Douglas-fir (Pseudotsuga menziesii) is one of the premier species of the Western forests. Not only is the wood exceptionally strong for its weight, but the trees are abundant. Trees can grow over 250 feet tall (with the first 150 feet being free of knots and branches) and over 6 feet in diameter. Some trees are well over 800 years old.
Certainly, harvesting of some of these “old-growth” trees today must be done cautiously to avoid environmental damage and avoid depleting the forests of these icons. However, forest fires in recent years, brought on in part by poor forest management in the past years, have been removing these trees and such removal means that man does not get to use the wood for housing, furniture, and so on. So should we remove the trees before they burn? If so, how many trees? Quite a battle is going on between environmentalists (preservationists) and conservationists on this issue.
Much of the Douglas-fir lumber today is coming from trees under 100 years old. These trees, called second-growth. Lumber properties may not be as high as with old growth, but the wood is still a superior product–strong, clear, nice grain appearance, good processing characteristics and so on. Most uses for Doug-fir today capitalize on the wood’s high strength; one important use is for ladders and scaffolding. Yet the beauty of this wood makes it favorable for millwork, furniture, cabinets and flooring.
Douglas-fir has two varieties: Coastal Doug-fir, coming from the BC to California, with the best growth west of the Cascade Mountains, and Interior Doug-fir coming mainly from the Rocky Mountains. Most wood processors prefer the Coastal variety, as it is stronger, stiffer, clearer (free of knots), and processes better. Data shown here is for Coastal. Note that there can be wide differences in weight, and therefore strength, and color in wood from different geographic locations. The clear wood is desired for its high strength and so is quite expensive. Lower grades are much more reasonable. Oftentimes, small sawmills will be the best source of supply for furniture and cabinet makers.
Density. Douglas fir has a green specific gravity (SG) of 0.45. At 6 percent MC, the SG is 0.50 (which means it is half as dense or heavy as water). The weight, when dry, is 32 pounds per cubic foot or about 2.0 pounds per board foot (planed to 3/4-inch thickness). Most Doug-fir lumber will be sold dried and planed (S4S, surfaced 4 sides). The standard thickness for such lumber is thinner than for hardwoods: 4/4 is 0.75 inch actual; 8/4 is 1.50 inch actual.
Strength. For dry wood, the ultimate strength (MOR) is 12,400 psi, stiffness (MOE) is 1.95 million psi and hardness is 710 pounds. Comparative oak values are 14,300 psi, 1.82 million psi and 1,290 pounds. Although oak is a little stronger, oak also weighs 30 percent more.
Doug-fir, when quite dry, does have a tendency to split, so predrilling of holes for large-diameter fasteners may be required at times.
Drying and stability. The wood dries rapidly with little risk of quality loss. Most suppliers will sell only KD stock, rather than green.
Shrinkage in drying is fairly low. Overall shrinkage from green to 6 percent MC is 6.9 percent tangentially (the width in flatsawn lumber) and 3.8 percent radially (the thickness of flatsawn lumber). Once dried, the wood does not move much even with large RH changes or if the MC is not matched to the environment’s EMC conditions. A typical, desired, final moisture range is 9 to 10.5 percent MC. This final MC range, although high for hardwoods, facilitates machining.
Once dry, it takes a 4 percent MC change to result in 1 percent size change tangentially and 7.5 percent MC change radially. It is not prone to warping in use.
Machining and gluing. This wood machines well, unless the MC is too low (under 9 percent MC). Then the wood appears to be quite brittle. This wood glues without much difficulty.
Grain and color. The color of Doug-fir varies from a reddish color (more common in second growth) to yellowish (more common in old growth). The color darkens somewhat when exposed to light. Each annual growth ring has light earlywood and darker latewood, which gives the wood an obvious heavy grain appearance. This heavy grain may cause some problems with some finishing materials, especially paints. However, the natural beauty of the wood suggests clear finishes may be best.
Have something to say? Share your thoughts with us in the comments below.