LVT Subfloor Prep: What You're Responsible For (and What It Costs) in 2026

LVT is thin. That's most of what makes it look good and most of what makes it unforgiving. A ridge, dip or bit of grit under carpet just gets absorbed into the pile. Under LVT, it telegraphs straight through the surface - and on a glue-down floor, it can be the difference between a floor that lasts fifteen years and one that starts lifting at the joints within twelve months.

If you've bought LVT online or from a retailer and you're hiring someone fitting-only to lay it, subfloor prep is the part of the job most likely to catch you out. It's rarely fully included in a labour-only quote, it's genuinely specialist work in a lot of cases, and it's the single biggest reason a fixed-price quote turns into a bigger number once the fitter actually sees your floor. This guide covers what needs checking on timber and concrete subfloors, roughly what each stage costs, what you can reasonably do yourself, and what tends to go wrong when it's skipped.

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Why LVT Is Less Forgiving of a Bad Subfloor Than Carpet

Carpet has pile and underlay to absorb small imperfections. LVT doesn't - it's a thin, semi-rigid material that follows the exact contour of whatever's underneath it, so a small ridge or dip that would be completely invisible under carpet can show through an LVT floor, cause click joints to separate, or stop glue-down planks from bonding properly.

The UK code of practice for installing resilient floor coverings like LVT is BS 8203, which sets out a framework for assessing subfloor surface regularity rather than a single number that applies to every product. It defines regularity classes - commonly referred to as SR1, SR2 and SR3 - based on the maximum gap allowed under a 2-metre straightedge: roughly 3mm for SR1, 5mm for SR2, and 10mm for SR3. Installers commonly work to a high target like SR1 for thin or fully bonded LVT where the manufacturer specifies it, but there's no single rule that automatically applies to every LVT format - a floating click system and a fully glued-down floor don't have identical requirements, and the manufacturer's own installation instructions should always be the final word.

Not sure which format you need? Check out our comparison of click-lock vs glue-down LVT in our LVT flooring guide.

Timber Subfloors: The Plywood Overlay

Suspended timber floorboards - common in older South London and Surrey properties - flex, creak and have gaps between boards that will telegraph straight through LVT if you lay it directly on top. The standard fix is a plywood overlay: flooring-grade plywood screwed down in a staggered grid pattern (typically at around 150mm centres) to create a flat, rigid base that doesn't move.

Use flooring-grade plywood that complies with BS 8203:2017 Annex A and whatever the specific LVT manufacturer's installation guide calls for. You'll sometimes see this referred to on trade sites as "SP101" - that's a real, branded plywood product (SP101 Flooring Plywood®) marketed as meeting the BS 8203 Annex A specification, but it's a proprietary product name, not the name of a British Standard in its own right, so don't take "SP101" alone as a guarantee of suitability without checking the actual panel against the manufacturer's requirements.

Thickness isn't a single fixed number either. Manufacturer guidance commonly specifies plywood in the 5.5-6mm range for overlay applications, but the right thickness depends on the existing floor's condition, joist spacing, and what the specific LVT range you've bought requires - always check the LVT manufacturer's own installation guide rather than assuming one thickness fits every job.

One thing plywood won't do: fix genuinely structural problems. If your floor has significant joist sag, deflection or movement, an overlay sits on top of that problem rather than solving it - the boards need stabilising and, in some cases, structural work needs doing first. A flat piece of ply over an unstable floor will still move.

Concrete Subfloors: Moisture Testing and Levelling

Newer extensions and ground-floor concrete slabs need a different kind of prep: moisture testing before anything else. Laying LVT over a concrete floor that's still holding excess moisture traps that moisture underneath the flooring, which can cause adhesive failure, mould, or the floor lifting months after it was fitted.

The commonly cited UK benchmark, per the Contract Flooring Association's Guide to Contract Flooring, is a maximum reading of 75% relative humidity (RH) for solid cementitious or concrete bases before most resilient floor coverings are installed, tested with a calibrated hygrometer under the correct method. If a reading comes back above that, a damp-proof membrane (DPM) is applied before the floor can be levelled and covered. It's worth being precise here: that 75% RH threshold applies to concrete and cementitious bases specifically - timber and plywood subfloors are assessed differently, by moisture content and physical condition rather than the same RH-percentage test, so don't assume a single number applies across every subfloor type.

Once moisture is confirmed to be within range, an uneven concrete floor typically needs self-levelling compound poured across it to create a flat surface before LVT goes down. This isn't a job to eyeball - the compound needs mixing and pouring correctly to cure evenly, and getting it wrong can leave soft or uneven patches under the finished floor.

