Wood Characteristics

Hardwoods and Softwoods

Hardwoods come from broad-leaf (deciduous) trees. Softwoods come from needle-bearing (coniferous) trees. Most hardwoods are harder than most softwoods, but there are exceptions. For example, Balsa is a very soft hardwood and Australian Cypress and Heart Pine are hard softwoods. Many people believe that the stability of a wood (the amount that it expands or contracts in response to humidity changes) relates to its hardness, assuming that harder woods are more stable. In fact, stability and hardness are not related, and some of the hardest woods are some of the least stable, while some of the softer woods are extremely stable.

Wood Structure

The structure of wood resembles a bundle of long, tiny "straws" that transport nutrients and water (sap) up and down the tree. When wood is milled, these straws look different when viewed on the side or face of a board than they do when viewed from their cut ends, such as on the ends of a piece of lumber.

Heartwood and Sapwood

Trees grow in thickness from their center by adding layers of cellular "straws" at the outside of the tree. As the tree grows and ages, the cells closer to the inside of the tree cease conducting sap and die. The dead cells become the heartwood and the newer cells that still conduct sap form the sapwood. In many woods, the sapwood and heartwood are clearly differentiated, the sapwood often being much lighter in color.

Growth Rings

Growth rings are concentric rings that radiate out from the center of the tree, each ring representing one year of growth. The term "grain" usually refers to the way that growth rings appear on the surface of a piece of wood. That appearance will vary depending on how the wood is cut from the log.

Wood Expansion and Contraction

The “straws” that make up wood structure absorb or lose moisture with changes in humidity. The moisture content of wood will always gradually equalize with the moisture of its surrounding environment. All wood expands when it takes on moisture and shrinks when it loses it. When it does, it changes size across its width and thickness, but only slightly in its length. This expansion and contraction continues forever, no matter how long ago the tree was cut down.

Unlike solid wood, Bamboo expands and contracts somewhat along its length, as do planks of engineered flooring (due to expansion across the width of the cross-ply layers). Installers who are not familiar with this aspect of Bamboo and engineered flooring will sometimes fail to leave adequate expansion space at the ends of rows of installed planks. With solid hardwoods, you can generally get away with this, but with Bamboo and engineered hardwoods, you must leave expansion space on all sides of a floor.

Wood and Moisture

Because moisture changes cause wood to move, especially wood flooring and most other wood products, moisture and wood DO NOT mix well. Most failures of wood flooring are due to moisture related problems, including wet mopping, flooding, inadequately cured concrete slabs, and HVAC systems that create job site conditions that are either too dry or too moist. Radiant heat subfloors and regions that have significant fluctuations in humidity also pose challenges to wood floors that can eventually cause significant damage.

Wood Hardness

The standard measure used in the U.S. for the hardness of wood flooring is the Janka test. It measures the force (in pounds per square inch, or PSI) required to drive a steel ball 0.444 inches in diameter into a given wood to the depth of half of the ball's diameter. Red oak, the wood flooring industry standard, is 1290 PSI on the Janka scale. Maple, often used for sports floors, is 1450. A number of exotic species are 3000+ on the Janka scale. In general, most tropical woods are harder than most domestic woods.

Wood Durability

All wood will dent if sufficient force is applied. Women’s high heel shoes exert tremendous force via the heel, so a 115 lb woman will exert about 2500 lbs of force. In high traffic applications, it is best to select harder woods. Oak would be a minimum (1290 on the Janka scale), but the harder exotics (2000+ Janka hardness) or EcoTimber woven bamboo (about 3000 Janka hardness) are the best choices from a durability standpoint.

Wood Color Change

Wood is a natural product and some degree of color variation is inevitable. Some species have a much higher degree of color variation than others. The only real ways to achieve uniform color in wood are to select out much of the material, which is a poor environmental choice, or to apply a stain. Some wood species change color dramatically as they age (oxidize). Others change color when exposed to light (UV radiation). Some do both, and sunlight generally accelerates the oxidation process. People who use wood products should expect the wood to change color over time unless it is heavily stained. In conditions of prolonged, intense direct sunlight, some species that normally darken with time will actually bleach to a lighter color.