Woodworking in Washington: Wood Moisture and Outdoor Builds
Month-by-month outdoor wood moisture for 5 places in Washington, from 8.6% in Spokane in August to 20.4% in Quillayute 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 Seattle-Tacoma as the reference city.
Outdoor wood moisture in Washington, month by month
| City | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Seattle-Tacoma | 15.6 | 14.6 | 15.4 | 13.7 | 13.0 | 12.7 | 12.2 | 12.5 | 13.5 | 15.3 | 16.3 | 16.5 | 14.3 |
| Olympia | 18.9 | 17.0 | 15.3 | 14.5 | 14.0 | 13.9 | 13.3 | 13.3 | 14.2 | 16.6 | 19.1 | 19.9 | 15.8 |
| Quillayute | 19.6 | 18.0 | 17.2 | 16.6 | 16.2 | 16.5 | 16.1 | 16.6 | 16.4 | 18.1 | 20.1 | 20.4 | 17.7 |
| Spokane | 17.5 | 15.5 | 12.9 | 11.4 | 10.9 | 10.3 | 8.7 | 8.6 | 9.8 | 12.1 | 17.1 | 18.7 | 12.8 |
| Yakima | 15.5 | 13.7 | 11.1 | 9.9 | 9.5 | 9.4 | 8.8 | 9.2 | 10.1 | 11.6 | 14.5 | 16.5 | 11.7 |
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.
Woodworking in Seattle-Tacoma: wood moisture by month
- 14.3%Year average
- 12.2%Driest: Jul
- 16.5%Wettest: Dec
- 1/16 in.Deck board movement
Wood sits on the damp side most of the year, averaging 14.3% with a peak of 16.5% in December. Kiln-dried stock left in an unheated garage will pick up moisture, so store it indoors. The driest months, June, July and August, are the best time for staining and painting.
Woodworking in Olympia: wood moisture by month
- 15.8%Year average
- 13.3%Driest: Jul, Aug
- 19.9%Wettest: Dec
- 3/32 in.Deck board movement
Wood here swings 6.6 points, from 19.9% in December down to 13.3% in July and August, enough to change a 5½-inch deck board's width by about 3/32 inch. Fasten tabletops and wide panels so they can move. On average it is 1.5 points wetter than Seattle-Tacoma. The driest months, June, July and August, are the best time for staining and painting.
Woodworking in Quillayute: wood moisture by month
- 17.7%Year average
- 16.1%Driest: Jul
- 20.4%Wettest: Dec
- 1/16 in.Deck board movement
Quillayute is on the coast of the Olympic Peninsula. Outdoor wood stays damp, averaging 17.7% and reaching 20.4% in December. Keep furniture stock indoors, keep outdoor lumber covered with stickers between layers, and wash off mildew before refinishing. On average it is 3.4 points wetter than Seattle-Tacoma. The driest months, May, July and September, are the best time for staining and painting.
Woodworking in Spokane: wood moisture by month
- 12.8%Year average
- 8.6%Driest: Aug
- 18.7%Wettest: Dec
- 5/32 in.Deck board movement
Wood here swings 10.1 points, from 18.7% in December down to 8.6% in August, enough to change a 5½-inch deck board's width by about 5/32 inch. Fasten tabletops and wide panels so they can move. On average it is 1.5 points drier than Seattle-Tacoma. The driest months, July, August and September, are the best time for staining and painting.
Woodworking in Yakima: wood moisture by month
- 11.7%Year average
- 8.8%Driest: Jul
- 16.5%Wettest: Dec
- 1/8 in.Deck board movement
Wood here swings 7.7 points, from 16.5% in December down to 8.8% in July, enough to change a 5½-inch deck board's width by about 1/8 inch. Fasten tabletops and wide panels so they can move. On average it is 2.6 points drier than Seattle-Tacoma. The driest months, June, July and August, are the best time for staining and painting.
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 Seattle-Tacoma the year-round outdoor average is 14.3%.
- 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:
| Change | Moisture change | Width change |
|---|---|---|
| Wettest to driest month | 16.5% to 12.2% | about 1/16 inch |
| From 19% ("dry" lumber) to the local average | 19% to 14.3% | about 1/16 inch |
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 Washington
- Bring it home early. Buy lumber a week or more before you build.
- 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.
- Store it where the project will live. Outdoors under cover for outdoor projects; inside the house for indoor furniture.
- 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 Washington have wet winters and dry summers. In Quillayute outdoor wood reaches 20.4% in December, and in Spokane it drops to 8.6% in August. 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.
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 Seattle-Tacoma, 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.
Sources
- Monthly EMC for Seattle-Tacoma: USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-190), Table 13-1. The same values appear in Simpson 1998, Table 2.
- Monthly EMC for Olympia, Quillayute, Spokane and Yakima: Simpson, W.T. 1998. Equilibrium moisture content of wood in outdoor locations in the United States and worldwide. FPL-RN-0268, Table 2
- How we got the numbers: Simpson's Table 2 is printed as a scanned image, so we transcribed it from a high-resolution render and read every value three independent ways (two separate text-recognition passes and a visual check). All three agreed on every value, and for the cities that also appear in the Handbook's text table the values match it exactly.
- Both tables were calculated by the Forest Products Laboratory from NOAA National Climatic Data Center records (Comparative Climatic Data): the average of morning and afternoon relative humidity and the normal daily temperature for each month, mostly from at least 30 years of observations.
- Recommended moisture content (Table 13-2, Figure 13-1), shrinkage coefficients (Table 13-5), decay resistance (Table 14-1) and finish service life (Table 16-5): Wood Handbook (FPL-GTR-190)