Please note: This older article by our former faculty member remains available on our site for archival purposes. Some information contained in it may be outdated.
A short answer to an often-asked question.
I’m somewhat amazed by the intense and frequent discussions surrounding this question. Seldom does a week pass when I don’t hear reasons why deck boards should be installed either bark-side up or bark-side down. In truth, the answer is very simple. Lay deck boards so that the best-looking face is facing up!
Original Moisture Content
Wood is stable when its moisture content is held 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, lumber that 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, a wet piece of lumber installed with the bark-side up, will cup to hold water on its surface as the board dries. However, kiln-dried lumber is usually surfaced (imparting a true rectangular shape) after the lumber has been dried. A dry piece of lumber installed bark-side-up will cup to shed water from its surface as it gains moisture — because if the growth rings straighten out as wood dries they will curve as the wood becomes wet. So, the original moisture content of a piece of lumber when it is installed – and – the conditions to which it will be exposed after installation control a boards final shape.
Another reason for contemplating bark-side-up/bark-side-down is decay resistance. Heartwood is often more resistant to decay than sapwood of the same species. Clearly you would install lumber bark-side down to expose the more resistant heartwood region to the elements. But not so fast. Heartwood is difficult to impregnate with wood-preserving chemicals. Sapwood is easy to treat. So if you are using treated lumber and you want to expose the most resistant surface — you got it — bark-side up.
Growth rings have two parts: earlywood and latewood. The inner layer of each growth ring (closest to the center of the tree) is formed during the early part of the growing season. The outer layer as you might guess 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 and is usually limited to flat-sawn softwoods like southern yellow pine and Douglas fir that is laid bark-side down.
Here we are dealing with appearance and probability. Knots originate at the center of the tree. Knots sometimes show up on the pith side of the board, but not on the bark-side surface. In fact, you are less likely to have exposed knots on the surface of a deck if all boards are laid bark-side up.
Have you ever noticed that some boards are missing wood along an edge or at a corner? Sometimes you see bark still clinging to an edge. This defect is called wane. And you can install a board only one way to expose wane: bark-side up.
It’s no secret that the dimensions of wooden objects are influenced by relative humidity. And the lumber used in wooden decks is no exception. The underside of a deck built close to the ground experiences higher relative humidity than the upper, exposed surface. Wind and sun dry the top, while the earth works effortlessly to keep the underside moist. The differential in relative humidity causes the bottom dimension of the boards to be larger than the top dimension. The result is cupping.
During our last rainstorm, I inspected a deck that I had built 3 years ago. The deck was built with pressure treated southern yellow pine and suspended about 8 inches above grade. The boards were installed with total disregard for growth-ring orientation. Every board had a slight cup and the only boards that didn’t hold water were those that had twisted, allowing the water held in the cupped portion to run off.
So as I said in the beginning, the answer is very simple: Pick the best face and install your decking best-face up. Securely fasten the deck boards and apply an annual coating of water repellent.