Fitting guide
Subfloor preparation
How to prepare a subfloor for LVT and SPC: BS 8203 flatness and moisture limits, the tests that matter, subfloor by type, and the checklist to run before the first tile goes down.
The floor you can see is decided by the floor you cannot
Almost every complaint about a finished floor traces back to the subfloor. Tiles that telegraph a ridge. Joints that open. A hollow patch that clicks underfoot. A floor that lifted three months in. None of those are product faults, and all of them were decided before the first tile came out of the box.
Glue-down LVT is 2mm to 2.5mm thick. It does not hide anything. It takes the exact shape of whatever it is bonded to, and raking light across a floor finds every trowel ridge, screw head and dip. Click SPC is more forgiving, but only up to a point, and past that point the joints carry the load and fail.
Subfloor preparation is not the boring part of the job. It is the job.
The standard to work to
UK resilient flooring installation is covered by BS 8203:2017. Two numbers out of it matter most.
Flatness
SR1
Maximum 3mm departure under a 2m straightedge. This is the standard for glue-down LVT.
Moisture, screed
75% RH
Cementitious and anhydrite screeds, measured with a hygrometer box or in-situ probe.
Moisture, timber
13% MC
Measured with a pin-type meter, not a surface reading.
If you take nothing else from this page: 3mm over 2m, 75% RH, 13% MC.
Flat is not the same as level
A floor can be perfectly level and still be unfit to lay on, and it can slope noticeably and be perfectly fine.
Level means horizontal. Nobody cares, within reason. Old houses slope. Flat means no dips or humps across the surface. This is the one that matters, because a floorcovering follows the surface it sits on.
How to check it
Lay a 2m straightedge on the floor and move it across the room in both directions and diagonally. Anywhere a 3mm feeler or a coin slides under it, mark the spot with chalk. You will end up with a map of the work. Dips get filled with smoothing compound. Humps get ground back, or the whole floor gets skimmed.
A floor with lots of both is faster to skim entirely than to patch. Patching a bad floor takes longer and looks worse.
Moisture: the test people skip
A dry-looking slab can be soaking. Concrete gives up moisture for a long time, roughly a day per millimetre for the first 50mm, so a new slab can be months away from ready.
Concrete, screed and anhydrite
Use a hygrometer box or an in-situ probe, and leave it in place long enough to reach equilibrium, normally around 72 hours. Surface meters are a rough indicator only, useful for finding a wet patch, not for signing off a floor. Below 75% RH and you can proceed. Above it, either wait or apply a surface damp proof membrane.
Timber
Pin-type meter, 13% or less. Check several boards, including near external walls and under any pipework.
No damp proof membrane in the slab?
Any ground-bearing slab without a working DPM needs a liquid surface DPM before flooring goes down, whatever the reading says today.
Underfloor heating
Commission the system fully and run it through a heating cycle before testing, then turn it down or off 48 hours before fitting. Testing an unheated screed tells you nothing about what happens when it warms up.
Sound and clean
Sound means the surface will not come away under the floor. Loose screed, friable topping, powdery laitance on an anhydrite screed, hollow tiles, springy floorboards: all of it has to be dealt with, because an adhesive bond is only as strong as what it is stuck to. Anhydrite screeds almost always need the laitance sanding off and priming.
Clean means free of anything that stops a bond forming:
- old adhesive residue, especially bitumen
- paint, plaster splashes and jointing compound
- curing agents, release agents and surface sealers
- dust, and then dust again after you think you have finished
Vacuum, do not sweep. Sweeping moves dust around, and a fine dust film under adhesive is a bond failure waiting to happen.
Temperature and humidity
Conditions during the fit are part of the preparation, and they are the ones most often ignored on a rushed job.
| Ambient temperature | 18°C to 24°C, for at least 48 hours before fitting |
|---|---|
| Hold conditions after fitting | At least 72 hours |
| Minimum subfloor temperature | 10°C under BS 8203 |
| Minimum for UZIN KE 49 HT adhesive | 15°C at floor level |
| Relative humidity in the room | 35% to 65% |
| Acclimatise LVT and SPC | 24 to 48 hours, flat, in the room |
Where two figures disagree, work to the higher one. The adhesive’s requirement always beats the general standard, because the adhesive is the thing that has to cure. Keep the material out of direct sunlight and away from heat sources while it acclimatises.
