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Old 02-02-2011, 08:05 PM   #1
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How to deal with expansion of deck boards?

I've read that one should have gaps between deck boards so they can expand width-wise with heat.
It isn't necessary to worry about lengthwise expansion, is it? When we had our deck repaired the carpenter just nailed them in. So those nails won't work loose because the boards try to expand along their length, right?

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Old 02-02-2011, 08:17 PM   #2
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Correct. They won't grow in length.
By the way... your boards won't expand. They shrink in width as they dry.
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Last edited by Willie T; 02-02-2011 at 08:25 PM.
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Old 02-02-2011, 09:10 PM   #3
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The cause of dimensional changes in wood is not heat, it is the moisture content of the wood. Different species have different expansion rates, but typically about a 2 percent dimensional change across the grain is maximum over the course of a season. So if the boards were dry when they were installed (i.e. kiln dried boards installed in the dry season), they will expand up to 2 percent in the summer.

A six inch wide board can change up to about 1/8 inch, hence the need for spacing between the boards. Some species expand more, you can check on the Forest Products Laboratory (WI) for the expansion rate for your particular deck board type. Boards do not typically change dimension along the grain.
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Old 02-02-2011, 09:33 PM   #4
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I built a 800 plus square foot deck this past summer in my back yard and the PT lumber was clearly wet. I jammed them tight together and by October there was a 3/16ths to 1/4" gap between all the boards. So to confirm what has been said, it's the moisture in the wood.
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