Subfloor preparation: why it matters before any new floor goes down

New flooring is only ever as good as what it's fitted onto, and a large share of flooring problems that show up months after installation — boards lifting, gaps opening up, adhesive failing, a persistent musty smell — trace back to a subfloor that wasn't properly prepared or tested before the new floor went down. It's a less visible part of the job than the flooring itself, which is exactly why it's worth understanding before work starts.

Moisture is the single biggest hidden risk with a concrete or screed subfloor, since concrete retains water for a long time after it's laid and doesn't necessarily look or feel damp even when it isn't dry enough underneath. A moisture meter reading above roughly 75% relative humidity on a concrete subfloor is commonly cited as the threshold at which a damp proof membrane, or DPM, is needed before flooring goes down; more specifically, residual moisture above around 65% RH is commonly flagged as a concern for wood flooring, and above around 75% RH for resilient coverings such as vinyl or LVT. Newly laid or recently screeded concrete is a particular risk here — it's commonly recommended to leave new concrete to cure for at least around two months before flooring is installed on top of it, since fresh concrete retains significant moisture that needs time to escape naturally.

A damp proof membrane is essentially a waterproof barrier placed between the concrete or screed subfloor and the new flooring, designed to stop moisture migrating upward into the floor above. This can take the form of a polythene sheet — commonly specified at a minimum of 1200 gauge under British Standard guidance — or a liquid-applied membrane painted directly onto the prepared concrete surface, depending on the situation and the flooring being installed. Either way, the purpose is the same: to break the path moisture would otherwise take from a subfloor that isn't fully dry into a new floor that could be damaged by it.

Moisture aside, a subfloor also needs to be structurally sound and physically clean before new flooring goes on top. Dust, debris, grease and any old adhesive left over from a previous floor covering all interfere with how well a new floor bonds or beds down, and are commonly cited as needing to be fully removed as a baseline step regardless of what's being fitted. Just as important is levelness — a subfloor with dips, humps or other defects needs to be brought as close to flat as possible, commonly using a self-levelling compound where needed, since fitting flooring over an uneven base is one of the most common causes of movement, creaking, or premature wear once the floor is in use.

Skipping proper subfloor preparation doesn't always cause an immediate, obvious problem — which is part of why it gets overlooked — but the consequences tend to show up later and can be expensive to fix. Boards can cup, gap or lift as trapped moisture works its way out over time; adhesive bonds on glued-down floors such as LVT or engineered wood can fail, causing tiles or planks to lift at the edges; and trapped moisture beneath a floor can lead to a persistent musty smell or, in worse cases, mould developing in the void below. Most flooring manufacturers also explicitly require proper subfloor preparation, including moisture testing where relevant, as a condition of their product warranty, so skipping this step can void a warranty even if the flooring itself was never faulty.

For anyone getting a flooring job quoted, it's worth asking directly whether the fitter plans to test the subfloor's moisture level before starting, whether a DPM is likely to be needed for your specific subfloor and flooring choice, and whether any levelling compound work is included in the quote or would be charged separately if the subfloor turns out to need it. A fitter who raises these questions unprompted, rather than needing to be asked, is generally a good sign of someone who takes subfloor preparation seriously rather than treating it as an afterthought.

Frequently asked questions