Subfloor by type
Concrete and sand-cement screed
The standard case. Test moisture, check flatness, prime, smooth where needed.
Anhydrite or calcium sulphate screed
Same moisture limit, but sand off the surface laitance and prime with a compatible primer. Anhydrite reacts badly with cement-based products, so check the smoothing compound is rated for it.
Timber joists
Never lay straight onto joists or bare floorboards. Overlay with plywood certified to EN 636-3, fixed at close centres, then check flatness again over the ply. Acclimatise and moisture-test the ply before it goes down.
Chipboard
Usable if sound, dry and well fixed, but it moves at the joints and swells if it ever gets wet. Overlaying with ply is the safer route.
Existing ceramic tiles
Can be laid over if they are sound and well bonded. Every hollow tile has to come out, the grout lines have to be filled, and the whole thing skimmed. Glazed tiles need priming.
Existing vinyl or LVT
Take it up. A bond is only as good as the layer under it, and you have no idea what is under the old floor or how well it is stuck.
Asphalt or bitumen
Specialist case. Needs a compatible primer and often an isolating membrane. Get advice before ordering.
Underfloor heating
Fully commissioned, run up and back down, off or turned right down 48 hours before fitting. Bring it back up in 3°C to 5°C steps per day afterwards, to a maximum 27°C at the adhesive line.
Glue-down or click: what changes
| Glue-down LVT | Click SPC | |
|---|---|---|
| Flatness | SR1, 3mm under 2m | Around 3mm under 1m, check the product |
| Moisture test | Essential, bond depends on it | Still required, the subfloor is still damp |
| Priming | Usually | No |
| Smoothing compound | Often | Sometimes |
| Over an existing hard floor | Rarely | Often, if sound and flat |
| Consequence of getting it wrong | Bond failure, telegraphing, lifting | Failed click joints, hollow spots, movement |
Click is more tolerant of an imperfect surface. It is not tolerant of a damp one, and it is not a way to avoid preparation. It just moves the failure from the adhesive to the joints.
What you will need that we do not sell
We are straight about this: we do not sell smoothing compound, primer or damp proof membrane. They are heavy, awkward to ship, and you will get them cheaper and faster from a local builders merchant.
- primer suited to the subfloor type
- latex or cement-based smoothing compound
- surface DPM, if the slab needs one
- gauging rake, trowel and mixing paddle
What we do sell is everything from the adhesive up: high-temperature LVT adhesive, wood adhesive and spreaders.
The checklist before the first tile
- Old floorcovering removed, residues off
- Moisture tested and recorded: below 75% RH, or 13% MC on timber
- DPM in place if the slab needs one
- Straightedge run across the room in both directions and diagonally
- Dips filled, humps ground, floor skimmed if it needs it
- Loose, hollow or friable areas made good
- Primed where the substrate or the compound requires it
- Vacuumed, then vacuumed again
- Room at 18°C to 24°C, floor at 15°C or above, for 48 hours
- Material acclimatised flat in the room
- Underfloor heating turned down 48 hours ago
- Kitchen units, islands and joinery already installed
- Enough material on site, including the wastage allowance, all one batch
Common questions
How long does a new concrete slab take to dry?
Roughly a day per millimetre for the first 50mm in good drying conditions, so a 100mm slab can be four months or more. Test it, do not estimate it.
Can I lay LVT straight onto floorboards?
No. Overlay with EN 636-3 plywood first. Boards move, cup and open at the joints, and every one of those movements will show.
Do I need to prime?
Almost always for glue-down, and always if the manufacturer of the smoothing compound says so. Priming controls absorbency, stops pinholing and improves the bond.
Is a surface moisture meter good enough?
For finding a problem, yes. For signing off a floor, no. Use a hygrometer box or an in-situ probe on anything cementitious.
The subfloor is only slightly uneven. Can I get away with it?
On click, maybe. On glue-down, no. At 2mm thick, “slightly uneven” becomes “visible from the doorway” the first time low sun crosses the room.