 
Cupping and Crowning
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.
Rapid cupping may occur when an impervious surface finish is applied, cutting
off evaporation through the surface. The cause will still be moisture
accumulating in the back sides of boards.
Extent of moisture changes can be illustrated by the following example when
checking the moisture content of the flooring with a moisture meter having
insulated needles:
: Surface of flooring -- 9% - 10% : Subfloor -- 13% - 15%
: Back of flooring -- 11% - 12% : Joists -- 14% - 16% These are
approximations, intended to illustrate that higher readings occur as the
insulated moisture meter probes are driven deeper into the construction. Actual
readings may be lower or higher, depending on how far the moisture condition has
progressed.
Usually when readings 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. A sling psychrometer or digital thermometer hygrometer may be needed to
establish humidity levels beyond question. 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.
CUPPING & CROWNING CONTINUED
HERE
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