Roof Talk-101 Don’t Ask If A Product Is Fire Rated, Ask If The System Is Fire Rated
Joel King
Director
of Technical Services / Product Development
US Ply, Inc.
Director of Technical Services / Product Development
Over the years, I have made several trips to Underwriters
Laboratory in Chicago, to construct roof system configurations and test them
for fire resistance. It is amazing to watch how intense a fire can be and to
appreciate the seriousness of life safety.
Even standing behind the glass in the safety room, you can feel a
portion of the heat radiating from a fully engulfed specimen as it is consumed
by the 1400°F fire. After ten minutes has elapsed the gas is turned off and
quickly the fire is extinguished. “Did we pass?” I have asked sheepishly – “yes,
you did well,” is the response. How can that be? The answer is in understanding
what a fire rating really means.
So when the question comes up, “is this product fire rated?”
An answer without much explanation might leave the wrong impression or lead to
choosing a system that does not meet the criteria for the desired “fire
rating”.
First of all, most roofing products are “fire classified”, but
not many products carry a class A, B, or C fire rating as a “stand-alone”
product and for good reason. Fire classifications of A, B, or C are intended to
be system ratings and not individual product ratings. Thus every
product used within a rated system has a classification for specific inclusion
within certain systems, but does not necessarily yield a certain rating by
itself. To be more clear; the composition of a material may have certain
ingredients that adds “fuel contribution” and certain ingredients that do not
add fuel but rather “resist” burning. Anything that either resists burning or
can effectively slow the rate of burn/spread is advantageous within achieving
the goal of a particular rating.
For example, organic and petroleum based products are fuel
contributors. Put another way: asphalt, polymers and other poly______ (fill in
the blank) will burn! Other materials which comprise a given product may either
burn at a slower rate or do not burn. Some of these materials are ceramic
granules, metal clad materials, fiberglass mats, fillers such as limestone and
other proprietary fillers. The general idea is to assemble the system in an
effective combination that will sufficiently resist a fire within the specified
time frame.
So how does that work? Before I answer that question, here
is some useful information about fire rating classifications:
Fire rating classifications are determined by testing a roofing
system in accordance with ASTM E 108, “Standard Test Methods for Fire Tests of
Roof Coverings”. This test method is used by Underwriters Laboratory (UL
Standard 790) and Factory Mutual within their own test protocols. The result is
that the successful passing system is given the rating as tested as a
system, not as a product.
To achieve a Class A fire rating there are two sets of
criteria. One is for an assembly over “combustible” decks (plywood, oriented
strand board and wood plank or wood board) and the other is over “non-combustible”
decks (steel, concrete, gypsum, lightweight concrete and structural wood fiber
aka Tectum®). Note: Factory Mutual only publishes Class 1 non-combustible decks
and which also include an additional calorimeter test.
The criteria over non-combustible decks is more straight
forward – the system must successfully pass a “spread of flame” test which
measures how far the fire spreads in a given time period (Class A is tested
with a 1400°F flame with a 12 mph wind and cannot exceed a spread of 6 feet in a 10 minute span, Class
B is tested in the same manner and is limited to maximum spread of 8 feet in a
10 minute span, and Class C is tested with a 1300°F and a 12 mph wind and is
limited to maximum spread of 13 feet in a 4 minute span.)
For combustible decks, there are two additional tests that
are applicable; one is a “burning brand” test which measures the depth of the
fire penetration in a given time. This is to ensure that the fire will not burn
through to the deck during the test period. The other is known as an “intermittent”
test, in which the fire is cycled on and off for an hour to also determine if
the combustible deck becomes inflamed. That said, Class A is more resistant
than a Class B and Class B is more resistant than a Class C.
The other factor is the slope. Fire spreads more rapidly as
the incline increases, therefore; each rating has a slope limitation unless the
rating specifically indicates it is for an unlimited slope. For low sloped
roofs, it is more common to see ratings up to a maximum incline of ½” per foot,
once the slope goes beyond ½” per foot, the system combinations get more
difficult to pass with standard roofing materials.
The next time, you hear the question regarding whether a
product is fire rated – the answer in short is probably yes – but whether it can
be included in the desired system combination over a certain deck type on a
given slope? That is the more appropriate question and the answer is listed in
the Underwriter’s Laboratory Fire Classification Directory or online at www.ul.com. Additionally, the manufacturer usually
has an index card file of system classifications which they have successfully
tested.
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