complaints concerning hardwood flooring are related to changes in the
moisture content of flooring and surrounding wood products. When
moisture changes are severe, the result can be gaps between strips,
cupping, buckling, movement, and/or cracks/splits in the flooring. A
combination of these results may be present in the same floor. This
information is to help inform the customer of the normal
characteristics of hardwood floors in the presence of “normal moisture
changes”, not related to flooding, accidental discharge of water,
hydrostatic moisture, etc.
SHRINKAGE – EXPANSION
Wood is a hygroscopic material.
This means that when wood is exposed to air, it will dry or pick up
moisture until it is in equilibrium with the humidity and temperature
of the air. Moisture absorption causes wood to swell. Moisture loss
causes wood to shrink. Shrinkage will cause small gaps to form between
boards that may vary from 1/32” to 1/16” in some cases. Expansion
will cause slight cupping under the presence of normal moisture
changes. This should be considered a normal characteristic of
hardwood floors and part of their natural beauty.
CUPPING OF WOOD FLOORS
Kiln-dried wood boards, which are
subjected to moisture only on one side, will expand on that side, and
will warp by bending away from the moist side. This can be easily
demonstrated with a narrow piece of paper; simply moisten one side.
The paper will immediately “cup” away from the wet side, creating a
convex surface on the wet side and a concave surface on the other
side. Similarly, hardwood flooring will cup for one reason and one
only -- from gaining or losing moisture on one side faster than on the
Some cupping should be considered
normal, especially in wide planks -- 5", 6", 7" and wider -- and
particularly in plain-sawn boards. In such boards (as opposed to
quarter-sawn; see Fig. 2) the growth rings of the tree travel in a
slightly curved pattern from one side of the board to the other. This
curved pattern produces, with normal moisture content changes, a
slight convex or concave cup, depending on how the rings curve within
individual boards. This type of cupping is usually not noticeable
unless the floor is viewed across the boards and against a strong, low
light source, such as a patio door or window wall. It is often noticed
while the house is still unoccupied but furnishings usually make the
cupping seem more normal as the strong light reflection is softened
and angles of view are changed.