Bark Up or Bark Down?
The debate about whether deck boards should be placed "bark side up" (growth rings curving down) or "bark side down" (growth rings curving up) has raged for years. Many people feel that bark side up is always the answer, but this is not accurate. In reality, you can decide which way is best for orienting the boards, whether you're installing a new deck surface or replacing a damaged wood deck board.
Types of Edge Grain in Deck Boards
Wooden deck boards typically come in three different cuts, or grain configurations, which you can see when looking at the end of a board (sometimes it helps to make a fresh cut with a circular or power miter saw):
- Flat grain: The most common cut, characterized by grain (growth rings) running roughly parallel to the wide faces of the board, but usually in a rainbow shape
- Edge grain: Grain running roughly perpendicular to the wide faces of the board
- Heart: Cut from the center of the tree, so the growth rings are in the center of the board edge; results in dimensionally prime pieces of lumber that are relatively rare
Why Boards Warp
The question about which way to orient a deck board is relevant primarily with flat grain boards because the orientation of their growth rings affects how the wood reacts when wet. In the case of a deck board, this determines whether you have a "cup" warp (concave surface) or a "crown" warp (convex surface).
When deck boards get wet, they do not get wet consistently. The wood on the top of each board is exposed to more moisture than the wood on the bottom. As a result, the wood fibers on the top side expand more than the fibers on the bottom. This creates dimensional changes in the wood. Wood warps in the opposite orientation of the growth rings. Because growth rings are usually curved, this means that when the wood warps, the rings tend to flatten out.
Heart Cut and Edge Grain Boards
With heart cut and edge grain boards, the issue of warping is very small. The wood distorts less significantly than with flat grain cuts, and the issue of "bark side" does not matter here. The more heart cut and edge grain lumber you can select when picking boards, the better.
Bark Side Up
"Bark side up" is the conventional wisdom. The main reason proponents like this method is, ironically, to reduce cupping. But when it rains, and the board warps, water collects in the concave shape and pools there. Wood develops a memory over time, and that's why you see deck boards in the permanently cupped position even when they are dry (when installed bark side up).
Another reason this method may be used is to avoid a defect called shelling. Shelling is more likely to occur with the bark side down method. It is caused by latewood growth separating from early wood growth, resulting in a flat splinter-like defect after repeated wetting/drying cycles. This is more common in Douglas fir and Southern pine lumber than with many other deck woods. But in general, you can largely avoid this problem by being more selective when choosing each piece of lumber.
It's also important to note that the bark side up method was popularized when heavy green-treated lumber was used for deck boards, and that lumber was unevenly saturated, causing warping problems. Today's quality lumber is kiln-dried before it is cut, minimizing uneven drying.
Bark Side Down
"Bark side down" orients the board so the end grain growth rings curve upward. This creates a crowning warp when wet. As a result, water is shed from the board, as opposed to pooling on it. Also, the exposed board face is heartwood (closer to the tree's center) versus sapwood, and heartwood is more decay-resistant. Bark side down is also the preferred method because it minimizes the chances of trip hazards due to cupping.