Oak is a hardwood and Douglas-fir is a softwood, which means that oak is harder, right? That reasoning doesn't apply in every instance, but in this case, it does. Red oak wood is almost exactly twice as hard as Douglas-fir wood, and white oak wood is even harder.
How Do You Measure Hardness?
When discussing different types of wood and their uses, the question of hardness is an important one. It's not the same as wood strength, which is a general evaluation of a number of properties that include hardness, modulus of elasticity, modulus of rupture and compressibility.
Wood suppliers have a specific definition of hardness. It's the force required to compress a 1/2-inch steel ball halfway into a plank of a specific species. That force is measured in pounds-force (lbf) or newtons (N).
Wood suppliers use this test, known as the Janka test, to measure hardness, and the resulting number is the Janka rating. Woodworkers and suppliers rank species according to their Janka ratings to create a chart. When comparing wood species for hardness, one glance at this chart is all you need.
How Does Douglas-Fir Compare to Oak?
Janka ratings can vary slightly depending on who performs the test, but according to the Wood Database, the rating for red oak is 1,220 lbf and that for white oak is 1,350 lbf. Compare that to 620 lbf for Douglas-fir, and it's apparent that both species of oak are harder than Douglas-fir.
Oak scores higher on other qualities related to strength as well, including modulus of rupture, elastic modulus and crushing strength. It's not inaccurate to say that oak is stronger than Douglas-fir as well as being harder. Oak is also denser, and it shrinks less.
Benefits of Douglas-Fir
You might wonder why woodworkers often choose fir if oak is harder and stronger than Douglas-fir. One reason is workability. Fir's relative softness makes it easier to cut, sand and shape than oak. It's an attractive blond color with a sinuous, wavy grain, but if you decide to stain it, it takes the stain better than oak, which has more density variations.
Fir is a structurally stable species. Owing to the fact that the trunks of fir trees are long and straight, it's easy to find boards that are virtually knot free to use for trim and molding. Fir trees are large, and there are a lot of them in North America, so fir is a natural choice for studs, beams and other building lumber, and that's where most of the blemished boards end up.
Whitewood vs. Douglas-Fir
"Whitewood" is a generic term that refers to construction-grade boards that may be pine, spruce, hemlock or fir. These are generally utility boards, and depending on the project, you might not care about the species.
If wood strength and hardness matter to you, choose Douglas-fir wood over whitewood. Fir is harder and stronger than pine, spruce or hemlock, and it's less likely to have knots or other blemishes. It may be more expensive but probably not as expensive as a hardwood like oak unless it's clear fir. Fir boards that are completely blemish free are more expensive than oak.
Hem-Fir vs. Doug-Fir Strength
If you're looking for a versatile but inexpensive building material, you might find that your lumberyard stocks hem-fir, which is a hybrid of Western hemlock and one of the true fir species. This material isn't as strong as Douglas-fir, but because it's a budget material, you wouldn't expect it to be.