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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.
CONVEX CUPPING ("crowning")
of the floor surface may also occur for all the reasons previously
described, but most often the reason is a different one. It usually
follows cupping of the concave type, which often occurs before
the house is ready for floors to be sanded and finished. If the
flooring is sanded with boards cupped and edges high, the high
edges of boards are cut flat by the sanding machine if the operator
does his job correctly. In profile, after sanding, the boards
will then have abnormally thin edges -- flat on top, with edges
of the reverse side of boards still curved upward, or cupped.
If these boards later dry and flatten to their original position,
the thin edges recede, leaving the top of boards convex (edges
lower than the centers) and the back again flat against the subfloor.
(Fig.l).
NORMAL CUPPING 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.
SOLUTIONS -- CUPPED FLOORS:
Cupped floors have gone through a site-related moisture escalation.
Re-dried and repaired, the floor already in place is, in most
cases, the best choice for a trouble-free floor in the newly-established
environment. Replacing a cupped floor is usually the worst choice,
especially if replacement is made before the moisture condition
causing the problem has been corrected. The replacement is almost
certain to react to the moisture situation by cupping, like the
first floor, and the whole process must be repeated.
There are, of course, exceptions, when the only solution is
to replace the flooring. But the problem has usually reached
a far more serious level before replacement is needed.
The first step in repairing a cupped floor is to remove the
source of moisture. To cure it the source must first be found.
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In crawl-space or concrete slab subfloors, be sure all outside
drainage moves rainwater away from the house. Water flows through
many types of soil almost as rapidly as on the surface of the
ground. If the building sits on a hillside the natural flow of
water may take it under the foundation in large volume, where
some will evaporate. Close this source by installing soil tile
on the high side of the building to drain subterranean water
around the house. Generally regrade if necessary to move water
away from the house. |
Figure 2. (A) Plain-sawed
boadrs have grain (the tree's growth rings) flate with the board.
Slight curves make wide boadrs cup slightly. (B) Quarter-sawed
boards have grain vertical to the boards wide dimension. Expansion
follows the grain direction about 4-1, making quartered boards
more stable. |
In crawl-space construction lay 6-mil polyethylene film over
the entire area of earth (or concrete or whatever surface exists),
weighted down with bricks to prevent its shifting in a breeze.
Be sure the entire area is ventilated on all walls and that vents
are open. If there are dead ventilation areas provide mechanical
means, either temporary or permanent, to circulate air. A humidistat-switched
automatic fan, with a tunnel to an outside vent, is one such
arrangement; coupling the fan to the air conditioning/furnace
fan is also a good arrangement.
In basement construction, the soil drainage solution applies,
but it may be necessary to dig out and waterproof the exterior
of basement walls, and install drainage near the bottom of footings
as well as intermittently up the side of the basement walls.
Because of the expense involved this should be considered only
as a last resort. Mechanical dehumidifiers in the basement plus
summertime ventilation may ease the problem enough to allow the
subfloor and surface hardwood to dry.
The next step is to allow the floor and all underfloor construction
to dry thoroughly. The process by which hardwood floors take
on moisture and expand takes many weeks, unless water in liquid
form has been in the picture. By the same token, its removal
may also take several weeks, or even months. Once a program of
drying has been set up, evidence that it is working can be seen
within a short time. Its progress should be monitored by taking
moisture readings on a bi-weekly or monthly basis, and no repairs
should be attempted until the readings have remained balanced
between face and back for 30 days to be sure that cupped floors
have flattened as far as they are going to. (Floors with a surface
finish react much more slowly to moisture changes.)
WHEN THE FLOOR RE-FLATTENS:
If cupped floors flatten when they have dried, new considerations
of a complete repair may arise:
Fastenings -- nail installation: The cupping action
may have loosened nails to some extent. If so, this will express
itself by squeaks or looseness when the floor is walked on. Face
nailing or fastening from underneath with wood screws will correct
the problem.
Adhesion -- mastic installation: Some types of wood floor
mastic have re-tack properties which will allow the mastic to
re-adhere even after the floor has been pulled loose and reset.
If walking on the floor produces popping sounds, or the floor
sounds "hollow" when tapped, adhesion has probably
been lost. In this event the affected parts of the floor must
be removed and replaced to accomplish an effective repair.
If an asphalt cut-back mastic was used originally, the dried-out
floor can be removed and replaced (a few pieces at a time), as
the mastic can be reactivated with a light spray of kerosene.
Headless pins of hardened steel can be used also where mastic
adhesion has been lost. These pins can be driven into either
wood or concrete subfloors.
Once fastenings are secure, the floor can be filled where cracks
exist, and either given a new coat of finish, after screening,
or buffed with No. 00 steel wool, cleaned, and re-waxed. If the
original finish was a surface type (i.e., Polyurethane), it can
be re-coated only if it has not been waxed.
WHEN THE FLOOR REMAINS CUPPED
after thoroughly drying it has most likely set new stresses and
most boards will remain cupped indefinitely. In this case the
only practical repair is a complete resanding and finishing job.
Cracks should be filled as a normal part of the finishing process
and fastenings checked and repaired before sanding. |
