Sub floor over concrete

Q. We had a small extension built for our home on a "slab foundation". We have now carpeted the area & find that the sub-floor the contractor laid is buckling up in spots (edges / corners). Not the entire floor but of course the main walk area. Besides the bumps in the floor -- should we be concerned about "mold" developing?

A. The buckling of the sub-floor is because it is swelling or expanding due to excessive moisture. If the slab doesn't have a vapor barrier under it then moisture vapor will travel from the ground, up through the slab, and into the sub-floor.

Having moisture on the surface of your slab and under the sub-floor, it is possible mold could develop.

If you could lift a small area of the flooring to see if the concrete slab is dark or moist, this would tell you if the moisture is coming from the slab and not just from the humidity in the air.

If the moisture is coming from the slab, you will have to remove the flooring and treat the slab with a moisture vapor blocker. This will slow or stop the transmission of moisture up through the slab.

You can find moisture vapor blockers at a concrete supply store in your area. They are sprayed on or rolled on similar to a sealer.