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 growth rings. The combined effect of these different rates of movement causes lumber to deform.

Flat-sawn lumber, which is cut so its wide face is parallel to the growth rings, cups as it gains or loses moisture. A good way to visualize the typical distortion is to imagine that the growth rings straighten as wood dries. Therefore, wet lumber will tend to cup toward the bark side as it dries. Kiln-dried lumber, on the other hand, is usually surfaced after the lumber has been dried, and will tend to cup away from the bark side as it gains moisture.

Heartwood and sapwood.

Heartwood is often more resistant to decay than sapwood of the same species. But heartwood is difficult to impregnate with wood-preserving chemicals, while sapwood is easy to treat. As a result, the most resistant face of treated wood is often the bark side.

Shelling. Growth rings have two parts — earlywood and latewood. The inner layers of each growth ring (closest to the center of each ring) are formed during the early part of the growing season. The outer layer grows later in the season. Repeated cycles of wetting and drying sometimes cause the layers of earlywood to separate from the layers of latewood. This separation, called shelling, occurs infrequently. When it does, it is associated with flat-sawn softwoods like Southern Yellow Pine and Douglas fir, that is laid barkside down.

In-use conditions. No matter which face is up, the ultimate shape of a board will be influenced by its moisture content. If one side is wetter than the other, that side will expand and cup the board. Since the underside of a deck built close to the ground experiences higher relative humidity than the upper surface, which is exposed to wind and sun, the boards often cup upward.

Pick the best face. When all is said and done, you needn’t remember all these factors when trying to decide which face to install up. In truth, the answer is very simple: Pick the best looking face, selecting out the faces with the most knots and wane, and install your decking best face up. Secure fastening and an annual coating of water repellent will do the most for keeping the boards flat.

— Paul Fisette is a wood technologist and director of the Building Materials Technology and Management program at the University of Massachussets in Amherst, Mass.