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Q.When installing vinyl flooring on a below-grade slab, how do I check for excessive moisture in the slab?

A.Jim Hamrick responds: There are two "subjective" tests that can be used to check for the presence of excessive moisture levels in a slab on grade.

The simpler is the "Rubber Sheet/ Plastic Mat Test." Using duct tape, carefully tape the edges of a 2x2-foot piece of 4-mil poly to the slab for 72 hours. If you observe any moisture droplets or surface color darkening of the slab, excessive moisture is present.

A more definitive approach is the "Bond and Moisture Test." Install a 3x3-foot piece of the actual flooring material per specs, using the proper adhesive. Seal the perimeter of the test area tightly to the floor with duct tape and allow it to cure for at least 72 hours. Remove the tape, and pry the flooring from the slab. You have a moisture problem if the tile comes up easily, the adhesive appears damp and stringy, or the adhesive releases from the concrete and stays on the flooring material.

A more scientific test is the "Calcium Chloride Test," where moisture migration rates are measured using a petri dish covered by a plastic dome. If this test is to be used, contact the flooring manufacturer for the range of acceptable values.

A good reference is the Armstrong Engineered Installation System manual, available from the Armstrong company (Armstrong Installation Products Division, P.O. Box 3001, Lancaster, PA 17604; 717/397-0611).

Jim Hamrick is an architect in York, Pa., and an instructor at Stevens State School of Technology in Lancaster, Pa.