Hardwood and Moisture

Most problems with 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, cracks, or splits in the wood.  Often, combinations of these symptoms occur at the same time.


Homeowners should be aware of, and educated on, the following items to ensure that the floor performs to its maximum potential:

1. The expected moisture content of wood flooring in a particular area after acclimation
2. The moisture content of flooring both at time of delivery and installation
3. The moisture content of flooring system and moisture conditions at the jobsite (i.e. moisture in subfloor)
4. The type of HVAC system and how often it will run
5. Construction factors which influence moisture content of adjacent materials such as masonry, drywall, and/or the location of the floor in relation to the ground level outside the home
6. How variations in climate effect relative humidity and, therefore, moisture content of wood.

Shrinkage/Expansion

Wood is a hygroscopic material. This means 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 and desorption causes it to shrink. Shrinkage of wood begins at 25-30% moisture content (the fiber saturation point) and continues until wood reaches 0% moisture content (oven dry state). Conversely, swelling occurs as wood increases in moisture content from 0% to 25-30% moisture content until it reaches the fiber saturation point. Above 25% - 30% wood is dimensionally stable.

Wood does not shrink or swell equally in all directions. A change in the moisture content of a board from oven dry to fiber saturation point will increase the size of the piece approximately 0.1% longitudinally (along the length), 2% to 8% perpendicular to the annular growth rings (radially or in strip height for plain sawn lumber and width for quarter sawn flooring), and 5% to 15% change in size parallel to the trees annular growth rings (also known as tangentially or in width of the strip with plain sawn flooring).


* CUPPED  -Flooring absorbing excessive moisture on the underside causes expansion and cupping with the edges raised.

* FLOORING SANDED FLAT  -Floor is sanded flat while at the higher moisture content, resulting in strips with thinner edges.

* CROWNED FLOORING  -Flooring loses some excess moisture, shrinks on the underside and flattens, leaving the edges of strips lower than the center.

Wood Floor Cupping: Causes

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 wetted side and a concave surface on the other side. Similarly, hardwood flooring will cup for one reason - from gaining or losing moisture on one side faster than on the other.

Normally when cupping is noticed, the surface of boards will be concave -- edges higher than the center of the boards. This means that the backs of the boards are absorbing water vapor to an extent expansion has begun. If left unchecked, heavy expansion may ensue followed by buckling. Often, however, only enough moisture is present to cause the cupping, and this will be the extent of damage.

Usually when signs like the above are found some evidence of the moisture source will be obvious, especially in a crawl space, when the full inspection procedure is followed. In a basement the moisture source may not be quite as obvious_ An observant inspector will usually notice high humidity, however, because the air in the basement will feel cooler than its dryer counterpart in the rooms above. Lack of visual evidence of evaporation below the floor does not disprove its presence by any means. Concrete basement walls and floors are ready evaporators, as can be demonstrated with a moisture meter.

If cupped boards are dried soon enough, they usually return to a flat position. However, if they remain cupped long, the stresses within boards may change to the extent drying will not remove the cupping. (This is more common in wider boards.) In that event, moisture readings taken during the inspection can be at or near normal.