Dull surfaces such as new concrete disguise sub-floor surface imperfections. Shiny, reflective surfaces such as new vinyl or rubber flooring do the opposite.

According to National Floor Covering Association (NFCA) specifications, the minimum industry standard for sub-floor flatness (surface waviness) when installing resilient flooring is 3/16" over 10' for slab on grade and 1/8" over 10' for suspended slabs.

These images show an example of what finished flooring can look like when installation proceeds over a sub-floor surface that doesn't meet tolerance, in this case, 1/2" over 4' (using a 10' straight edge).

Construction budgets should include an allowance for effective grinding and patching work (not in the scope of work for the flooring contractor according to industry standards). Specifications should clearly direct responsibility for this work to the owner of the slab and/or construction manager, and conversation around expectations and scheduling for such work should be had well in advance of floor installation start date.

NFCA's third party independent inspection service, the 'Quality Assurance Program' (QAP), draws attention to such issues giving busy construction teams the chance to understand, plan and solve issues fairly and in advance of the all important deadline. The alternative is the same old same old disputes over extras, delays, poor quality, hold backs and dissatisfied clients.

Concrete Sub-Floor Preparation: Minor discrepancies in new or existing surfaces can be adjusted by using patching compounds. This is considered, within reason, part of the flooring contractor's work. Where flatness (surface waviness) discrepancies do not meet NFCA or floor manufacturers' tolerances (3/16” over 10’ using the straight edge test method), conditions must be corrected using a Hydraulic Cement Underlayment (HCU), the application of which shall be done by others (general contractor or owner) This may also be undertaken by the flooring contractor as a billable extra. Moisture testing of the parent concrete slab, using the preferred RH In-Situ Probe test method in accordance with ASTM F-2170, must take place prior to application. A surface temperature of more than 10c (50f ), or per manufacturers’ installation guidelines, must also be confirmed before proceeding.
To find NFCA members specializing in the supply of Hydraulic Cement Underlayment click here.