WATER SPOTS THROUGH SLAB

H.  WATER SPOTS THROUGH SLAB:
Moisture can wick through concrete slabs.  Damp spots (Fig. H1, item B) that are randomly located within carpeting laid over a slab may be due to wicking moisture.  Professional carpet installers will often refuse to install a carpet with such spots showing.  Slabs used for living spaces should be underlain with plastic vapor barriers to prevent water damage via transpiration up into the carpet.  It is common for these vapor barriers to get trampled or torn during the installation of the concrete.  If the water table is very close to a slab, it can readily rise through old wooden grade stakes (Fig. H1, Item A) that may have been used to set as reference points for leveling the slab.  Carpet stains often appear above these old stakes.  Old wooden grading stakes are ideal entry points for subterranean termites.

It is common for moisture to exist under slabs.  Often, poor surface drainage around slabs contributes to water beneath them.   Even though a house may not have a crawl space or a basement to seep water, it is still important to maintain proper drainage around its foundation.

WHAT TO:  HOW TO:
You can sometimes confirm that moisture is rising through a slab by securing a large piece of tin foil to the surface of the slab.  Seal the edges tightly with tape and return in several days to see if moisture is BENEATH the foil.  It is occasionally possible to use specialized moisture detecting devices to help localize the center or sources of water spots.

If external drainage improvements do not improve the situation, break through the slab at the suspicious spots to see what is actually underneath.  Drilling through concrete slabs is quick and easy with a hammer-drill.  If hollows are found, the water and loose material can be removed and then the void can be backfilled with gravel, a vapor barrier laid, and then the repair patched over with concrete.

In mild cases a workable “cure” can often be used by installing a plastic vapor barrier over the concrete floor, or by painting a concrete sealer on the concrete floor and reinstalling the carpeting.

When the suspected source of moisture is from faulty landscape or roof drainage correct the exterior drainage problems and allow sufficient time for the under slab areas to dry.  Proper drying can take months.  Be patient and proceed step by step.