Comparison of strength properties and failure characteristics between fire-retardant-treated and untreated roof framing lumber after long-term exposure: a South Carolina case study

The degradation of strength properties related to the presence of fire retardant treatment (FRT) in wood has been previously documented. This degradation process is directly associated with environmental conditions of temperature and humidity. The FRT chemicals react with the wood during cyclical changes in temperature and humidity causing changes in pH such that the wood becomes brittle. This process is most commonly associated with plywood roof sheathing, where exposure to radiant heat is most significant. However, a recent case study in South Carolina indicates that the effects on southern pine dimension lumber used in roof framing can be equally dramatic. This study included strength testing of both FRT and untreated roof framing lumber after 22 to 31 years of exposure. Analysis of modulus of elasticity (MOE) and the bending modulus of rupture (MOR) of these samples revealed no appreciable loss of bending strength in the untreated lumber. In contrast, the treated wood samples had significant losses in both MOE and MOR. The failure of the FRT wood samples occurred suddenly with a marked absence of toughness, which is a reflection of their brittle condition.

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