A. Paul Fisette responds: Depending on the wood type, moisture content, and exposure, deck boards can cup, twist, or decay. Theoretically, you can ensure that a board will cup downward to shed water, and thereby reduce deterioration, by exposing one face or the other. In fact, seldom does a week pass that I’m not involved in a discussion about whether to install deck boards either bark side up or bark side down. But such recommendations usually only account for one of a handful of factors that control cupping.
Original moisture content. The original moisture content of a piece of lumber controls a board’s final shape. Wood is stable when its moisture content is above 30% (fiber saturation point). As wood dries below 30%, it shrinks. Wood shrinks and swells twice as much in the direction parallel to the growth rings as it does perpendicular to the...
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