Subfloor preparation: the half of the job you never see

The tolerances your floor needs

Different floor coverings tolerate different amounts of unevenness. Glue-down LVT is the least forgiving: the industry standard is no more than 3mm deviation under a 2-metre straightedge, because a thin, flexible plank conforms to whatever is underneath it and every ridge shows in raking light.

Rigid click products such as SPC tolerate around 5mm over 2 metres. Carpet over a good underlay is the most forgiving, hiding minor undulation entirely, though a sharp step or a proud nail will still show through and eventually wear the backing.

The practical test is simple and worth doing yourself before you get quotes: lay a long spirit level or straight timber across the floor in several directions and look for daylight underneath. If you can slide a pound coin under it anywhere, the floor will need attention.

Concrete subfloors: levelling and damp

Old concrete in South East London houses is rarely flat. Kitchen extensions, removed hearths, patched service runs and decades of settlement leave dips and humps. The usual remedy is a latex or cement-based self-levelling compound, applied over a primer, which flows out to a smooth plane and cures in a few hours.

Levelling compound is not a general-purpose fix. Deep hollows need filling in stages or with a repair mortar first; a thick single pour cracks. Old bitumen adhesive residue needs a specific bitumen-tolerant primer, because standard latex will not bond to it and will delaminate later.

Damp is the other concrete issue. Ground-floor slabs in properties built before the 1960s frequently have no damp-proof membrane, or a failed one. Before any impermeable floor goes down, the slab should be moisture-tested with a hygrometer. If readings are above 75% relative humidity, a liquid DPM is applied before levelling — otherwise the moisture is trapped under the new floor and the adhesive eventually fails.

Timber subfloors: ply overlay and squeaks

Suspended timber floors present the opposite problem: not undulation but movement and gaps. Original floorboards flex between joists, cup at the edges, and the joint lines telegraph through thin floor coverings within months.

The standard solution is a plywood overlay — typically 6mm WBP ply for LVT, screwed at 150mm centres across the whole sheet, with joints staggered and edges sanded. Nailing is not adequate; nails work loose and cause squeaks. On a floor with significant movement, 12mm ply may be needed.

Before overlaying, the boards themselves should be secured. Screwing loose boards down to the joists removes squeaks permanently, whereas overlaying a squeaking floor simply hides the noise source under a new floor you would have to remove to fix it.

• Glue-down LVT: 3mm over 2m, primed and level, usually latex or 6mm ply
• Click SPC/laminate: 5mm over 2m, foam underlay acceptable
• Carpet: most forgiving, but fix proud nails and sharp steps first
• Any impermeable floor on ground-floor concrete: moisture test first

What preparation costs

Latex self-levelling typically adds £10–£18 per square metre, including primer and labour, and more where the depth is significant. A 6mm plywood overlay runs £14–£22 per square metre supplied and fitted. A liquid damp-proof membrane adds £8–£14.

Lifting and disposing of old flooring adds £3–£6 per square metre plus tip charges, and old adhesive removal can be considerably more if it is stubborn or contains bitumen.

These figures are why two quotes for the same room can differ by several hundred pounds. The cheaper one has usually assumed the subfloor is fine. Ask directly: 'What preparation is included, and what happens if the subfloor needs more?'

Warning signs to check before you order flooring

Springy or bouncy floors indicate joist or board problems that must be resolved structurally, not covered up. A new rigid floor over a springy subfloor will develop joint failures.

A damp, musty smell at floor level, or dark patches and salt bloom on concrete, means moisture that will need addressing before anything impermeable goes down.

Existing tiles that sound hollow when tapped are debonded and will move under a new floor. They need lifting, not overlaying.

Finally, check the finished floor height. Preparation adds thickness — ply plus LVT can be 8–10mm, and levelling can add more. Doors, thresholds, kitchen plinths, radiator pipes and stair bottom risers all need checking against the new height before work starts, not on fitting day.