Woodworking in Oregon: Wood Moisture and Outdoor Builds

Month-by-month outdoor wood moisture for 7 places in Oregon, from 5.9% in Burns in July to 20.2% in Eugene in December, 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 Portland as the reference city.

Outdoor wood moisture in Oregon, month by month

City by city

Each city below gets its own numbers and advice. Deck board figures are for a 5½-inch flat-sawn pine board going from the wettest to the driest month.

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 Portland the year-round outdoor average is 14.2%.

• 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: about 8% (6% to 10%), the figure for most of the country.

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 Oregon

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.

Dry air: expect lumber to keep shrinking

Outdoor wood in the driest parts of the state settles well below 19%, the most moisture that lumber stamped dry is allowed to hold. Boards bought near that limit will shrink noticeably before they stop moving, so let them sit for a few weeks before tight joinery, expect gaps between deck boards installed tight to open as the boards dry, and expect joints and paint lines to show some movement in the first season.

A big swing between seasons

Parts of Oregon have wet winters and dry summers. In Eugene outdoor wood reaches 20.2% in December, and in Burns it drops to 5.9% in July. 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.

In strong sun and dry air, film finishes crack and peel sooner; the Handbook notes clear varnish in direct sunlight generally needs refinishing every 1 to 2 years. Pigmented finishes protect the wood from sunlight better: semitransparent stain typically lasts 4 to 8 years on rough-sawn or weathered wood, solid-color stain 6 to 15 years and paint 10 to 20 years.

Best months for outdoor builds and finishing

In Portland, outdoor wood is driest in June, July and August and wettest in January, November 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.