Subfloor preparation for commercial LVT
Key takeaways
- Subfloor condition and preparation are the biggest single source of LVT failure. The tile is rarely the problem; what it was laid on usually is.
- Moisture testing is the step that is skipped most often and costs the most when it goes wrong. Relative humidity is measured, not judged by looking at the slab.
- LVT is thin and rigid, so it telegraphs defects rather than bridging them. The flatness tolerance it needs is tighter than the one a sheet vinyl gets away with.
- Preparation is a sequence – assess, clean, repair, prime, smooth, prime again where required – and skipping a stage undermines everything after it.
- The adhesive is part of the preparation, not an afterthought. Open time, temperature and substrate porosity all change how it behaves.
Why subfloor preparation matters more for LVT
Luxury vinyl tile is thin, hard and laid in individual units and those three properties together make it the least forgiving common commercial floor finish. A carpet tile hides a multitude of substrate sins. A sheet vinyl bridges small imperfections. An LVT does neither. It presses down onto whatever is beneath it and takes the shape of it.
The result is that a defect which would be invisible under another finish reads through an LVT as a visible line, a shadow under raking light or a plank that rocks under foot. Once the floor is laid, none of that is fixable without lifting it, so preparation is not the boring part of an LVT project – it is most of the project.
It is also the part most likely to be squeezed. Preparation is priced provisionally, discovered late and compressed when the programme slips, which is precisely why preparation-related problems are among the most common causes of complaint on commercial LVT installations.
Assessing the substrate before anything else
The first job is establishing what is actually there. In a new building that means the specification and the pour records. In a refurbishment it means opening up, because the drawings are frequently wrong and the floor has usually had two or three previous lives.
The questions that matter are what the base is – a power-floated slab, a sand cement screed, a calcium sulphate screed, a timber deck, an existing finish – how sound it is, how flat it is, how absorbent it is and what is on it. Old adhesive residues, curing compounds, laitance, paint, oil and plaster droppings all interfere with the bond and each is dealt with differently.
Calcium sulphate screeds deserve a specific check because they are common, they are easy to mistake for a cement screed and they behave differently. They must be dry, the laitance has to be mechanically removed and they need a compatible primer, because some cementitious products react badly with them. Getting this wrong produces a floor that debonds cleanly across a whole area.
Moisture – the failure that ends projects
Residual construction moisture in a slab will migrate upwards until it finds a barrier. An impervious LVT bonded to the surface is that barrier, so the moisture concentrates at the adhesive line, softens the bond and the floor lifts, bubbles or opens at the joints anywhere from a few weeks to a year after handover.
The measurement is relative humidity taken with a hygrometer left in place long enough to reach equilibrium, following BS 8203, rather than a surface reading from a hand-held meter taken as the fitter arrives. Readings should be taken across the area rather than at a single point, because a slab dries unevenly and the wet part is usually the part nobody tested – near external walls, under where the site hut stood, or where a leak happened during construction.
The accepted threshold for bonding an impervious floor covering directly is 75 per cent relative humidity. Above that, the options are to wait, which on a construction programme usually means months, or to apply a surface damp proof membrane. An epoxy DPM is applied in one or two coats and many systems cure fast enough to let the floor proceed quickly, though cure, recoat and loading times vary between systems and are read off the data sheet. It is cheap enough against the cost of relaying a finished floor that it is often specified as a precaution rather than a remedy.
Two further points get missed. A slab with no effective structural damp proof membrane beneath it can keep taking up moisture from the ground, so it may never reach a condition suitable for direct bonding and a surface DPM is the practical route. And underfloor heating changes the picture – the heating has to be commissioned through a full heat-up and cool-down cycle before any measurement is meaningful.
Surface regularity and flatness
BS 8204 defines surface regularity as SR1, SR2 or SR3, measured as the maximum permissible gap under a two metre straightedge. SR1 is the tightest and SR3 the most relaxed. Those classifications describe a screed rather than setting the requirement for a floor covering. For a thin bonded finish such as LVT the requirement comes from BS 8203 and the manufacturer’s installation instructions and in practice it is normally met by SR1 or a tight SR2.
This is worth writing into the specification as a number rather than a description, because it turns a subjective argument at handover into a measurable one before the floor goes down. Straightedge readings should be taken and recorded before the finish is ordered, not after it has been laid and the client has noticed a hollow in the middle of a reception.
The other half of flatness is local rather than general. A long gentle undulation of two or three millimetres over two metres is often acceptable; a sharp ridge of half a millimetre is not, because the eye reads a line far more readily than a slope and a rigid tile bridges the ridge and rocks. Grinding out local high spots is usually quicker and better than trying to fill up to them.
Cleaning, repair and mechanical preparation
Whatever is on the surface has to come off before anything goes on. Laitance on a power-floated slab is a weak layer that will take the floor with it when it fails and it is removed by mechanical preparation – shot blasting, grinding or scarifying, chosen for the substrate and the site conditions.
