Concrete Floors and Moisture

New and existing concrete subfloors must meet the requirements of the latest edition of ASTM F 710, “Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring,” available from the American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428; 610/832-9500;

Note: Regardless of the type of concrete or other cement-like material used as a base for resilient flooring, in the event of underlayment failure, the responsibility for warranties and/or performance guarantees rests with the concrete or cement-like material manufacturer and not with the manufacturer of resilient flooring.

Concrete Floors and Moisture
Any concrete subfloor can be a source of moisture-related flooring failures, including above-grade concrete floors. By its very nature, concrete starts as a water-saturated mass which must cure and then dry sufficiently to allow the installation of flooring. Above-grade floors normally have only the mix water to contend with although rain, spills and water leaks can add more water. Roughly one-half of the mix water is consumed by hydration of the cement during the curing period, with the rest being slowly reduced by evaporation. Once dry enough for installation, there is little chance of future moisture related problems on above-grade concrete slabs. Concrete on-ground, or below-ground floors, have not only the mix water to consume and dissipate, they also have a potentially inexhaustible source of moisture from the ground. When covered with resilient flooring, a slab that is directly on sub-grade soil will become approximately as moist as the soil on which it is placed.

To reduce this ingress of moisture, a well-designed floor system will have a capillary break and an effective and intact moisture vapor retarder in place. Slabs on and below-grade can be affected by both water vapor and capillary rise. Below-grade slabs are closer to the water table, have poorer ventilation for drying, and have the added risk of hydrostatic pressure. On-ground concrete slabs and below grade slabs must have an effective and functional vapor retarder directly beneath the concrete to prevent ingress of moisture from the sub-base and sub-grade soil.

Resilient flooring products, whether sheet, plank or tile, function as moisture vapor retarders on top of the floor slab; if more moisture is rising from beneath the concrete than can be accommodated by the flooring and adhesive, then failure of the installation is inevitable.

Too much ground moisture can create problems for on-grade and below-grade areas of commercial and residential buildings over and beyond those relating to the installation and use of resilient flooring. These problems vary from merely slight, but unpleasant dampness to actual structural damage. Moisture near the surface of as concrete slab varies as the weather changes and moisture within the slab usually approximates the dampness of the subsoil.

Note: Water/cement ratio is the most important factor regarding moisture migration, permeability and rate of drying of a concrete slab. Water/cement ratios as low as 0.45 to 0.5 are practical and recommended by the concrete construction industry for slabs to receive resilient flooring. A water-cement ratio of 0.5 is an achievable and reasonable requirement for slabs on- or below-grade. Significantly higher water-cement ratios may lead to slower drying and problems with moisture movement through the slab, causing flooring failures.

Updated on September 20, 2022