Better The Enemy You Know
Vol 21 No 3
During pre-planning surveys, firefighters would certainly recognize the fire spread dangers associated with wood deck roofs in industrial facilities. However looking at a metal deck might not conjure the same concerns about fire spread.
In an earlier article, I stated that "A topic that gets nowhere near enough attention, in spite of the repeated efforts of noted building construction expert Frank Brannigan, is that of combustible metal deck roofs. If a built up roof has too much asphalt on the metal deck (too much means more that 0.12 pounds per square foot) the asphalt will vaporize when the metal deck reaches 800? F. This vapor will force its way through seams in the roof and burn under the deck and heat up more of the deck in a self-sustaining reaction. Burning asphalt will fall through the seams and start additional fires."
In this article, we will examine the issue of combustible metal deck roofs in more detail. For additional information, see Frank Brannigan's text Building Construction for the Fire Service, 3rd edition, published by the National Fire Protection Association (NFPA), GE GAP Guideline 2.1.5 Roof Core Samples, and FM Global Data Sheets 1-28 & 29/R and 1-31. This article will focus strictly on the spread of fire along the underside of the roof and not collapse hazards posed by wood or steel trusses or other types of roof support. Those hazards are significant but have been discussed thoroughly by others.
NFPA standards and the model codes do not emphasize the hazards of interior fire spread under combustible metal deck roofs. They tend to focus on exterior hazards such as flying brands. In my experience, it is the Highly Protected Risk (HPR) loss control consultants that encourage their customers to install noncombustible metal deck roofs.
The terms combustible and noncombustible metal deck roofs are sometimes confusing to people not in the HPR industry. Combustible does not refer to the use of a combustible metal like magnesium. It is not the metal deck that burns. It is the adhesive (commonly asphalt) and/or unlisted combustible insulation that burns. It is the hot metal deck that causes the asphalt to vaporize and then ignite. Some vapor can be expected to burn in a "noncombustible" metal deck roof, but propagation will be limited. Brannigan's text points out that "noncombustible" metal deck roofs still contain some combustibles and in some circumstances might still contribute to a fire.
Metal deck roofs that use a significant amount of asphalt as an adhesive to hold the insulation in place or as a vapor barrier are vulnerable to fire spread that can be even more severe than with wood deck roofs. This is because the burning dripping asphalt can start additional floor level fires as it spreads along the roof.
This is not a new issue. The first major industrial fire where the spread was attributed almost solely to the combustible metal deck roof occurred in 1953 at an automotive transmission plant in Michigan. (See photo 1) Sprinklers were not provided because it was felt that the contents were not combustible. The combustibility of the metal deck roof was not well known at the time.
There have been at least three other major industrial fires where the presence of a combustible metal deck roof was a major contributing factor. Two were federal government installations and the other was in the private sector. One of the government building fires occurred in 1956 in a building that was also non-sprinklered because there was a very low combustible loading. The 70,000 square foot roof collapsed and pulled the walls down with it.
Sprinklers were provided in the other two facilities. In 1985 at an Air Force base in Oklahoma, sprinklers were not provided in a concealed space between the facility ceiling and the combustible metal deck. The fire spread unimpeded in this space, fed by the vapors given off by the asphalt. The fire was finally stopped in day three of the operation by what is probably the largest trench cutting operation in history. The last location was also sprinklered but due to numerous factors, the sprinklers were not effective and the facility was a total loss. See photo 2. Events like these may not appear to be very common but as you can see, they can be major events when they occur.
What Can Be Done?
The best thing to do is to install roofs so that they qualify as noncombustible. One way to do this is to use mechanical fasteners (figure 1) instead of asphalt, in conjunction with other listed materials. Another way is to limit the amount of asphalt to 0.12 pounds per square foot of roof area, in conjunction with other listed materials (figure 2.) This limited asphalt must be distributed according to its listing. Roofs that qualify as noncombustible are called Class I roofs in the FM Global system and Fire Classified under Underwriters Laboratories (UL) terminology. Insurance diagrams, which are useful to emergency responders for a variety of reasons, will state the roof construction.
If you see a steel deck on a preplanning survey, try to find out if it can be considered a noncombustible metal deck. In my experience if you ask the installing contractor if the roof qualifies as an FM Global Class 1 roof and they have no idea what you are talking about, the roof is probably not a Class 1 roof. Be careful if the contractor says the roof has a UL fire rating. They frequently mean that the roof is a UL exterior fire rating. This protects the roof from flying brands and is important but it is not the same as a roof that is fire classified for protection against interior fire exposure. If you ask for this clarification and they have no idea what you are talking about, you probably do not have a fire-classified roof.
What Should You Do If You Have A
Combustible Metal Deck Roof?
The proper way to prevent interior fire spread along the roof deck is through a properly designed sprinkler system that does not allow the roof deck to get hot enough to set up this process. Although it is possible to stop the process by cooling the deck with hose streams from beneath, in an industrial fire, the area could be too large for that by the time the hose streams can be set up. Once that happens, the fire is probably going to burn to the firewall if there is one.
Of particular concern is when a building does not have sprinklers because the occupancy is mostly non-combustible or where a building has a sprinkler system designed for ordinary hazard manufacturing and small areas of higher hazard exits. In the case of the non-sprinklered building, all it takes is a stack of pallets large enough to get the roof deck to 800?F. One test indicated that a single six-foot stack of pallets under a 22-foot high roof is enough to initiate this process. Once this happens, this building with a seemingly noncombustible occupancy can burn to the ground. Just refer back to photo 1. In a sprinklered building, a small area with a hazard greater than the sprinklers can protect can have the same result.
As always, awareness of the hazard and planning for the results is the best defense.
GE Global Asset Protection Services, LLC specializes in property loss prevention services & consulting, and provides risk management solutions designed for property risk managers. We offer a contiuum of fully customizable services, such as support placing property insurance requirements, minimizing property loss frequency & severity, data management and integrated consulting recommendations and solutions. GE Global Asset Protection Services, LLC, is a global service provider with 115 years of experience, offering industrial, manufacturing, service, mining and utility occupancy expertise, with online, real-time web tools and reports around the clock, and around the globe. Please visit www.gegapservices.com.