Synopsis: A brief discussion of the importance of controlling moisture content in exterior decking lumber by a trio of scientists at the USDA Forest Products Laboratory in Madison, Wisconsin. Includes good basic information before you lay a new deck.

If you’ve ever inspected, repaired or torn off an old wood deck, you know what can go wrong with one. Nail heads stick up. Deck boards decay, cup, or twist, and joints that once were tight open up and loosen. Bad construction, the use of unsuitable lumber, the wrong fasteners or a lack of maintenance are often the sources of the problems. However, there is another important factor that can affect deck performance.

Here at the United States Department of Agriculture Forest Service Forest Products Laboratory in Madison, Wisconsin, we’ve learned from extensive research into wood behavior that the origin of many of these problems often can be traced to the moisture content (MC) of the wood at the time the deck was built or to the effects of moisture during its lifetime.

The effects of moisture in deck lumber determine how good a deck will look, how well it will hold up, and, often, how long it will last. Obviously, it’s impossible to control the amount of humidity and rain a deck is exposed to. (You can limit the amount of moisture that comes in contact with the wood only by applying a proper water-repellent finish or by purchasing lumber that has a water-repellent finish.) However, you can control the amount of moisture in the wood. Too much or even too little moisture in wood eventually can lead to structural problems.

Moisture content can affect a deck for years to come

To minimize warping, splitting, checking, shrinking, and failing finish, the deck boards at the time of…