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Controlling Moisture in Deck Lumber

By Bob Falk, Kent McDonald and Jerry Winandy

Many problems associated with deck deterioration can be traced to the original moisture content of the lumber.

Look for the mark to be sure what you’re getting. Grade marks, such as the ones on these two pieces of pressure-treated lumber, indicate the maximum moisture content of the wood, which is information that’s good to know if you’re going to use the wood to build a deck. Some pressure-treated wood will carry two marks, one from the mill and one from the pressure treated.

Properly dried lumber is worth the trouble. To dry lumber properly, align the stickers, and use spacer strips between wood over pile supports to promote even weight distribution and optimal drying. Place weights on top of the stack of wood to stop the top boards from warping.

If you’ve ever inspected, repaired or torn off an old wood deck, you know what can go wrong:
*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. 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 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.)

You can, however, 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 construction should be uniform and less than about 20% MC, regardless of the species or whether the wood has been pressure treated. In most areas of the United States. we expect lumber in aboveground, protected. exterior applications to reach an equilibrium moisture content (EMC) around 12%. If you’re specific site is normally either very wet or very dry, the EMC will be higher or lower, respectively.

In general. the moisture content have most treated lumber is high–in the 35% to 75% MC range–and the wood is still wet when it arrives at the job site, unless it has been kiln-dried after treatment and marked KDAT. If the wood is stamped KDAT, its moisture content should be about 19% or less. Because redwood and cedar aren’t treated with preservatives. they’re usually marketed as kilndried or as air-seasoned, which means they will have about a 20% MC. Most deck builders install deck boards on delivery. Although this way is easiest. pressure-treated boards probably will vary greatly in moisture content and often will shrink unevenly. In the case of preservative-treated wood. We recommend KDAT lumber. when available. because many problems that eventually surface in deck construction are the result of using wet lumber.

Another option is to air-dry the treated lumber yourself. In both cases. you’ll be able to identify problem deck boards before installation and exclude them from your project. Air-dry pressure-treated lumber to equalize moisture content–Treated lumber that’s not marked KDAT should be air-dried for several weeks, depending on the type of weather and the extent to which the lumber is exposed. Usually, pressure-treated wood comes directly from the treater and is bound and shipped wet to the lumberyard, where it often is stored outside and unprotected. Air-drying for several weeks will help even out the moisture-content differences between the pieces of wood and, on average will lead to a more consistent moisture content at installation.

In the long run. it is worthwhile to order the lumber to arrive at the job site a few weeks early to allow time for air-drying. Air-drying also is recommended if you build a deck with redwood or cedar that contains a moisture content much greater than 20%. The air-drying method we recommend is stacking the lumber in layers separated by narrow strips of wood, or stickers. to allow air to move freely between layers ). Care should be taken to align the stickers vertically within the pile. Alignment helps to distribute the load evenly and to minimize warping during drying. Also, it’s a good idea to place weights, such as concrete blocks. on top of the pile to help minimize twisting of lumber during dying. (Avoid iron weights because they can stain the wood if they get wet.) If the pile is protected from the weather–either by a shed or by plastic sheeting–and is allowed to dry several weeks, the lumber should reach a moisture content of close to 20%.

Using an electronic moisture meter is a simple method of measuring moisture content in wood. This kind of meter typically measures the electrical resistance between two metal pins driven into the wood. Moisture meters must be calibrated depending on wood species and temperature. You can purchase a moisture meter for around $40.

Moisture content also can be determined by weighing a few representative small pieces of wood. drying the pieces in an oven at 200°F for 24 hours and weighing them again after they’re oven-dry. Divide the difference between the original weight and the oven-dry weight by the ovendry weight, then multiply by 100 to get the moisture content in the form of a percentage. Shrinking can be used to your benefit–Wood shrinks only when moisture content falls below about 30%. A 6-in. wide treated southern pine deck board should shrink by about 3/16 in. if it reaches 12% EMC, so laying wet decking boards tightly against each other should result in a 3/16-in. gap when the boards dry (photo top right). For redwood or cedar purchased at 20% MC, a nominal 6-in. decking board will shrink only about 1/10 in. when a 12% EMC is reached. If the lumber installed is drier than the local EMC, and if the boards are laid tight, there’s potential for the wood to pick up moisture. swell and buckle. Depending on the lumber species and moisture content-as well as the desired gap between boards-a gap between deck boards can be planned based on the amount of expected shrinkage. We suggest a final gap of about 3/16 in. to ¼ in.–not big enough to catch a small heel, but big enough to allow dirt, leaves and other debris to fall through.

Warping and cupping usually are caused by uneven shrinkage between the-top surface and the bottom surface of deck lumber. The cupping of individual boards is aggravated because the top surface is usually at a lower moisture content-because of exposure to the sun and wind-compared with the protected bottom surface. This situation means that deck boards installed wet are likely to warp the most, especially when installed during hot months. This shrinkage difference is more pronounced if the bottoms of the boards remain damp, such as when the deck is built low to the ground or near wet soil. Finally, we don’t recommend using deck boards wider than 6 in. because cupping and warping can become excessive. Don’t forget to finish the job-Even though you’re using naturally decay-resistant or pressure treated wood. The horizontal surface of a deck is exposed to foot traffic. sun and rain. which makes finishing a deck with a water-repellent preservative a necessity. This exposure will degrade the wood’s surface, and unless the wood receives the proper finish, discoloration and checking often result leading to a rough, uneven deck surface and decay in untreated wood (photo bottom right). Applying and maintaining a finish on your deck will help minimize problems. It used to be recommended to wait a year, or a season, to finish a deck. In our experience, this amount of time is too long because surface problems that cannot be corrected later may develop (i.e., checking, cracking, splintering). For a new deck. apply the finish after the surface of the wood has dried to about 20% MC. Wood stamped S-DRY, KD (kiln-dried), MC-15 (average moisture content 15%) or KDAT has been dried and can be finished immediately.

Gaps between boards shouldn't be this wide. The gaps between these deck boards are
much too great and can result in injury for women wearing high-heeled shoes or for small
children. Gaps this wide can result when wet boards are laid with a gap between them. The
gap becomes larger as the boards shrink. Evenly dried, properly fastened boards resist warping. Warping and cupping are usually caused by uneven shrinkage between the top surface and the bottom surface of wood. The wrong types or placement of fasteners also can cause wood to warp and cup.

All decks need regular maintenance. Even cedar, redwood and pressure-treated lumber require
regular application of a water-repellent preservative (including mildewcide). But lumber
that is not naturally durable or that is not preservative-treated will quickly decay, resulting
in a short-lived and vulnerable deck.

Bob Falk, PE., is a structural engineer and Kent
McDonald and Jerry Winandy are wood scientists
at the USDA Forest Products Laboratory in
Madison, Wis. They are coauthors of “Wood
Decks: Materials. Construction, and finishing,” a
handbook published in cooperation with the
Forest Products Society (2801 Marshall Court,
Madison, Wis. 53705-2295: 608-231-1361.) Photos
by Steve Schmieding, Jim Vargo and the authors
except where noted.

PORCHES, DECKS &
OUTBUILDINGS
The Taunton Press

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Use a moisture meter to know when your wood is dry enough to coat.