These "home improvements" change the way a fire behaves within a building by limiting the amount of available oxygen (resulting in higher carbon monoxide levels) and slowing the transfer of heat to the outside, resulting in increased speed of fire growth toward flashover. Nearly every BTU produced by the fire, and all the products of combustion, remain inside the fire building until firefighters arrive. The terminology which has been used to describe this phenomenon is called "tight building syndrome." A relatively new product has increased this tight building effect in an area which is critical to fire operations: membrane roof coverings.
Membrane roofing has been in use for at least 15 years, replacing rolls of so-called "tar paper" as the covering of choice on many large-area flat roofs. Tar paper is a sheet of felt or paper which is impregnated with various asphalt coatings. This combination of paper and petroleum products created a highly combustible product but one which did a necessary task: repelling water from roof surfaces.
Tar paper has a number of drawbacks from the builders' and owners' perspectives. It is relatively fragile, it tears easily (which makes it able to be cut rather easily with either our ax or power saw) as well as being penetrated by hail, people walking on it or objects being dropped on it. Tar paper also has a relatively limited life span. It develops leaks after only a few years if conditions are not right. It also is relatively complex to install, requiring additional coatings of hot "tar" to be mopped or sprayed on over the top for sealing purposes.
Membrane roofing is designed to eliminate many of the drawbacks of conventional tar paper. Instead of a paper or felt base, membrane roofing uses fiber resin or polyester fabric as the membrane, which is then coated with various asphalt compounds to provide a tough, flexible and watertight seal over a roof area. The fabric provides tear resistance, which makes cutting with an ax more difficult than cutting conventional tar paper.
Membrane roofing, however, introduces several other problems for the fire service which can cause even greater difficulties and dangers. The first is the contribution membrane roofing makes to tight building syndrome. Since it does such a great job of keeping out water, it also does a wonderful job of keeping in smoke and, in conjunction with the insulation that is placed beneath it many times, heat. This contributes to an increasing number of backdrafts.
The composition of the membrane itself may also be contributing to backdrafts or "smoke explosions" in the cocklofts of fire buildings. The asphalt and plastic mixture gives off large quantities of flammable gases when heated. The gases which are thus distilled accumulate in the cockloft until the roof either burns through or is cut open.
The membrane is highly flammable, igniting readily on exposure to a match. This is the cause of many fires, since the material is applied and sealed by heating with a propane torch. This flammable membrane also produces the danger of rapid flame spread across the surface of the roof whenever it is exposed to flame, such as when the roof burns through, or when the saw blade or chain creates an opening over the fire.
This poses a real threat to the safety of the roof crew as well as to any nearby exposed buildings. The asphalt and plastic mixture liquefies and burns with the intensity of a Class B fire. A hoseline must be stretched to the roof position and operated as needed to protect the vent team and exposures, as well as for controlling flame spread across the surface. Naturally, standard practices to prevent hose streams from being operated into any roof ventilation openings still must be followed.
Membrane roofing poses another threat to firefighters operating at roof level. The reinforcing fabric mesh gives the material significant tensile strength, which means it does not tear easily when pulled from end to end or side to side. When the material is applied and sealed into place, it forms a relatively rigid surface which maintains its shape even when the roof deck beneath it is gone, such as from burning away.
The membrane acts like a surface of a bass drum, holding its shape with no support in the middle. Some manufacturers claim their fabrics have held up to over 200 pounds per feet before breaking. Of course, that's not counting on exposure to fire. The danger to firefighters, though, is that the material does not necessarily show any telltale sag if the roof decking below is weakened. In fact, at the scene of one recent fire, I watched several members step through a roof covering that looked perfectly sound but which had only roof joists left below for support. Thankfully, this was the day after the fire had been extinguished and when members returned to the scene to critique the fire. No one was injured but it taught us all a lesson: even in broad daylight, with no smoke or need for speed, it is impossible to judge the condition of these roofs by sight. You must probe ahead each and every step with a substantial blow from a tool. Remember to think of these roofs as being just like the skin of that bass drum.
It is nearly impossible to differentiate between a membrane roof and an older tar paper and hot tar roof once each has been in place for more than a few months. The time to identify them is when they are being installed. This too is harder to detect than older roofing methods. In the past, the old tar kettle (a source of many fires in itself), with its fragrant aroma, would indicate a roofing or roof repair in progress. Membrane roofing does not require a layer of hot tar to be mopped or sprayed on it. Like that tomato sauce advertises, "it's in there."
If you are unsure of the type of covering, membrane or conventional, your fire tactics should prepare for the worst-case scenario. Fires in top floors and cocklofts should prompt a precautionary hoseline to be stretched to the roof as soon as possible.
As always, roof crews should have at least two separate and remote means of egress. They should be aware of the wind direction and avoid getting caught on the down-wind side of a hole that is showing fire without hose line protection. They must, however, not hesitate in their roof venting duties, otherwise the gases accumulating in the cockloft are likely to wreak havoc on all those on the top floor, firefighters as well as civilians. Cut early and cut large!
John Norman, a Firehouse® contributing editor, is a captain with the FDNY, assigned to Rescue Company 1 in Manhattan. He is also an instructor at the Nassau County, NY, Fire Service Academy and lectures nationally on fire and rescue topics. Norman is the author of Fire Officer's Handbook of Tactics, which may be ordered by calling 800-752-9768.
