The Metal Deck Roof Debate
STRATEGY AND TACTICS
Is the hose or the saw the better way to handle a fire in a metal deck roof? Two authorities offer this exchange of views.
Metal deck roofs built up with combustible materials are popular for use in constructing noncombustible, light commercial buildings because they’re easy to install and they can cost 20 percent less than comparable wooden roofs. But if a fire involves this kind of roof, the blaze will be selfsustaining.
The key is the layer of tar put between the corrugated metal base and the layers of tar, insulation, and composition (layers of asphalt-saturated felt and pitch roofing material) covering it. (See Figure 1.)
When the metal is heated, that first layer of tar liquefies. (Dripping hot tar is a good indicator of a metal deck roof fire, but it will be less readily observed if there’s a ceiling beneath the roof.)
The liquid soon turns to gas, which, unable to escape upward, pushes down through the joints formed where sheets of metal overlap. (See Figure 2.) As the gas burns, it generates more fuel by heating larger and larger areas of tar; this lets the fire keep burning independent of the original source of ignition.
Although any roof that’s exposed to fire can be hazardous to firefighters, metal deck roofs are espedally dangerous because of their ability to propagate and conceal fire which can rapidly travel under the roof and between the metal corrugations and the composition covering. It’s not only hidden, but hard to get access to.
Add to this the fact that steel—of which the trusses supporting the roof will probably be made—begins to lose its strength at 1,000° F, and you have the danger of quick collapse. Elongating as much as 9 inches in 100 feet at this temperature, such trusses have been known to push down walls, causing the roof to collapse in as little as nine minutes.
There are discussions of the metal deck roof fire problem in Frank Brannigan’s Building Construction for the Fire Service, 2nd edition, pages 247-250 and 256-257; in the National Fire Protection Association’s Fire Protection Handbook, 16th edition, pages 7-67; and in the Factory Mutual Handbook of Loss Prevention, 2nd edition, pages 5-8.
Teaching at the National Fire Academy last summer, Brannigan lectured on metal deck roofs and proposed a solution. John Mittendorf was in attendance and proposed an alternative solution. Their opinions appear on the following two pages.
