Lumberyard in the Sky: Fire Retardant Treated Lumber

A mammoth residential building was under construction next door to our fire station, and we all noticed that the contractor was using red lumber in some of the framing. The contractor told me that this was lumber that had been treated with a fire retardant. After acquiring a sample of the product, we put the flame to it—and he was right! It would char, but it would not sustain combustion. Soon, we were seeing this material being used all over our area in large, multistory residential buildings that normally would have used steel. This sparked my curiosity, and so I investigated further.

With the rising price of steel, many builders are turning to fire retardant treated (FRT) lumber instead of fire-rated noncombustible assemblies incorporating steel or concrete (photos 1 and 2). FRT lumber is a new generation of construction material for use in weather-protected conditions. Some of the past treatment methods have created a very corrosive wood and problems with hygroscopicity (the capacity of the lumber to react to the moisture content of the air by absorbing or releasing water vapor). Of decisive significance for water vapor absorption or release is the product’s water content. This was a problem in extremely humid climates. This new generation of chemicals and process claim to have reduced these effects dramatically to a point where the FRT lumber is no more corrosive than normal wood. This significant improvement ensures that the treatment remains intact and does not leach out. Long-term exposure to weather will diminish the effectiveness, however, so the FRT lumber is used for interior and exterior unexposed components.