basing our survival on the drywall.
Most of the lightweight wood trusses use plywood as the final external layer, whether as flooring or as roof decking. Its use as roof decking makes the process of opening up these roofs with power saws relatively easy. To make cuts and get off the roof as soon as possible, I recommend the louver cut. Not only does it work well with the plywood, but it is less time-consuming to just “flip” the cuts vs. trying to pull the sections.
Other new ideas for the construction trade include metal “C” channels used as both rafters and beams. They are similar to the galvanized studs that are frequently used, but they are of a much heavier gauge. There has not been much documented experience with them under fire conditions, but expect them to perform just as other unprotected metal components–early failure.
It is in your best interest to be aware of what goes on in industries that affect the fire service. It is also important to know what buildings are going up in your district. Once they are on fire, it is too late to try to guess what is holding the building up. n
2. Brannigan, Francis L. Building Construction for the Fire Service, Third Edition (Quincy, Mass.: National Fire Protection Association, 1992), 541.
A lightweight open web steel bar joist building with corrugated metal decking. This assembly as shown has no fire rating. The rating comes from the ceiling and roof that are used. (Photos by author unless otherwise noted.)
Roof turbines are visible along the roof line of this multiunit apartment complex. If fire enters the attic area, a rise in temperature will cause the turbines to spin faster, spreading the fire. Remove all turbines to avoid this.
(Left) A typical built-up roof on metal decking consisting of multilayers of material: two layers of tar and gravel with insulation in between, about three inches thick. (Photo by Tim Melton.) (Right) A triangle-shaped vent hole is cut in a metal decking roof. The triangular shape seems to reduce the flap problem. This roof needed to have the covering, about three inches worth, removed before the deck could be cut. A chain saw was used to cut this material. Then a rotary saw with a metal cutting blade was used to finish the opening. Remember to cut inside the original dimensions of the hole (about three inches) so that the handles or guards on the saw do not prevent the blade from reaching proper depth. (Photo by Tim Melton.)
(Left) All new townhouse construction featuring lightweight wood roof trusses, floor trusses, and metal studs. In some cases, the metal studs are nonfirestopped, giving a channel for smoke and heat to spread unchecked throughout the building if the fire breaks through the drywall skin. (Right) Closeup of a typical gang nail. Notice how the bottom chords do not line up perfectly and how regular nails are used to secure the plate.
(Left) Smoke pushes from two of those “in between” areas. At the line between the first- and second- floor windows–at what should be floor level–smoke is pushing through the siding; at the upper righthand exposure 4 side, smoke is pushing from the area where the two buildings meet. Both are indicators that the fire has developed from a room and contents fire; has gotten through the protective skin (drywall or plaster); and is now in the voids of the building, attacking the structural components. (Right) “C” channels, heavy gauge metal in the shape of a capital “C,” are installed as roof rafters in this modern church. They get their lateral strength from the plywood decking. Like most steel structural components, these channels are susceptible to elongation, twisting, and early failure under heavy fire conditions.
BOB PRESSLER, a 22-year veteran of the fire service, is a firefighter with Rescue Company No. 3 of the City of New York (NY) Fire Department. He created and produced the videos Peaked-Roof Ventilation and SCBA Safety and Emergency Procedures for the Fire Engineering video series “Bread and Butter” Operations. Pressler has an associate`s degree in fire protection engineering from Oklahoma State University, is a frequent instructor on a wide range of fire service topics, and is a member of a volunteer department.