Subfloor Preparation: The Most Important Step in Any Flooring Project

Ask any experienced flooring fitter what causes most installation failures and the answer is almost always the same: poor subfloor preparation. Whether you're laying luxury vinyl tiles, solid hardwood, or laminate, the performance and longevity of your new floor depends entirely on what's underneath it. This subfloor preparation guide homeowners can rely on covers everything you need to know — from moisture testing concrete to levelling uneven timber — before a single plank goes down.

Why Subfloor Preparation Cannot Be Skipped

New flooring laid over a defective subfloor will fail. It's not a question of if — it's when. Boards will creak, click systems will separate at the joints, adhesive bonds will break down, and moisture damage can cause warping within months. These failures are rarely covered under product warranties, because manufacturers rightly expect the subfloor to meet minimum standards before installation begins.

The cost of doing it properly upfront is a fraction of the cost of lifting and re-laying a failed floor. Spend the time here and everything else follows far more easily.

Concrete Subfloors: Moisture Is Your Biggest Enemy

Concrete is common in modern homes, extensions, and ground floors. It looks solid and ready, but it holds moisture — and that moisture will destroy many types of flooring if not properly managed.

How to test for moisture:

• Electronic relative humidity (RH) meter: The most practical option for DIYers. Drill into the slab to the correct depth, insert the probe, and allow it to equilibrate. Readings are taken as a percentage of relative humidity.
• Calcium chloride test: A sealed dish of calcium chloride is left on the surface for 60–72 hours, then weighed. This measures moisture vapour emission rate (MVER). More commonly used by trade fitters.
• Plastic sheet test: Tape a sheet of polythene to the slab for 24–48 hours. Condensation underneath suggests moisture is present. This is indicative only — not a reliable substitute for proper testing.

Maximum acceptable moisture levels by floor type:

• Solid wood and engineered wood: 75% RH maximum
• Luxury vinyl tile (LVT) and rigid core flooring: 85% RH maximum
• Ceramic and porcelain tile: generally tolerant of higher moisture, but check adhesive manufacturer guidance
• Laminate: typically 75% RH maximum

If your concrete slab exceeds these thresholds, you must apply a damp proof membrane (DPM) before proceeding.

Applying a Damp Proof Membrane (DPM)

A DPM is a liquid-applied resin that forms an impermeable barrier over a concrete slab. It prevents moisture vapour rising through the floor covering and causing damage. Most products are applied by roller in two coats at right angles to each other, with a drying period between coats.

Key points when applying DPM:

• The surface must be clean, free from dust, grease, and loose material before application
• Allow each coat to fully cure before applying the next — typically 3–6 hours depending on conditions
• Pay close attention to edges, where moisture can track up from the perimeter
• Once cured, a DPM-treated slab can accept self-levelling compound, adhesive, or direct overlay

Do not assume a DPM removes the need for moisture testing — it doesn't. You test first, then decide whether a DPM is required.

Timber Subfloors: Fix Problems Before You Floor Over Them

Suspended timber subfloors — common in older properties, particularly those built before the 1970s — present a different set of challenges. The issues to address are movement, noise, and level.

Fixing squeaky and loose boards:

Squeaking is caused by boards moving against each other or against the joists beneath. Nails work loose over time — the fix is to use screws. Drive 50–60mm countersunk wood screws through the floorboard and into the joist below. This pulls the board tight and eliminates movement. Do not simply nail over the top of existing nails — it rarely solves the problem long term.

Walk the entire floor systematically before any new covering goes down. Every squeak and flex needs addressing. A floor that moves underfoot will transmit that movement directly to whatever is laid on top.

Checking for rot and structural damage:

Probe any boards that look discoloured or feel soft underfoot. Rot — particularly around external walls and near pipework — must be replaced, not covered over. If in doubt, lift a board and inspect the joists.

Getting the Level Right

This is where many DIY installations go wrong. Flooring manufacturers specify maximum tolerances for flatness, and the standard rule across most product types is no more than 3mm deviation over 1.8 metres. This is measured by placing a straight edge or spirit level across the surface and checking for gaps beneath it.

On timber subfloors, high spots can often be sanded down. Low spots and larger undulations require a plywood overlay or self-levelling compound.

Plywood overlay for timber subfloors:

• Use a minimum of 6mm exterior-grade plywood for LVT; 12mm for solid or engineered wood
• Stagger the sheets so joints do not align — this prevents ridges telegraphing through the finished floor
• Fix with screws at 150–200mm centres across the sheet and 100mm around the edges
• Leave a 2–3mm expansion gap between sheets to allow for movement
• Feather the joints with a floor-levelling compound once fixed

Self-Levelling Compound: When and How to Use It

Self-levelling compound (also called floor leveller or smoothing compound) is used to correct surface irregularities on both concrete and timber subfloors. It flows to fill low spots and dips, creating a flat, smooth base for the new floor covering.

When to use it:

• Concrete slabs with surface imperfections or previous adhesive residue
• Over a plywood overlay on timber floors where joints need feathering
• Any area where the 3mm in 1.8m tolerance cannot otherwise be met

How to apply it correctly:

• Prime the substrate first — skipping the primer causes the compound to dry too quickly and crack
• Mix to the manufacturer's specified ratio — too much water weakens the finished surface
• Pour and spread with a gauge rake, working quickly as most compounds have a pot life of around 20 minutes
• Use a spiked roller to release trapped air bubbles
• Allow to fully cure before installing the floor covering

Typical drying times for self-levelling compound:

• Light foot traffic: 2–4 hours
• LVT or laminate installation: 24 hours minimum
• Adhesive-fixed wood flooring: 24–48 hours, or until moisture readings are within tolerance

Temperature affects drying significantly. Below 10°C, curing slows considerably. Do not apply in cold or damp conditions unless using a product specifically formulated for low temperatures.

Minimum Subfloor Requirements by Floor Type