Hardwood Flooring Physical Properties

A. Moisture Content and Dimensional Stability
Moisture plays a large part in how wood behaves, both during the machining process and after installation. Installers would do well to study moisture's effect on wood in some detail: however, a brief discussion is worthwhile here.

Moisture content is defined as the weight of water in wood expressed as a percentage of the weight of oven-dry wood. Weight, shrinkage, strength and other properties depend on the moisture content of wood.

Wood is dimensionally stable when the moisture content is above the fiber saturation point (usually about 30 percent moisture content). Below that, wood changes dimension when it gains or loses moisture.

The ideal moisture content for flooring installation can vary from an extreme of 4 to 18 percent, depending on the wood species, the geographic location of the end product, and the time of year. Most oak flooring, for example, is milled at 6 to 9 percent. Before installation, solid wood flooring should be acclimated to the area in which it is to be used, then tested with a moisture meter to ensure the proper moisture content. (Note: Engineered flooring tends to be more dimensionally stable than solid flooring.

Different woods exhibit different moisture stability factors, but they generally shrink and swell the most in the direction of the annual growth rings (tangentially), about half as much across the rings (radically), and only slightly along the grain (longitudinally). This means that plainsawn flooring will tend to shrink and swell more in width than quartersawn flooring and that most flooring will not shrink or swell much in length.

The individual species descriptions that follow include an indication of dimensional stability, from below average to excellent, as well as a comparison to red oak. For example, mesquite is rated as excellent, with notation that it is 65 percent more stable than red oak - that is, mesquite is likely to shrink or swell 65 percent less. The percentages noted are based on comparing a factor called the dimensional change coefficient of each species with that of red oak. Red oak was chosen as the benchmark because of its widespread familiarity and use in the flooring industry.

B. Hardness and Durability
Probably the most important strength property for wood used in flooring applications is its side hardness, also known as Janka hardness. Side hardness represents the resistance of wood to wear, denting, and marring. It is measured by the load required to embed a 0.444 - inch steel ball to one-half its diameter in the wood. Janka hardness ratings are generally based on an average of tests on both tangential and radial (plainsawn and quartersawn) samples. The higher the number the harder the wood, this should only be used as a general guide when comparing various species of wood flooring.

D. Machining
The workability of a wood depends on several factors, including density, extent of interlocked or variable grain, hard mineral deposits, and tensions in wood that may cause fibrous and fuzzy surfaces. The degree of checking and separation present will also affect machining ease.

Interlocked grain is characteristic of many tropical species and causes problems in planing quartersawn ___ boards unless feed rates, cutting angles, and sharpness of knives are carefully controlled. Hard deposits in the cells, such as calcium carbonate and silica, may have a pronounced dulling effect on cutting tools. This effect generally increases as wood is dried to normal working requirements.

E. Nailing
When nailing some of the denser woods with hand or air nailers, installers may encounter splitting tongues, as well as failure to secure the fastener even after repeated attempts. This can sometimes be corrected by changing the angle of the nails point of entry. On certain exceptionally dense species, pilot holes may have to be drilled to ease nailing. Blunting the ends of fasteners may also help prevent splitting.

Though dense, heavy woods normally offer higher nail-withdrawal resistance, less dense species allow the use of more and larger diameter fasteners to compensate for their lower holding ability.

F. Sanding
Some wood species are highly resinous and tend to clog sandpaper. When working with such species, it may be necessary to use a coarser grit of sandpaper than normal, or to change the sandpaper more often than with other species.

Also, the wood dust created by sanding some species tends to cause an allergic reaction in some people. This is more likely to occur with imported species than with domestic. However, even North American oak has been known to cause a skin rash or respiratory difficulties in some people. Where applicable, known tendencies to cause allergic reactions are noted.

As a precaution, flooring mechanics should wear long sleeves, dust masks, and eye protection when sanding.