This article concludes the two-part summary of the 54-page Chapter 3, "Wood Construction," of the 667-page third edition of my book, Building Construction For The Fire Service . Part 1 of "The Building Is Your Enemy" was published in Firehouse® February 1996, part 2 in August 1996, part 3 in...
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Bear in mind that all the wood in the interior of the building has been drying since the day it was installed. Sprinklers which are not hitting fire may pour scalding water on firefighters and otherwise interfere with operations. It is a real unanswered question as to whether such sprinkler systems should be allowed to continue operating when fire is in the voids. If any reader has experience to relate, I would like to hear of it.
The building code answer to the void space problem is firestopping and its lesser cousin draftstopping. To the best of my knowledge, no firestopping method for combustible buildings has ever been tested.
One difficulty with the concept is that firestopping must be perfect. Fire gases are under pressure and the slightest opening can become a "nozzle" to send a jet of flaming gas past the barrier. It is difficult to obtain a perfect barrier, and even good barriers are often violated for passage of utilities.
Firestopping and its lesser cousin draftstopping are not the same. Firestopping has consisted generally of a two-inch thickness of lumber and is used in relatively small spaces. Draftstopping is used in larger voids and generally is made of gypsum board, plywood or chipboard. Even if installed correctly under the eye of a vigilant knowledgeable inspector, they are both subject to penetration for any reason.
Tightly fitted heavy wood firestopping is the best. In one case it was removed and not replaced to accommodate a fire main, thus fire in the truss voids will follow the fire main throughout the building.
By my definition there are two types of firestopping:
Inherent: This grows out of a necessity of construction. A wet masonry panel wall built on a concrete floor automatically is fire-stopped.
Legal: This is firestopping which (to the builder) serves no purpose but to meet the code. This is more likely to be deficient.
Firestopping and draftstopping cannot be relied on. "Undress the building," by which I mean "see" the interconnected hidden voids and assign units to deal with the problem. Because the voids may be complicated and stream obstructed, as by cross pieces of trusses, it appears that the better tactic might be to locate hot spots by use of a thermal detecting device and penetrate the wall or ceiling sheath with a piercing nozzle. Opening the void gives the fire the oxygen it needs, and the fire may be beyond the reach of hose streams. In addition all hose streams, but particularly fog streams, carry air to the fire.
Wood is subject to insect infestation, wet and dry rot, and other forms of decay, which may cause weakening without being apparent to the casual observer.
Deterioration of this nature in structural members should be carefully noted on pre-fire plans.
Protecting Wood From Ignition
Wood burns, and much effort has gone into overcoming this characteristic. There are coatings which when heated intumesce (swell up like marshmallow) and delay ignition. These coatings must not be applied like paint, but each gallon to a specified area. If the wood is furred (spaced) off from the wall, the reverse side is not covered.
Wood can be impregnated with mineral salts. This will retard surface ignition. Some materials used can leach out and corrode metal connectors.This FRT (fire retardant treated) wood is harder to cut and its strength is decreased. In some codes it is "accepted as non-combustible." Wood cannot be made non-combustible, but its ignition can be delayed.
Do not confuse this wood with wood treated to resist rot and insects. As noted earlier, such wood gives off toxic smoke.
Plywood Roofing Problems
Some years ago, to avoid parapetting fire walls through the roof, builders adopted a practice of placing FRT plywood, one sheet wide, on the roof, on both sides of a firewall, which does not penetrate the roof. At the time, I set forth potential deficiencies of this method of stopping the extension of the fire over the top of the firewall since the plywood delaminates (layers separate) and fire passes over the top of the wall.
Recently, it also has been found that plywood treated with certain chemicals decays from heat and is subject to failure if walked on. The problem is compounded if treated plywood is used not only along the line of the fire wall, but randomly throughout many roof areas. In some cases, the entire roof is treated plywood. There have been huge lawsuits.