Twenty firefighters were evacuated from a hot, dense smoke situation in a strip mall 60 seconds before the roof collapsed. The roof was a Combustible Metal Deck Roof (COMR) generally known as a "built-up roof", because of its layers of tar, insulation and roofing generally topped by gravel and supported by steel-bar joist trusses.
When you look up in the interior, you see steel-bar joists and steel decking. It appears to be a non-combustible void. It may be classified as non-combustible. Some building departments and other regulators may not know of this hazard since it is well known only to Highly Protected Risk (HPR) Insurers who insure well protected properties. But is generally unrecognized by building and fire officials. See the story of a four-fatality nursing home fire, BCFS3, p. 306.
If there is a built-up roof above, you have a very dangerous situation in the event of a fire.
- The burning contents of the building heats the steel deck.
- The tar above begins to boil.
- The gases form a bubble and cannot vent upward.
- The gas vents downward through the joints in the steel.
- The gas ignites.
- The gas fire heats more steel deck.
The fire is now a self-sustaining roof fire, independent of the building's contents, spreading in all directions. This situation requires only 8oo degrees Fahrenheit for five minutes on the steel.
Some fire may show above the roof but throwing water on the roof is not productive. The tactic is to hit the underside of the roof with a heavy smoothbore stream to cool the steel from a safe location, and thus cut off the gas.
Do not attempt roof ventilation. A huge vent would be required. In a factory mutual test building, 100 ft x 20 ft, no contents, just the roof burning, a 54 sq. ft vent was needed. And there is serious danger of collapse. A six-firefighter roof team of the Dallas Fire Department fell into the fire when the heated elongated bar joists pushed down the wall and the roof collapsed. Fortunately they were near the edge and were retrieved.
Factory Mutual Type 1 roofs and UL Classified roofs have much less tar adhesive and are intended to not be self propagating.
Sprinklers that are installed to cool the steel overhead can stop the fire. Sprinklers below the ceiling will not cool the steel.
The CMDR is most often not recognized in fires and thus you may never have read of it. Such fires have been laid to unburned gases from the contents and attic fires.
A CMDR roof was responsible for 162 of the 164 lives lost in the Beverly Hills Supper Club Fire in Kentucky and loss of several lives in a nursing home fire. If you see corrugated metal and some unburned roofing you have had a CMDR fire. Noting this in a report or article will help to inform people of this rather unknown hazard. REF BCFS3, pp.302-8
THE MEMPHIS TRAGEDY
Much of the valuable information provided in this column originates with observant firefighters who take the trouble to pass the information along. Two firefighters died in a roof collapse in a Memphis strip mall. An article on Firehouse.com reported a building department official saying that buildings like that often had bar-joist roofs. Will anybody who knows whether or not there was a typical built up roof on steel roofing on bar joists as described above, contact me at Fbrannign@aol.com. A picture showing some bare corrugated metal and some built up roofing would be ideal. If you prefer not to be credited just say so.
THE CHICAGO PORCH TRAGEDY
As I was writing this article, we learned of the Chicago porch tragedy.
It reminds us once again that the law of gravity is written in granite, no exceptions, and no deals, rich or poor. When the load on the structure exceeds capacity by one molecule, gravity puts the structure right where it wants it, down on the ground.