Most complaints concerning hardwood flooring are related to changes in the moisture content of flooring and surrounding wood products. Other problems may also be associated with wood flooring however, the following information relates to the effects of moisture on flooring. When moisture changes are severe, the result can be gaps between strips, cupping, buckling, movement, and/or cracks/splits in the flooring. Also, a combination of these results may be present in the same floor.
The enclosed information and comments should help explain some causes of shrinking and swelling, and some of the possible results. Knowledge of the following items is necessary to assure the proper performance of flooring:
- The expected moisture content of wood flooring in a particular area after acclimation.
- The moisture content of flooring both at time of delivery and installation.
- The moisture content of flooring system and moisture conditions at the jobsite.
Additional factors which contribute to the changing moisture content of wood flooring are things such as the type and operation of HVAC system, incomplete construction such as masonry, drywall, and the time of year.
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. Shrinkage of wood begins at 25-30% moisture content, the fiber saturation point, and continues until wood reaches 0% moisture content, an oven dry state. Conversely, swelling occurs as wood increases in moisture content from 0% to 25-30%, the fiber saturation point. Above 25% – 30% wood is dimensionally stable.
Wood does not shrink or swell equally in all directions. A change in moisture content of a wood piece from 0% to 28% (oven dry to fiber saturation point) will increase the size of the piece approximately 0.1% longitudinally or along the length/grain, 2% to 8% perpendicular to the annular growth rings (radially or in strip 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).
Water Damage
Excess moisture from the flooded condition must be removed immediately. If cupping of flooring is the only adverse result, drying of the floor is necessary. The use of fans and dehumidifiers from the interior of the home or below the floor system (i.e. crawl space, if possible) may be all that is necessary to remove the cup. In homes with forced air heat, turn off any humidification and heat the residence to 76°- 80° F. Set furnace blower on manual to run continuously. After the flooring has flattened from the drying procedures, screening, filling, and recoating could be sufficient for repair, if no permanent staining has occurred. If the floor is water stained, re-finishing may be necessary.
If the water has been removed and cupping is prominent (buckling not present) light sanding directly across the grain of the flooring to remove the finish is required (this procedure should be done by a professional). Since the sanding
drum follows the contour of the cup, flooring edges are not removed, which helps prevent crowning later. The sanding procedure allows the excess moisture in the flooring to dissipate faster. Do not sand the floor to bare wood; simply remove a majority of the finish. Drying of the floor as described in #1 should be initiated. After drying has occurred and the floor has stabilized and flattened, some compression cracks, some loose strips, and cracks due to movement from the initial flooding might be expected. At this time re-finishing may begin. Loose areas should be re-attached by nailing or screwing and cracks should be filled. This repair is a judgment call as the more severe the initial cause the more remedy #3 is indicated.
If the floor has cupped and buckled, excess moisture probably remains in the flooring and floor system. Since the floor has also separated from the sub floor (buckled) and has loosened, replacement of the damaged and surrounding area is generally the procedure for repair. The excess moisture remaining in the under floor system should be completely removed before re-installation and finishing. If the flooring is installed over plywood attached to a slab, removal of the entire system is normally required.
An attempt to cosmetically repair by simply re-nailing, sanding out the cup and re- finishing could backfire. Later problems with staining, crowning, cracks, finish failure, or loose floors could develop. By the time these later problems occur most insurance claim releases have been signed, and either the home owner or flooring contractor is saddled with the final repair. Additional repair, partial replacement and refinishing could be the least severe result. The worst case would be to completely replace the floor.
In any case quick action to remove the induced flooded moisture is the first step in successful repair. Patience is also required as finished flooring and floor systems do not react and dry out overnight. It may take weeks for the flooring to flatten and/or stabilize.
Cupped Flooring
Step 1. Flooring absorbing excessive moisture on the underside
causes expansion and cupping with the edges raised.
Flooring Sanded Flat
Step 2.Floor is sanded flat white at the higher moisture content,
resulting in strips with thinner edges.
Crowned Flooring
Step 3. Flooring loses some excess moisture, shrinks on the underside
and flattens, leaving the edges of strips lower than the center.
Cupping of Wood Floors – 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 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.
Normally when cupping is noticed, the surface of boards will be concave-edges higher than the center of the boards. This will mean that the backs of the boards are absorbing water vapor to an extent expansion has begun. If unchecked, heavy expansion may ensue, followed by buckling of the floor. 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 it’s 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.