Hardwood is not a technical description or category; it's more of a nickname that's usually used to distinguish "hardwoods" from "softwoods." Of course, the nicknames have some truth. Hardwoods as a group are harder and denser than most softwoods. But the real distinction lies in the type of tree—or more specifically—the type of seed that hardwoods come from.
Hardwoods Are Angiosperms
Hardwood trees are types of angiosperms, plants that grown from seeds that have a protective covering. The seeds may come in many different packages, such as inside the fruit of apple or cherry trees, in the seed wings of maple trees, or in the hard shell of a nut or acorn. By contrast, softwoods are gymnosperms, which have seeds without coverings. The cones of a pine tree are the most common example.
An easier, if somewhat less accurate, way to distinguish hardwoods from softwoods is to look at the leaves. Hardwoods typically are deciduous trees, which lose their leaves in fall or winter. Most softwoods are evergreen trees, which do not lose their leaves (pine needles are a type of leaf, but not all evergreens have needle-like leaves).
What Makes Wood Hard?
Hardness of wood generally is attributable to its density. The more dense and closely spaced the wood fibers, the harder the wood. Hardwood fibers tend to be tightly packed together, leaving little room for the fibers to move or the wood to compress. It might seem that dense wood also is non-porous, but there is not a direct correlation between porosity and wood density.
Measuring Wood Hardness
The hardness of wood is measured by a test known as the Janka hardness test. It involves pressing a steel ball into the surface of the wood and measuring how many pounds of pressure it takes to sink the ball halfway into the surface. The higher the pressure result, the harder the wood. A few examples of Janka hardness scores cherry is 950, white oak is 1360, hickory is 1820, and Ipe is 3680. Balsa wood, which is technically a hardwood (after all, it's an angiosperm) scores a mere 67.
By comparison, looking at a few common softwoods, you realize they aren't much softer than hardwoods like cherry: Southern yellow pine has a Janka hardness of 870, and cedar is 900.
Is Hardwood Stronger?
As the steel ball of the Janka test indicates, harder woods are more resistant to dents and dings than softer woods. This is the main reason why fine furniture, cabinets, and premium trim work are usually made of hardwood. Hardwood simply stands up better to everyday use and abuse.
But hardness does not necessarily correlate to strength, at least when it comes to "bending-without-breaking" strength. In fact, the density of hardwoods can make them brittle and prone to cracking or breaking. Wood strength is related more closely to the porosity of the wood and how the wood is cut (or split) and dried. Woods with small, diffuse pores, such as maple, are less elastic and resilient than woods with larger "ring" pores, such as ash.
Air-dried wood tends to be stronger than kiln-dried wood because the latter process dries out the fibers with heat, making them more brittle. Splitting, or riving, wood along its natural grain patterns is the best way to retain the wood's strength. By contrast, cutting across the grain robs the wood fibers of much of their holding power, weakening the wood.
Common Types of Hardwood
You can buy hardwood lumber and other products in hundreds of different wood species (if you look hard enough), as hardwoods are imported from all over the world. But there are plenty of beautiful, useful hardwoods local to every part of North America, and maybe even your backyard. Some of the most common species of hardwood include:
- Apple (several varieties)
- Maple (several varieties)
- Oak (several varieties)
Common Uses of Hardwood
Hardwood has many properties that make it a traditional favorite of furniture makers and woodworkers, including stiffness, tight grain, strength, and impact-resistance. The right hardwoods can be sanded to an almost glassy smoothness—something you can't get with pine or cedar. Looking around a house, you might see hardwoods used on the kitchen cabinet doors, interior trimwork, flooring, stair steps and railing parts, fireplace mantels and surrounds, and, of course, furniture.
Where you won't see hardwood is in a home's structure (with the exception of some historic homes). Construction lumber, such as wall studs and floor joists, as well as engineered structural members and trusses, are almost exclusively made from softwoods, such as pine, larch, Douglas fir, and spruce. Hardwoods aren't used for structural materials for several reasons: they're expensive, they're heavy, they're hard to work with, and they're relatively brittle.
Working With Hardwood
Each hardwood species has its own properties and peculiarities, but generally speaking, hardwoods are harder to cut, drill, sand, and glue than most standard softwoods. Hardwoods dull saw blades and router bits quickly and are almost impossible to drive screws into without a pilot hole. On the upside, all of that hardness can result in cleaner cuts, sharper edges and corners, neater pilot holes, and overall better precision in the finished product.
To minimize struggles when working with hardwood, always use sharp blades and bits for cutting, routing, and drilling. Also, use traditional wood screws instead of drywall screws. Wood screws are stronger and have a smooth shank section under their heads that allows the screw to slip though the upper piece of wood (where you don't need threads). Drywall screws, on the other hand, are threaded over their entire shank, and they're brittle and are prone to breaking, especially in hardwood. To make sure screws go in easily and without splitting the wood, always drill the right size pilot hole for the wood and the screw, including a large "clearance hole" for the smooth portion of the screw shank.