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Moisture Problems Plague Flooring from Concrete Slabs on Grade


September 11, 2012


Water or water vapor rising out of a concrete slab on grade can cause adhesion loss, warping, peeling and an unacceptable appearance of resilient floor coverings. Beneath seamed, continuous flooring, moisture can also cause adhesive deterioration, ripping or separation of seams and air bubbles or efflorescence deposits. Other harmful effects include damage to flat electrical cable systems, buckling of carpet or carpet tiles, odors, and fungi growth.

Excessive moisture in concrete slabs on grade usually originate from one or more of three sources:

  1. Residual water from the construction process in or below the slab.
  2. Naturally occurring ground water from a permanent or seasonal high water table
  3. Water from irrigation systems, broken plumbing pipes or other recurring man-made sources

A slab may appear dry but actually have a detrimental level of moisture passing through it. Moisture passing through the slab can carry damaging alkaline salts from the ground or concrete itself. The time required for slabs to dry out sufficiently so that floor coverings and finishes are not adversely affected ranges from six weeks to six months. To receive resilient flooring, concrete slabs must be dry enough to permit the adhesives to bond property. Asphalt and vinyl composition tile may be applied on concrete slabs that are in direct contact with the earth. Vinyl and rubber tile and sheet vinyl products should not be used under these conditions. Sheet vinyl with a moisture-resistant backing and vinyl composition, rubber and asphalt tile may be applied, however on slabs that are in contact with the earth and waterproofed with membrane.

Slabs placed on earth are never really dry unless a membrane waterproofing layer is installed beneath them. Therefore, some resilient flooring manufacturers limit the products that may be used without a waterproofing membrane (to asphalt and vinyl composition tile). Even those materials should not be installed until at least three months’ after the concrete is placed, and even then, not until the slab is tested and proven to comply with the finish product’s manufacturer’s requirements.

Grading specifications must require the contractor to keep any below-grade excavations free of water. This is important to avoid potential reservoirs of moisture that can migrate upward through slabs, to prevent structural degradation of soil bearing strengths, and to avoid swelling and subsequent shrinkage if soils contain excessive moisture characteristics. Vapor retarders, the last defense against sub-slab moisture, are a necessary but problematic component of slab-on-grade moisture problems are often traced to punctured vapor retarders. Vapor retarders must be able to withstand the rigors of construction traffic between the time they are installed until the slab has been poured. They must also maintain long-term integrity by resisting attach by moisture, chemicals and organisms in the soil.

During the floor covering selection and specification process, identify all floor covering products to be in contact with slabs on grade. Contact manufacturers or pertinent trade associations to determine the substrate moisture conditions required for the acceptable performance of these products. Perform bond and moisture tests on the concrete as recommended by the finish manufacturer to determine if the surfaces are sufficiently dry and free from harmful substances.

Due to excessive moisture transmission, it may be necessary to consider remedial measures after construction is complete. One method is to eliminate excessive sub-slab water sources resulting from naturally occurring ground water or irrigation by retrofitting subdrains at the building perimeter. A second alternative is to reduce the rate of moisture emission from concrete slabs-on-grade by utilizing those products whose manufacturers claim can stop or limit moisture movement out of the slab. A third possibility is changing the floor finish to a material that is vapor permeable and not adversely affected by moisture.

Water and water vapor accumulation in concrete slabs may not seem like a big problem until carpets begin to buckle, vinyl flooring discolors, flat electrical cable systems begin to malfunction, or odors and fungi growth occur. Fortunately, by taking the proper steps, these unpleasant conditions can be avoided.

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