Read more on what's included vs not in fitting-only.

What Does Subfloor Prep Actually Cost?

As a broad 2026 domestic budgeting guide - these are indicative prices from UK cost guides, not fixed trade rates - plywood overlay typically runs around £15-25/m² supplied and fitted, self-levelling compound around £8-20/m², and a damp-proof membrane around £5-12/m². For a typical 20m² room, that works out to roughly £250-600 in prep costs alone, depending on which of these your floor actually needs.

A few things worth knowing before you budget against these numbers: small rooms often cost more per square metre than the ranges above suggest, because minimum labour and delivery charges don't scale down with room size. And if your floor genuinely needs more than one treatment - plywood plus joint filling plus a DPM, for instance - the combined cost can run higher than the £250-600 typical-room figure.

Can I Prep My Own Subfloor Before a Fitter Arrives?

Some of it, yes - most of it, no, if you want the LVT to actually last.

Clearing the room, uplifting old carpet or flooring, and giving the floor a basic sweep and visual check are all reasonable to do yourself before your fitter arrives, and it can genuinely save time on the day. Moisture testing needs a calibrated hygrometer and knowing how to interpret the reading correctly. Fixing a plywood overlay needs the right fixing pattern and joint layout to actually create a rigid, stable base rather than just a flat-looking one. And self-levelling compound needs mixing and pouring correctly to cure evenly - get any of these wrong and you risk voiding the manufacturer's warranty on the LVT itself, on top of having to redo the work.

Before hiring anyone, read our checklist on how to vet a labour-only fitter.

What Actually Goes Wrong When Subfloor Prep Is Skipped or Done Badly

It's easy to treat "prep your subfloor properly" as generic advice. Real UK homeowner reports show it's a genuine, recurring problem, and the same failure patterns come up again and again.

One homeowner reported LVT tiles detaching from their latex base less than a year after installation, with inadequate preparation or an adhesive/primer issue raised as a likely cause. Another described plywood laid over old pine floorboards with visible ridging at the plywood joints - the floor eventually had to be lifted and feather-edge compound applied to smooth out the joints properly. A third reported uneven plywood directly causing their LVT to crack at the click joints. Another described click joints repeatedly separating over a floor they identified themselves as "not level enough." And in a fifth case, the homeowner questioned whether 6mm plywood alone was actually enough to correct significant existing variation in a wavy chipboard subfloor - a useful reminder that plywood evens out a floor, it doesn't fix a genuinely uneven or structurally unstable one underneath it.

None of these reports are controlled technical investigations - they're homeowners describing what happened and what they believe caused it. But the pattern across them is consistent enough to be worth taking seriously: skipped or rushed subfloor prep shows up as ridging, joint failure, and tiles that come away from the base, and once it's happened, the usual fix is lifting the floor and starting the prep again properly - which costs more than doing it right the first time.

Frequently Asked Questions

Do I need to level my subfloor before LVT?

Only if it's out of tolerance for the specific LVT format you've bought. Click-lock/floating systems generally tolerate more variation than fully bonded glue-down LVT, but neither format tolerates a genuinely uneven floor - check against the manufacturer's own installation guidance rather than assuming either format is maintenance-free.

What happens if I skip subfloor prep to save money?

The most common consequences are joint failure, lifting, hollow-sounding patches, and voided manufacturer warranties. In most cases the floor ends up needing to be lifted and re-laid once the problem shows up, which costs more overall than doing the prep properly the first time.

Is subfloor prep included in a fitting-only quote?

Not automatically. It's the kind of work that should be itemised separately once the fitter has actually assessed your subfloor in person, rather than assumed to be bundled into a flat labour-only rate quoted sight unseen.

How long does subfloor prep add to the job?

A straightforward plywood overlay or a self-levelling compound pour typically adds around half a day to the job. Ask for a specific time estimate once your subfloor's actually been assessed rather than assuming a flat rule applies to every room.

Conclusion

LVT looks and performs its best when the floor underneath it is genuinely flat, dry and stable - and getting there is rarely as simple as "just lay it down." Whether that means a plywood overlay on old floorboards or moisture testing and self-levelling compound on a concrete slab, it's real, billable work that should be priced once someone's actually seen your floor, not guessed at over the phone.

If you've bought LVT and need it fitted properly - subfloor prep included and priced honestly upfront - get in touch for a free site survey and a proper moisture check before you commit to a delivery date.