Woodworking in Idaho: Wood Moisture and Outdoor Builds
Month-by-month outdoor wood moisture for 3 places in Idaho, from 7.3% in Boise in July to 15.7% in Lewiston 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 Boise as the reference city.
Outdoor wood moisture in Idaho, month by month
| Month | Boise | Lewiston | Pocatello |
|---|---|---|---|
| Jan | 15.2% | 15.1% | 14.7% |
| Feb | 13.5% | 13.6% | 13.7% |
| Mar | 11.1% | 11.9% | 12.0% |
| Apr | 10.0% | 10.9% | 10.2% |
| May | 9.7% | 10.7% | 9.9% |
| Jun | 9.0% | 10.0% | 9.6% |
| Jul | 7.3% | 8.1% | 8.4% |
| Aug | 7.3% | 8.0% | 8.0% |
| Sep | 8.4% | 9.2% | 8.8% |
| Oct | 10.0% | 12.1% | 10.1% |
| Nov | 13.3% | 15.0% | 13.2% |
| Dec | 15.2% | 15.7% | 14.9% |
| Year | 10.8% | 11.7% | 11.1% |
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 Boise: wood moisture by month
- 10.8%Year average
- 7.3%Driest: Jul, Aug
- 15.2%Wettest: Jan, Dec
- 1/8 in.Deck board movement
Wood here swings 7.9 points, from 15.2% in January and December down to 7.3% in July and August, enough to change a 5½-inch deck board's width by about 1/8 inch. Fasten tabletops and wide panels so they can move. The driest months, July, August and September, are the best time for staining and painting.
Woodworking in Lewiston: wood moisture by month
- 11.7%Year average
- 8.0%Driest: Aug
- 15.7%Wettest: Dec
- 1/8 in.Deck board movement
Wood here swings 7.7 points, from 15.7% in December down to 8.0% in August, 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 0.9 points wetter than Boise. The driest months, July, August and September, are the best time for staining and painting.
Woodworking in Pocatello: wood moisture by month
- 11.1%Year average
- 8.0%Driest: Aug
- 14.9%Wettest: Dec
- 3/32 in.Deck board movement
Wood here swings 6.9 points, from 14.9% in December down to 8.0% in 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. The driest months, July, August and September, 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 Boise the year-round outdoor average is 10.8%.
- 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 | 15.2% to 7.3% | about 1/8 inch |
| From 19% ("dry" lumber) to the local average | 19% to 10.8% | about 1/8 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 Idaho
- 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 Idaho have wet winters and dry summers. In Lewiston outdoor wood reaches 15.7% in December, and in Boise it drops to 7.3% in July and 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 Boise, outdoor wood is driest in July, August and September and wettest in January, February 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 Boise: USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-190), Table 13-1. The same values appear in Simpson 1998, Table 2.
- Monthly EMC for Lewiston and Pocatello: 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)