Subfloor Preparation for Professionals: Getting It Right First Time
Subfloor preparation is the stage of floor installation that most determines whether the finished result is excellent or problematic. No matter how good the floor covering product or how skilled the installation, a poorly prepared subfloor will cause problems. For professional floor fitters, systematic subfloor assessment and preparation is the foundation on which everything else depends.
Concrete Subfloor Assessment
Assess a concrete subfloor for three things: flatness, moisture and surface condition. Flatness is checked using a long straightedge, typically 1.8 metres or 3 metres for commercial assessments. Mark any high spots (areas that raise the straightedge) and low spots (areas where the straightedge bridges and there is a gap beneath) across the whole floor area. Any deviation exceeding 3mm under a 1.8 metre straightedge requires remedial action before floor covering is installed.
Moisture testing, as covered in the moisture testing guide, must be completed and documented. Surface condition assessment involves checking for contamination (oil, grease, old adhesive, paint), surface laitance (the weak dusty layer that forms on top of concrete as it dries) and any cracking or structural movement that would cause problems after the floor covering is installed.
Grinding and Levelling Compounds
High spots on concrete are best removed by grinding using a disc grinder or rotary grinding machine with diamond grinding discs. For large areas, a walk-behind planetary grinder is significantly more efficient than a handheld grinder.
Low spots, dips and general unevenness are addressed with self-levelling compound. Mixing the compound correctly is critical: too much water produces a weak, porous result; too little makes the compound difficult to work and can cause surface cracking. A 30-litre forced-action mixer (not a drill-mounted paddle mixer) produces the most consistent results. Pour the mixed compound into the dips and allow it to self-level; guide the flow with a gauging trowel if needed to reach distant areas, and use a spiked roller to release any trapped air.
Timber Subfloor Preparation
For floating and glue-down installations on timber subfloors, check each board for squeak and movement. Fasten any loose boards with screws counter-sunk below the surface. Fill screw holes and any gaps wider than 3mm with a flexible filler before the floor covering is installed over them.
Check the overall flatness of the timber subfloor. Sheet materials (tongue-and-groove plywood or OSB) laid over existing floorboards improve flatness significantly and provide a more stable base for glue-down products. For very uneven timber subfloors, a floor-specific levelling compound with the appropriate primer can be applied over plywood or hardboard to achieve the required flatness.
Primers and DPM Application
Many levelling compounds and adhesives require the application of a compatible primer to the concrete subfloor before use. The primer seals the surface, reduces the absorbency of the concrete, and improves the bond of the compound or adhesive. Skipping the primer on a very porous or dusty concrete surface can cause the compound to dry too quickly, producing surface cracking or poor adhesion.
- Assess concrete for flatness, moisture and surface condition before any work
- Grind high spots; fill low spots with levelling compound
- Use forced-action mixer for levelling compound
- Fix loose boards on timber subfloors before any overlay
- Apply primer where required by the levelling compound or adhesive data sheet
- Document all preparation work with photographs for project records
Thorough subfloor preparation is investment in a problem-free installation. The time and material cost of proper preparation is small relative to the cost of addressing failures caused by a poorly prepared substrate. Professional floor fitters who take preparation seriously have fewer callbacks and fewer warranty claims.
