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Flat Roofing

Modified bitumen roofing membranes were introduced to the North American market in the mid-seventies. Their usage has grown through the eighties and nineties and now represent a large sector of the roofing industry in Canada. These two-ply roofing systems utilize a polymer modified base and cap sheet. In this country the polymer of choice is SBS (Styrene Butadiene Styrene) with very little APP (Atactic Polypropylene) in use.

Modified bitumen sheets are very versatile and may be applied in a number of different ways. The two most common techniques for their attachment to the substrate and themselves is torch melting or fusion, and mop applied with hot molten asphalt. Gaining acceptance is the use of cold adhesives; mechanical attachment and self-adhesive peel and stick membranes.

The torch-applied process is generally regarded as the optimum method of attachment. The torch flame literally melts and fuses each sheet together or to the substrate. However, torch usage brings with it an element of risk as the open flame may come into contact with flammable surfaces and materials. Both the Canadian Roofing Contractors Association (CRCA) and the National Roofing Contractors Association (NRCA) do not recommend torch application directly to combustible substrates, such as wood decks or wood-fibre roof insulation.


In fact, both organizations do not recommend the use of open flame torches near any flammable material. In reality it is next to impossible not to find burnable materials on the rooftop. Wood, wood shavings, sawdust, cobwebs, propane and many roofing materials are highly combustible.


Of equal concern is the potential for “flame travel” into voids, cracks and crevices. Those areas of gap in construction components provide a place for flame to travel into the building and come into contact with bits of material thus promoting smoldering or fire. This smoldering may last several hours, ultimately resulting in a fire; long after the roofing contractor has left the job site. Buildings that maintain a negative pressure exacerbate this phenomenon as an open flame is literally sucked into the structure.


No matter how experienced the applicator, flame travel to undesirable places is unavoidable. Industry safety practices along with specialized training, including the recommendation to implement “two to four hour fire watches” with infrared guns after the completion of torching, has improved the situation somewhat, but in reality have proven largely ineffective.


Fires with these types of roofing systems have become alarmingly commonplace and result in very large property losses. The insurance industry is just now woken up to the fact that torch applied systems are responsible for several building fires. The time may come when the insurance companies may force the roofing industry’s hand by insisting on an outright ban on torch-applied systems. Until that happens though, we must demand that the designers of roofing systems give more thought to how they go about specifying and detailing these materials. System design, product selection and the potential for open flame travel must be reviewed more thoroughly before projects are tendered.



It is generally recognized that the torching of both the base sheet and cap sheet (torch-torch) offers the greatest potential for a fire. Using an alternative method to apply the base sheet is recommended as this will encapsulate the roof substrate thereby allowing the cap sheet to be torch applied over top. While these other techniques of application do have limitations attached to their installation, they offer a viable solution to the open flame problem. For example, peel and stick products and cold adhesives have temperature restrictions for their application in cold conditions. Hot asphalt mop applied base sheets have proven to be a successful alternative. Similarly, modified bitumens may be torch applied to fibreglass felts that are solidly mopped or adhered to the substrate.


Also worthy of consideration is the use of a thermal barrier, such as a layer of non-combustible or “torch safe” roof insulation. Careful consideration should be given to edge and parapet details where fire potential via travel of flame is greatest. The noncombustible insulation should be installed to both the horizontal and vertical surfaces and include a noncombustible cant to further protect the transition.


Alternative parapet and curb details utilizing noncombustible materials may also be considered. Metal curbs and upstands are safer than wood blocking for example. Masonry block and poured concrete, where appropriate, should be entertained. On projects that necessitate large quantities of combustible materials, such as wood blocking or on re-roofing jobs that expose dry, dusty substrates, other roofing systems may be more appropriate. Conventional built-up or single-ply roofing systems may prove safer during the installation process.



Those who design roofing systems – architects, engineers, specifiers or consultants – must carefully consider the potential for fire when designing or specifying torch-applied modified bitumen roofing systems. Roofing contractors who bid these projects must also review the tender documents and identify the risks associated with fire. In the end, the roofer is expected to install what the designer has specified. Unfortunately, they are often put in a position where they are asked to install these systems in less than ideal circumstances.


The roof system designer is also well advised not to get caught up in the word game that accompanies the specifying of torch-applied systems. Terms such as “fire-proof, fire-resistant, torch safe, non-combustible and low-flame” can provide a false sense of security. The terminology is often misleading and the words are used interchangeably. For example, fire-resistant and fire-proof have different meanings. Consult the product manufacturer and their literature to fully understand the material’s fire performance capabilities.


Worker training is an important aspect of ensuring a quality and safe application of modern industrial and commercial roofing systems. But no amount of training or experience will eliminate the potential for fire when installing these torch-applied systems. The open flame torch is a dangerous tool that has increased the roofing contractor’s likelihood for starting fires. The designer must design their torch-applied modified bitumen roofing system with this in mind.

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