Subfloor preparation and floor levelling

Nobody has ever been excited about a levelling compound. But almost every flooring complaint we are called to look at traces back to a substrate that was not flat enough, not dry enough, or not stable enough — and to a quote that priced the pretty part and hoped the rest would be fine.

So we assess the subfloor before we quote the floor, and we price preparation as its own line. If it turns out to need nothing, you pay for nothing.

What gets checked

Three things decide whether a substrate is ready.

The assessment

• Flatness — checked against a straight edge across the whole area, not just where it looks suspect. Every flooring product specifies a tolerance, and wide boards and herringbone are far less forgiving than narrow straight-lay.
• Moisture — concrete slabs tested before any direct-stick work; timber and particleboard read with a resistance meter, corrected for species.
• Soundness and movement — drummy tiles, loose particleboard, deflecting joists, and old adhesive residue all have to be dealt with before anything goes over them.

What preparation typically involves

Depending on what the assessment finds: grinding back high spots and adhesive residue, self-levelling compound over hollows, a liquid-applied moisture barrier where slab moisture is high, plywood overlay to create a stable, screwed-down substrate, or re-fixing and packing an existing timber subfloor.

On suspended floors we also look at what is underneath — ground clearance, ventilation, and whether there is a vapour barrier over the ground. In a lot of older Sydney homes, improving subfloor ventilation is the single most valuable thing that can be done for the floor above.

At a glance