Preparation of Concrete Subfloors

Subfloors & Underlayments

The surface of a concrete subfloor must be dry, clean, smooth and structurally sound. It must also be free of depressions, scale or foreign deposits of any kind.

The surface shall be free of dust, solvents, varnish, paint, wax, oil, grease, residual adhesive, adhesive removers and other foreign materials that might affect the adhesion of flooring to the concrete. If these contaminants are present on the substrate they must be mechanically removed prior to the installation of the flooring material

Many buildings built before 1978 contain lead-based paint, which can pose a health hazard if not handled properly. State and federal regulations govern activities that disturb lead-based painted surfaces and may also require notice to building occupants. Do not remove or sand lead-based paint without consulting a qualified lead professional for guidance on lead-based paint testing and safety precautions. For non-lead-based paint, a good paint remover for many concrete subfloors is a solution of Trisodium Phosphate and hot water, mixed and applied according to the manufacturer’s instructions and recommended safety precautions. Paints with a chlorinated rubber or resin base that cannot be removed by Trisodium Phosphate may be removed by grinding with a concrete or terrazzo grinder. AHF Products does not recommend the use of solvents to remove paints or old adhesive residues because the solvents can remain in the concrete and negatively affect the new installation. Whenever sanding, be certain the work site is well ventilated and avoid breathing dust. If high dust levels are anticipated, use appropriate NIOSH designated dust respirator. All power sanding tools must be equipped with dust collectors. Avoid contact with skin or eyes. Wear gloves and eye protection, and long-sleeve loose fitting clothes.

After the concrete has cured and is dry, clean construction joints, saw cuts, score marks and cracks, and fill with an appropriate cementitious underlayment. Repaired areas must be finished flush with the surface of the concrete and allowed to fully dry before the installation of the floor covering.

Actual expansion joints or other moving joints with elastomeric fillers are designed to absorb movement in concrete slabs. Cementitious underlayments, patches and flooring installed across expansion joints often crack or buckle when the slabs move. AHF Products does not recommend flooring products be installed across expansion joints or isolation joints. Expansion joint covers are available for use with various floor coverings and should be specified by the architect.

Dusty concrete slabs may be primed. Sweep or vacuum the concrete and apply the primer with as recommended. After drying, install the flooring in accordance with the recommended installation system. NOTE: A dusty concrete floor on-grade or below-grade may be a sign of alkali salts.

A rough concrete floor can be ground smooth with a commercial diamond or carbide-equipped grinding machine. If the concrete subfloor is extremely rough or uneven, it may be too great a job to smooth this way. In this case, apply a cementitious underlayment. A smooth, flat, uniform surface is necessary as a good base for flooring.

Updated on August 15, 2022