How Flat Should a Subfloor Be? Use the Selected System’s Tolerance

A uniformly sloped plane can be flat without being level. A nominally level room can still have local humps and depressions. Use the straightedge span and tolerance specified by the selected product or installation system. This guide does not invent one numerical tolerance for every material.

Map high and low areas and note substrate type, stability, cracks and moisture conditions. Different tile sizes, flooring systems and underlayments can have different preparation requirements. Check the actual applicable technical documents.

If a low area needs leveling, the compound quantity uses area and average required fill thickness. A deepest point of 1/2 in does not mean the entire floor needs 1/2 in. Divide varying areas into zones or establish an average from a measured profile.

For CUSTOM LevelQuik RS’s verified low coverage reference of 46 ft² per 50 lb bag at 1/8 in, proportional scaling gives 23 ft² at 1/4 in. A 100 ft² area at that average depth needs ceil(100 / 23) = five bags before a separate allowance. Use only where the product’s application permits the scaling.

Product minimum/maximum thickness, primer, substrate preparation, aggregate extension and curing matter. Grinding high spots and filling low areas are different scopes. Do not treat self-leveling material as a fix for an unstable or unsuitable substrate. The quantity calculator checks entered thickness limits, but the actual product and project requirements govern the work.