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.
TORCH-TORCH
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.
ROOF SYSTEM DESIGNER
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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