Douglas-fir: Exceptionally strong and abundant

Douglas-fir: Exceptionally strong and abundant

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 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 raised questions. Should we remove trees before they burn? If so, how many? 

Much of the Douglas-fir lumber today is coming from trees under 100 years old, which are 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 Douglas-fir today capitalize on the wood’s high strength, yet the beauty of this wood makes it favored for millwork, furniture, cabinets and flooring.

Douglas-fir has two varieties: Coastal Douglas-fir, coming from British Columbia to California, with the best growth west of the Cascade Mountains, and Interior Douglas-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. Clear wood is desired for its high strength and consistent look, and so is quite expensive.

Processing suggestions and characteristics
Density. Douglas-fir has a green specific gravity (SG) of 0.45. At 6 percent MC, the SG is 0.50. 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). 

Strength. For dry wood, the ultimate strength (MOR) is 12,400 psi, stiffness (MOE) is 1.95 million psi and hardness is 710 pounds. Douglas-fir, when quite dry, does have a tendency to split, so predrilling of holes for large-diameter fasteners might 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. 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. 

Machining and gluing. This wood machines well, unless the MC is too low (under 9 percent MC). This wood glues without much difficulty.

Grain and color. The color of Douglas-fir varies from a reddish color to yellowish. The color darkens somewhat when exposed to light. Light earlywood and darker latewood gives the wood an obvious heavy grain appearance. 

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