Forest fire damage on deck

Q. Our home and outside deck were subject to a lot of heat during the recent forest fires. What damage might this cause in terms of low MCs and wood strength damage?

A. I am sorry to hear that your home was close to the fires. In answer to your question, the heat can temporarily drive out some of the moisture in the wood, but this typically will not be a lasting effect. The low MCs experienced by the wood might make the wood more brittle if it were being machined, but for siding and decking, this loss of moisture will cause a little shrinkage and maybe open up a few cracks, but the wood will regain the moisture and swell back to its original size and close the cracks.

Of more concern is the effects of the heat on wood. At 212F, all the water evaporates, although this does take a little time. Once the water is gone, the wood temperature increases and will begin to turn darker brown and eventually develop char (black color). Wood can actually char at 212F, but this reaction takes years; the hotter the wood, the faster it chars. So, do you notice any darkening or char? Wood that has charred is certainly considerably weaker than before it charred.

Eventually, the wood reaches about 450F and at these temperatures, the wood molecules quickly begin to break apart into smaller molecules. These small molecules are gaseous and highly flammable; if there is a flame for ignition, then the wood will ignite and continue to burn on its own. In reality, the wood is vaporizing and the vapor is burning. It is at this point that plenty of oxygen must be available to assist in burning. If there is a lack of oxygen, the fire cools and not all the vapors are burned.

Apparently, the wood in your home did not see these temperatures, so it would appear that this short term heating that the wood was exposed to has not caused permanent damage.

Obviously, this is a general explanation and your specific situation could have been different, which a visual examination would determine.


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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.