Subfloor preparation: the part of a wood floor you never see

Most wood floor failures start underneath. What a proper subfloor survey checks, how we prepare concrete, screed and old floorboards in London homes, and why it's in the quote.

Why the bit underneath decides everything

A wood floor only ever does what the surface under it lets it do. If the subfloor is damp, the boards cup. If it's uneven, the joints open and the floor creaks or bounces. If it's dusty or weak, the adhesive bonds to the dust instead of the slab and the floor lifts in a year. Almost every failed floor we're called to look at in London was fitted on a subfloor that was never properly checked.

That's why subfloor preparation is a line in every BAJ quote rather than a surprise on the day. We'd rather tell you what your floor needs before we order the boards.

What we check at the survey

A survey takes about forty-five minutes and most of it is spent looking down. We're checking four things: moisture, flatness, strength and height.

• Moisture: a hygrometer reading on concrete or screed. Above 75% relative humidity, timber can't go down without a damp-proof membrane or more drying time.
• Flatness: a 2-metre straightedge. Engineered boards want no more than 3mm deviation under it; herringbone and chevron want better than that.
• Strength: we walk it, tap it and probe it. Soft screed, loose boards and hollow patches get flagged.
• Height: how thick the floor build-up can be before doors, thresholds, stairs and kitchen plinths become a problem.

Concrete and screed

New screed in a London extension looks ready long before it is. As a rule it dries at about a millimetre a day per millimetre of thickness, so a 65mm screed wants nine to ten weeks before a moisture test will pass. We test rather than guess. If it's borderline, a liquid damp-proof membrane lets the job go ahead safely.

Flatness is sorted with a self-levelling compound: primed, poured, and left overnight. High spots are ground off. Cracks are stitched before anything goes over them. Old concrete in a Victorian basement usually needs a membrane, a leveller and sometimes a plywood overlay before it's a surface worth gluing a floor to.

Timber joists and old floorboards

Most London terraces have original pine boards on joists. They're rarely flat and often loose. We screw down anything that moves, replace rotten boards, then overlay the lot with 6mm or 12mm plywood fixed at close centres. The ply gives a flat, continuous surface to bond to and stops the old boards telegraphing through the new floor.

If the original boards are worth keeping as the finished floor, that's a different job, and a good one: sanding and refinishing original pine or restoring a parquet. We'll tell you which makes sense in your house.

Underfloor heating and sound

Over wet underfloor heating, the screed must be commissioned (heated up slowly and cooled down) before we fit, so the moisture is driven off and the slab has moved as much as it's going to. We ask for the commissioning record. Over electric systems in a flat, the mat is encased in leveller first so there's nothing to snag.

In flats, most leases ask for an acoustic layer under hard floors. We use a bonded acoustic underlay or a resilient layer that meets the usual Part E figures and still lets the floor be glued, so you get the sound reduction without a bouncy floating floor.

Why it's worth paying for

Preparation is usually the least visible and most variable part of a flooring quote. A flat, dry screed in a new-build might need nothing but a prime; a Victorian ground floor with bouncy boards and a damp bay might be a day of work before a single board goes down. Either way the cost of doing it is a fraction of the cost of lifting a failed floor.

If a quote you've been given doesn't mention the subfloor at all, ask what they plan to do about it. Ours does, every time, with a 10-year workmanship guarantee that includes the preparation.

Questions

Updated 2026-10 · 6 min read · by the BAJ Flooring family