Substrate Smoothness (Flatness)
Concrete slabs change shape, curl, shrink and sag between the time they are poured and months later when the floor covering contractor is scheduled to start work.
Floor covering product manufacturers and minimum industry standards require a surface flatness of 1/8" (3mm) over 10' (3m) for elevated slabs and 3/16" (4.5mm) over 10' (3m) for slabs on grade using the straight edge test method.
The General Contractor shall provide substrate (concrete or wooden) surface level and smooth (flat) conditions acceptable for floor covering materials. This shall include grinding or sanding of ridges, undulations, projections, and areas of carbonation and scaling and filling and levelling of expansion joints, cracks, grooves and other irregularities. Where patching or levelling is required, the use of a Latex base-filling compound is recommended.
There are four main reasons for these strict tolerances.
The General Contractor shall provide substrate (concrete or wooden) surface level and smooth (flat) conditions acceptable for floor covering materials. This shall include grinding or sanding of ridges, undulations, projections, and areas of carbonation and scaling and filling and levelling of expansion joints, cracks, grooves and other irregularities. Where patching or levelling is required, the use of a Latex base-filling compound is recommended.
There are four main reasons for these strict tolerances.
Trip hazards
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Rolling loads
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Maintenance wear and tear
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Visual issues
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Substrate Preparation
Loose, Hydraulic Cement Underlayment
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Mechanical preparation
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New Hydraulic Cement Underlayment
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The following excerpt from PART A12 Substrate Preparation of the NFCA Floor Covering Reference Manual specifies related work responsibilities between the General Contractor and the Floor Covering Contractor:
- The General Contractor shall (except as noted below) provide a substrate surface acceptable for the installation of specified flooring materials. Unless otherwise defined by local trade jurisdiction, or agreed to between the General Contractor and the flooring contractor prior to commencement of the Work, the General Contractor will be responsible for:
- Provision of substrate surfaces within smoothness (flatness) tolerances defined under Part A10 - Acceptable Conditions.
- Filling and levelling all substrate surfaces including control and construction joints, structural cracks, grooves, gaps, and other irregularities.
- Grinding smooth all ridges, undulations, projections and areas of carbonation and scaling.
- Correcting low density or powdery concrete surfaces in order to provide substrate surfaces that are acceptable for the installation of flooring materials.
- Major patching or installation of an underlayment using products that are suitable for substrate surfaces, that are compatible with flooring materials to be used, and that are in accordance with NFCA requirements. Refer to Part A12 - Substrate Preparation.
- Note: Where the contractual relationship is between an Owner and the flooring contractor the contractor shall be responsible for providing surfaces acceptable for the installation of flooring including those items listed above.
- The flooring contractor shall be responsible only for minor substrate preparation that includes filling of small chips and dents, removal of minor protrusions and vacuuming of an otherwise acceptable surface in accordance with NFCA requirements and as defined by local trade jurisdiction requirements. General Contractor or Owner shall include for the additional substrate preparation work (shot-blasting, grinding, levelling, skim coating, crack filling etc.) as required to meet NFCA and manufacturer requirements.