Top Five Failed Building Practices: Part 2 of 5 Fire-Retardant Treated Plywood

Here is the second installment of the five-part series, The Top five Failed Building Practices. You can read Part 1: Aluminum Branch Wiring here.

Part 2: Fire-Retardant Treated Plywood

The second item on our list of the Top Five Failed Building Practices is Fire-Retardant Treated Plywood.

Townhouses and other connected homes are generally built with firebreaks to keep flames from spreading between units. Traditionally, before 1970, a brick or cement block wall would separate the homes and extend up beyond the roofline. This is known as a parapet wall.

This type of divider wall is still being used, but since the 1970s, many townhouses have shared a roofline without parapet walls. Instead, fire protection is supposed to come from the construction methods used between the units.

These methods vary, but modern firewalls often consist of a six-inch-wide, insulated metal framed wall with doubled sheet rock on both sides. A problem arose, however, when builders used fire-retardant treated plywood (FRT) in attics as roof decking between the units. This was not meant as a cost-cutting measure, but rather as a way to complete the firebreak.

Fire-resistant wood products for various industries have been around for  decades, but this roof decking is more recent and was commonly used in townhouses and condominiums in the 1970s and 1980s.

FRT plywood is made by forcing inorganic salts deep into the wood fibers, then drying the plywood. The chemicals reduce the chance of fire spreading to other units because the plywood will char during a fire but not burst into flame. The product tested and performed well for reducing the spread of fire.

However, there are two other kinds of problems. Both have to do with heat and humidity. When FRT plywood is installed in poorly ventilated attics, the highly acidic chemicals can leach out of the wood and degrade metal fasteners, including the roof nails and truss plates used to hold the roof structure in place.
The same fire retardant chemicals can also react with high heat and cause the wood itself to degrade, dry prematurely and fail. The plywood can become so dry and brittle that it would be unsafe to walk on. It can cause the roof to leak. In some cases, FRT plywood has failed in just a few years, when the house was still relatively new.

FRT plywood can best be identified in the attic. This is also the safest place to determine whether the wood has begun to deteriorate. It will be stamped or otherwise marked as fire resistant. Look at the underside of the roof decking, adjoining the firewalls. A clear indication of possible deterioration is a dramatic color change from the rest of the plywood. This product will often change to a very dark coffee or reddish color. Also, the surface can become very brittle and separate easily when probed with a screwdriver.

Severely deteriorated FRT plywood must be replaced, but there are ways to delay replacing less-damaged surfaces. One way is to add a layer of drywall to the attic side of the plywood. This and other methods, however, will only delay the inevitable removal of deteriorated plywood, usually when the roof shingles are being replaced.

There are newer versions of FRT plywood, but most builders now have incorporated the drywall method into their firebreak construction.

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Next up: Part 3: Polybutylene Pipes.

Read Part 1: Aluminum Branch Wiring