FRT PLWOOD: NOT AS SAFE AS IT SOUNDS

FRT PLWOOD: NOT AS SAFE AS IT SOUNDS

SAFETY & HEALTH

The Building Officials and Code Administrators (BOCA) code was changed in 1978 to permit the use of fire-retardant-treated (FRT) plywood as roof sheathing. Before this code change, a fire wall was required to compartmentize attached, row-type residences by fire walls. These walls were designed to contain a fire for some time within the compartment, retarding horizontal extension. The code change provides that if fireretardant plywood is used, the fire wall need extend only to the underside of the roof deck. FRT plywood has to be installed as roof sheathing four feet on either side of the firerated assembly serving as a fire wall. In 1981 the Standard Building Code accepted this change, and in 1982 the Southern Building Code did as well.

FRT plywood is regular plywood that is chemically treated to resist burning. It is pressure-treated with a liquid solution containing chemicals that remain in the wood after drying. These chemicals are activated by heat and water reacting with the wood, producing a charring action that slows the burning of the wood; hence, the plywood can be given a fire rating.

When the plywood was designed, the chemical reaction to retard the fire spread was to take place in the presence of temperatures near 300°F and with water vapor present. In other words, this reaction would only take place under fire conditions. The roof deck would be replaced after suppression.

In actual use, however, FRT plywood’s reaction to heat and humidity can be radically different over extended periods of time: It often begins to react in ambient attic temperatures of about 170°F, and normal humidity in the air provides enough water vapor to support the reaction. At this relatively low temperature (often reached in attic spaces) a chemical reaction takes place, and the sheathing loses much of its strength. This loss of strength could pose hazards for firefighters.

To date, there have been no reports of firefighter injuries on these weakened roofs. However, according to a FEMA safety alert, four maintenance workers have fallen through such roofs. Although none were seriously injured, the potential danger to firefighters is clear.

Be on the lookout for these weakened roofs during prefire planning. You already consider roof condition and building age in older structures but may tend to overlook them in newer buildings. Look for excess waviness in the roof near the fire walls. The shingles may be raised at the point where the roof contacts a side wall of an adjoining structure. This may indicate the beginning of cracking and delamination.

This problem is more likely to occur in townhouses and multi-unit housing, where FRT plywood is frequently used as roof sheathing. You can detect visible signs from the underside of the roof deck. (Since the FRT is only four feet on either side of the fire-rated assembly, you may need to get into the attic to see the problem.) After deteriorating, the plywood appears dark, as if charred. If caught in the early stages the reaction appears as a brown water stain and is the beginning of pyrolisis that will eventually cause char. Another sign is the presence of a white powder on the FRT plywood, suggesting the beginning of the chemical reaction. Alert personnel to these signs of deterioration.

In 1987 the American Plywood Association issued warnings on FRT plywood. By now some codes should have been amended to prohibit the use of FRT plywood as roof decking. To be sure, check with your local building code enforcement agency. The agency also may have records of where FRT plywood was installed.

In October 1989 the American Plywood Technical Services Division issued a report entitled Fire – Retardant -Treated Plywood Roof Sheathing, Preliminary Examination and ‘Testing. The report describes an additional test to measure strength reduction due to chemical reaction of the plywood: “[Pjiercc the [plywood] panel with a knife or ice pick and pry out small pieces. Normal plywood will splinter; degraded plywood will powder and fracture in a manner similar to charcoal.”

Groups representing plywood manufacturers, chemical treatment companies, construction trade associations, code enforcement agencies, and others connected with the installation of FRT plywood currently are trying to find a solution to this problem. They must decide, among other things, how much of the FRT plywood is a problem, because not all FRT plywood is deteriorating. They also will discuss how to replace the defective wood. Until they resolve the problems, the fire service must be wary of these weak roofs, be alert, and preplan strategically for responses to such a building.

waviness is evident on either side of the fire wall. The fire wall is under the shingles that have raised up in the center of the roof.the buckling of the shingles at the juncture of the roof and adjoining fire-rated assembly is further evidence of failure of the roof decking.

(Photos by author.)

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