Species surface hardness
May 31, 2014 | 7:00 pm CDT

When working with various woods, there is always the consideration whether we can substitute a less expensive wood for the species we are using and still get the same performance. When looking at a potential substitute, cost, availability, color and density can certainly be important factors.

The importance of density

Density is very important because this property is related closely to strength, stiffness and nail, screw and staple holding power. We can often offset low-density effects. If the strength is low, we can perhaps make the piece a little larger. If the fastener holding power is low, we can increase the fastener size or number of fasteners to achieve the required performance.

One additional variable that is hard to compensate for by changing the design or manufacturing process is surface hardness. A low hardness, or softer woods, means that the surface is easy to dent; a dent can damage the wood fibers and give a brittle finishing coating. Crushed fibers can often be restored by steaming the damaged area briefly; finish repair is much more difficult.

Softer woods, although they machine easier, are also harder to sand to a smooth, fuzz-less surface.

Comparing hardness

Surface hardness in the wood industry is measured by the load or force required to embed a steel ball, 0.444 inch in diameter, to a depth of one-half its diameter. The hardness of end grain is different from the two surfaces. I have tabulated the surface hardness for various lumber species and arranged the list from softest to hardest, even within the groups. This arrangement is similar to a ranking of lightest to heaviest.

The data, the average of both tangential and radial surfaces (flatsawn and quartersawn), but not end grain, are from the  U.S. Forest Products Lab  in Madison, Wis.

I have divided the list into two parts: hardwoods and softwoods. The hardwoods, which are not necessarily hard, are trees with leaves while the softwood trees have needles.

Special note: Soft maple, a lumber trade name, includes both red and silver maple. Note the difference in hardness between the two species that make this grouping. Also note the large differences between the pine, spruce and oak species.

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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.