Q.: In South Florida we have a recurrent problem with moisture on top of the slabs in houses and warehouses. We have devised a simple test that consists of using paper towels to dry up the water on top of the slab, then chipping off some concrete to expose material below the finished surface. We always find the concrete to be dry and that the dust from the chipping is dry when rubbed between our fingers.

We have also done another kind of test. We drill cores from the slab and sample the fill under the slab as well as the fill outside the house. Our results always indicate the moisture content of the fill under the slab to be lower than the fill outside the house by several percentage points.

We are sure that this is water that has condensed on the slabs from the air when the humidity is high (79 to 85 percent in spring and summer). We are convinced that it is not water that has percolated through the slab, but we have trouble convincing our customers. Can you publish something regarding this problem and its solution?

A.: We agree that your tests do show that the moisture is coming from the air, not from the subgrade.

There is another possible test that ought to be convincing. If you were to lay a piece of sheet metal, about two or three feet square, on the surface of the concrete and tape down the edges securely and leave it overnight, moisture should condense on the top surface of the metal if the air is cool and humid. When you then remove the tape and look at the underside of the sheet metal there should be no moisture on the bottom. This should convince the observer that the moisture is coming from above and not from below, since moisture would not go through sheet metal.

Under some circumstances you could get a small amount of condensation on the bottom of the sheet metal also, provided the slab had not yet dried out thoroughly and moisture were still coming from below. If so, you would have to agree that there is still moisture in the slab. In many cases, however, the top could be very moist while the bottom contains little or no moisture.