Woodworking in Alaska: Wood Moisture and Outdoor Builds

Month-by-month outdoor wood moisture for 11 places in Alaska, from 8.7% in Fairbanks in May to 20.5% in Barrow (Utqiagvik) in September, and what it means for buying lumber, gluing up and finishing.

Wood trades moisture with the air around it until it reaches a balance called the equilibrium moisture content (EMC). The numbers on this page are the average EMC of wood kept outdoors under cover (out of direct rain and sun) for each month, calculated by the USDA Forest Products Laboratory from at least 30 years of NOAA humidity and temperature records. Wood that gets rained on will be wetter for a while; wood inside a heated or air-conditioned house will be drier. The general advice below uses Juneau as the reference city.

Outdoor wood moisture in Alaska, month by month

City by city

The 10 cities whose climates differ most get their own section; the other 1 follow one of them so closely that they are listed with their numbers and the closest match. Deck board figures are for a 5½-inch flat-sawn pine board going from the wettest to the driest month.

Other Alaska cities with a similar climate

These cities track one of the cities above closely, so instead of repeating the same advice, here are their own numbers and the closest match.

• Homer: 13.1% in May to 15.8% in December, averaging 14.7%. Month by month it is within 0.4 points of King Salmon on average, so the advice there applies.

What moisture content to aim for

The Wood Handbook's rule is to install wood at the moisture content it will live at, so it moves as little as possible afterward. In Juneau the year-round outdoor average is 16.0%.

• Outdoor projects (decks, benches, planters, siding and trim): the Handbook recommends an average of 12% (individual pieces 9% to 14%), the Handbook's figure for most of the United States.
• Indoor furniture and trim: not given for Alaska: the Handbook's map covers the lower 48 only, so for indoor work rely on moisture meter readings taken in the room where the piece will live.

A pin or pinless moisture meter takes the guesswork out. Check a few boards from each batch, not just one.

How much a deck board moves here

Using the Wood Handbook's shrinkage coefficient for flat-sawn loblolly pine (a common treated-lumber species; Table 13-5), a board 5½ inches wide, the width of a 2x6 or a 5/4 deck board, changes width by:

These are estimates: real boards are rarely perfectly flat-sawn, and western redcedar moves about 10% less than pine. The point is the direction. Lumber bought at 19% will shrink, so gaps between deck boards installed tight will open as it dries.

Acclimating lumber in Alaska

1. Bring it home early. Buy lumber a week or more before you build.
2. Stack it with stickers. Lay straight spacer strips every couple of feet between layers so air reaches every face, and keep the pile off the ground.
3. Store it where the project will live. Outdoors under cover for outdoor projects; inside the house for indoor furniture.
4. Measure, don't guess. When moisture meter readings stop changing from one day to the next, the wood is ready.

A big swing between seasons

Parts of Alaska have wet winters and dry summers. In Barrow (Utqiagvik) outdoor wood reaches 20.5% in September, and in Fairbanks it drops to 8.7% in May. Wide boards, glued-up panels and tight joints feel this most. Fasten tabletops and wide panels so they can move, and avoid gluing wide boards edge to edge with their grain crossed.

Damp all year

Outdoor wood in parts of the state stays above 12% in almost every month. Kiln-dried furniture lumber (dried to 6% to 8% at the mill) stored in an unheated garage or shed will drift up toward the outdoor level. The Forest Products Laboratory notes that warming a closed storage space above the outside temperature lowers the moisture level: with an outdoor EMC of 14%, holding the space about 20°F above outside brings it to about 7%. Store furniture stock indoors or in a conditioned room, and build indoor pieces there too.

Damp weather also favors mildew on outdoor finishes. The Wood Handbook advises washing mildew off with a commercial mildew cleaner or diluted household bleach and detergent before refinishing, since new finish over mildew fails early.

Woods and finishes that hold up outdoors

For anything left outside, choose wood that resists decay or wood treated for it. The Wood Handbook's Table 14-1 rates the heartwood of the cedars (including western redcedar), white oaks, black locust and old-growth redwood as resistant or very resistant, Douglas-fir and young-growth redwood as moderately resistant, and most pines, spruces and firs as slightly or not resistant. Use pressure-treated lumber rated for ground contact for posts and anything touching soil.

From the Handbook's Table 16-5: a water-repellent preservative typically lasts 1 to 3 years, semitransparent stain 4 to 8 years on rough-sawn or weathered wood, solid-color stain 6 to 15 years and paint 10 to 20 years. Clear varnish is a poor choice for furniture left in the sun; it generally needs refinishing every 1 to 2 years there.

Best months for outdoor builds and finishing

In Juneau, outdoor wood is driest in April, May and June and wettest in September, October and December. Lumber dries fastest, and finishes go onto dry wood, in the drier months, so plan staining and painting for then. Avoid sealing wood during the wettest stretch, when it is holding the most moisture it will hold all year. The city sections above give the driest months for each city.

Keep reading

• The actual size of a 2x4, and why it matters
• Why your woodworking project doesn't fit
• How to build an Adirondack chair

Deciding what to build this season? Start with our list of 9 things worth building instead of buying.