A.Chris Donnelly responds: Wood tends to shrink and swell about twice as much along its growth rings as it does across those rings. To see how this effects deck boards, let me illustrate two scenarios:

In the first, think of a regular flat-sawn board with the rings forming long arcs along the end of each board. As the board dries, the growth rings contract along their length. The longer rings will shrink from the shorter rings. Effectively, the rings are trying to straighten out, which cups the board towards the bark side of the tree. If the...

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