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Most 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.


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.


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 other.


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.