The science and physics associated with wood’s affinity for water are complex. While the science can be difficult to understand, knowing a few basic fundamentals can make wood’s performance in different environmental conditions predictable. By grasping a few key aspects, we can address moisture-related issues in wood flooring—and also become better installers.
The three components of wood
Wood contains three primary components: cellulose (for strength), hemicellulose (for strengthening and growth) and lignin (for holding the cellular structure together). When evaluating wood dimensional change, cellulose plays a significant role. Cellulose is a long-chain carbon-based polymer oriented with its long axis parallel to the long axis of the wood cell. All chemically reactive sites for adsorbing water molecules are located laterally along the length of cellulose chains, making it a key player in wood’s dimensional response to moisture.
RELATED: Understand Wood Floor Moisture Content & Dimensional Change
The three planes
Three principal planes in wood are critical to defining its properties. They are cross-sectional (the surface exposed if cutting across the long grain), radial (the surface exposed if cutting from the center of the tree to the bark), and tangential (the surface tangent to the growth ring). Wood properties vary in each of the three principal planes, defining wood as an anisotropic material (different properties in different directions). Most of these differences are due to the shape and orientation of the wood cell in the wood matrix.
What wood cells look like
Wood cells are similar to drinking straws: long and slender with a hollow cavity. The cell’s long axis is oriented parallel to the tree length. Wood cells are rectangular in cross-section, with the widest dimension of the cell wall oriented in the tangential plane and the shortest in the radial plane. This difference in width explains why wood expands and contracts nearly twice as much in the tangential (flatsawn) direction than in the radial (quartersawn) direction.
By understanding these concepts, one can comprehend the rationale behind practices such as leaving gaps at vertical obstructions, the variation in gap sizes between planks due to seasonal shrinkage (side vs. ends), and the potential issues when a wood floor is exposed to water or fluctuating moisture levels in the air. Equipped with this knowledge, you can now consider all the variables that may impact your work as a wood flooring professional. 
