Concrete floors always emit moisture. Each flooring manufacturer allows a
specific level; follow their guidelines.
The test at right gives you a rough idea of the moisture emitted from your
slab. Generally, if the plastic is dry, moisture is within acceptable
levels. Always lay clown a vapor barrier—it's required over concrete. Then
lay underlayment and install laminate. If, however, the plastic is wet or
the floor is damp, moisture levels may be in the unacceptable range. Call a
professional for more specific tests and solutions.
Do a thorough outdoor site evaluation to make sure that moisture is not
leaking in. Check for crooked gutters and poorly positioned downspouts.
Taping down the test plastic
Cut a few 2-foot-square pieces of polyethylene. Duct-tape them to
various areas of the subfloor. Wait about 72 hours.
Reading the results
Lift up a corner of each test square. Beads of condensation on the
underside of any of them, or a dark, moist subfloor indicate a moisture
problem. Hire a professional for further testing and solutions.
Moisture can be a particular problem on floors above crawl spaces.
Each laminate floor manufacturer specifies a certain ratio of
ventilation per square foot of crawl space. Check that your area
meets or exceeds it. If there is a moisture problem and no obvious
way to remedy it, have a professional come in to help.
Some manufacturers also require a vapor barrier to be laid on the
ground of the crawl space. Even if it's not required, it's a good
idea. Install one before moving on.