Sweating Slab or Plumbing Leaks

[Skip Erwin]

This vinyl flooring on a slab has these black stains on it at the foyer and in the kitchen. They look just like the kind of stains you find on the vinyl flooring around leaking toilets. They measure above 20% moisture. I talked to my plumber friend and he said these might not indicate leaks. The slab could be sweating. What are your thoughts? Should I refer this to a plumber or a general contractor? And, how are they going to tell if it's sweating or plumbing leaks without causing damage to the floor? Thanks in advance.

[Marcel R. Cyr]

Never seen this.

If you think it is plumbing or radiant heat pipes, pressure test the pipes.

If you want to check for moisture, there are a couple of options.

Following are several industry-recognized concrete slab moisture tests that provide measurement of emission rate:

1. Polyethylene Film Test
   Tape a 2' x 2' square of 6 mil. clear polyethylene film flat to the slab, sealing all edges with moisture resistant tape. Suspend a 250-375 watt heat lamp 2' above the plastic film. After 24 hours, check the film -- if no condensation or "clouding" develops on the underside of the polyethylene, the test area shall be considered dry enough to install wood flooring.
2. Phenolphthalein Test
   Apply several drops of a 3 percent Phenolphthalein solution in grain alcohol at various spots on the slab. If a red color develops in a few minutes, the slab is too wet to begin installation of the finished flooring system.
3. Calcium Chloride Test
   Use a prepackaged calcium chloride test (widely available) and follow the manufacturer's instructions. The Calcium Chloride Test measures the quantity of moisture passing through a concrete floor. This measurement is stated as pounds of moisture over a 1,000 sq. ft. area during a 24-hour period. An acceptable level is 4.5 lbs. or less.

Wrong adhesive bleed through, might be another possiblity.

Hope this helps.

Marcel