Concrete subfloor

Part I – Concrete Subfloor Specifications

  • Subfloor Must Be Flat
    • Make sure the concrete slab is flat to a flatness tolerance of 1/8” to 3/16” in a 10- foot radius.
    • If the slab is out of specification, consider grinding, floating or both. Many high spots can be removed by grinding, depressions can be filled with approved patching compounds, and slabs also can be flattened using a self-leveling concrete product.
    • When sanding or grinding concrete, care must be taken to minimize the amount of silica dust produced. OSHA recomm ends using dust-collection devices, or applying water to the concrete before sanding. Approved respirators may also be used to minimize the amount of silica dust inhaled.
  • Subfloor Must Be Dry
    • Refer to Moisture Requirements and Moisture Testing.
    • Concrete moisture meters and other tests can be useful in identifying moisture problem areas. However, NWFA guidelines specify using relative-humidity testing (ASTM F-2170), calcium chloride testing (ASTM F-1869) or calcium carbide (CM) testing (ASTM D-4944-04 and MilSpec CRD-C154-77) to identify the moisture content of the slab.
    • If a slab tests too high in vapor emission to glue a floor down, consider using a vapor retarder type product, installing a vapor retarder and a plywood sub-floor or using an alternative installation method.
    • Moisture readings should not exceed 4.5 when using a Tramex concrete moisture encounter.
    • Concrete slabs with a calcium chloride reading of more than 3 require use of a vapor retarder with a perm rating of 1 or less. It is strongly recommended to use an impermeable vapor retarder with a perm rating of 0.13 or less, such as 6 mil polyethylene film.
  • Slab Must Be
    • Minimum 3000 PSI
    • Free from non-compatible sealers, waxes, and oil, paint, drywall compound, etc.
      - Check for the presence of sealers by applying drops of water to the slab, if the water beads up, there may be sealers or oils.
  • Do not attempt to glue a wood floor over a chalky or soft concrete slab.
  • Burnished, slick steel-troweled slabs may require screening with a 30-grit abrasive.
  • Specifications for Lightweight Concrete
    • Make sure the concrete is well bonded to the sub-floor. Check for hollow spots, cracks and loose areas.
    • As with on-grade concrete sub-floors, make sure the concrete is clean, flat to specification, and dry.
    • Over lightweight concrete (less than 3000 PSI), if the flooring adhesive used has a higher shear strength than the concrete, use the Floated Subfloor installation method. If the PSI of the concrete is unknown, use the Floated Subfloor installation method or contact the adhesive manufacturer.
    • Rule of thumb: Draw a nail across the top; if it leaves an indentation, it is probably lightweight concrete.