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In recent years, the rising costs of heating and cooling buildings has resulted in a variety of efforts aimed at improving the energy efficiency of the structures. Blown-in insulation, caulking of cracks, reduction of the number and size of attic vents and installation of "thermopane" glass double and even triple layers of glass have all had an impact on the firefighter's working environment inside a burning building.
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.
Photo by John Norman
Fire that was blowing down out of the cockloft seriously burned several of the firefighters who were operating at this top-floor fire.
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.
Photo by John Norman
A section of membrane roofing reveals little indication of the damage potential it possesses.
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.