Shot blasting is fast and leaves an excellent key but it is noisy and it needs the area cleared. Diamond grinding is usually quieter, better in occupied buildings and better for removing old adhesives, but slower and how quiet and how dusty it is still depends on the equipment and the extraction. Scarifying is aggressive and suited to heavy coatings and thick residues. On most commercial refurbishments the answer is grinding, because the constraint is usually noise and dust in a live building rather than the substrate itself.
Cracks and joints need judgement rather than a rule. A dormant crack is filled with a resin repair and moved on from. A live crack, or a structural movement joint, must not be bridged – the movement will find its way through any finish laid over it and the joint should be carried through the floor with a proper movement joint detail. Bridging a movement joint is one of the few preparation errors that is guaranteed to fail rather than merely likely to.
Priming and smoothing compound
Priming does three things: it controls the absorbency of the substrate so the smoothing compound does not dry too fast, it binds residual dust and it promotes adhesion. The primer has to match the substrate, so a porous screed, a non-porous power-floated slab, an epoxy DPM and a calcium sulphate screed all take different products at different dilutions.
The smoothing compound is what actually creates the surface the LVT is bonded to. For a commercial LVT it should be a high-strength compound compatible with the adhesive and the covering, normally protein-free, applied at the thickness the manufacturer states rather than feathered thin over high spots. Primer, compound, adhesive and floor covering are best taken from a single compatible system, because a mixed build-up moves the warranty question to whoever chose the combination. A compound applied too thin over a hard slab has nothing to grip and can shell off in sheets under rolling loads.
Two mistakes recur. The first is patch-smoothing rather than covering the whole area, which leaves changes in porosity and hardness that show through as visible patterns. The second is laying on the compound before the primer has properly dried, which traps moisture and produces pinholes and blisters. Both are programme pressure rather than ignorance.
Timber and existing floors
Timber substrates need their own approach. A boarded floor moves, flexes and has joints, all of which will telegraph, so the usual answer is an overlay of flooring grade plywood, screwed at close centres to prevent deflection with joints staggered and filled, then primed and smoothed. Chipboard and OSB are less suitable and need checking against the manufacturer’s guidance.
Ventilation beneath a suspended timber floor also has to be maintained, because sealing the surface with an impervious finish above an unventilated void is how damp problems start.
Laying over an existing finish is sometimes possible and often a false economy. An old, well-bonded ceramic tile floor can be a good base once the joints are filled and the surface is prepared, but an old vinyl or an old adhesive bed is only as good as its own bond to the slab and any adhesive containing bitumen needs proper assessment before it is disturbed. In any building constructed or refurbished before 2000, old adhesives, backings, tiles and boards should be assessed for asbestos before they are disturbed rather than assumed safe. Where an existing floor is retained the risk transfers to the new installation, so the decision should be a considered one rather than a way of saving a day.
Site conditions, adhesive and open time
The last stage is where good preparation gets undone. LVT and its adhesive are both temperature sensitive. The material should be acclimatised in the space at working temperature for the period the manufacturer states and the substrate temperature matters as much as the air temperature – a cold slab under a warm room will slow the adhesive and change its open time.
Adhesive open time is the point most often rushed. A pressure sensitive adhesive needs to reach the right tack before tiles go into it. Laid too wet, the tiles float and creep, opening the joints over the following weeks. Laid too dry, the bond never develops. Neither is visible on the day and both show up later.
Rolling the floor after laying is not optional either. It transfers the adhesive properly to the back of the tile and skipping it in the corners and edges is why joints lift there first. Where the building is going to be heavily loaded soon after, the adhesive needs its full cure before that traffic arrives.
What to confirm before an LVT project starts
Confirm what the substrate actually is by opening up rather than reading a drawing. Confirm the relative humidity by hygrometer across the whole area and get the readings in writing. Confirm the surface regularity requirement as an SR number in the specification. Confirm who is responsible for mechanical preparation and what noise and dust the building will tolerate. Confirm the acclimatisation period and the site temperature the programme can deliver. Confirm any movement joints and how they will be carried through.
Preparation products are made by manufacturers such as Uzin, Mapei, F Ball, Ardex, Bostik and Sika and a specialist contractor matches the primer, compound and adhesive to the substrate and the finish. Listing a manufacturer here does not imply a partnership, approval or accreditation.
The covering that goes on top is graded separately, covered in the LVT wear layer guide and preparation rates sit alongside covering rates in the commercial flooring cost guide.
Surface Specialists is a network of vetted specialist contractors covering subfloor preparation, commercial LVT and resin systems. Tell us the building, the substrate and the programme and we match the project to a contractor who works in that environment. Get in touch to arrange a site survey.
Frequently asked questions
What subfloor preparation does LVT need?
A sound, clean, dry and flat substrate, achieved by assessing what is there, mechanically removing laitance and contaminants, repairing cracks, priming to suit the substrate and applying a full-cover smoothing compound at the stated thickness. LVT is thin and rigid, so it telegraphs defects rather than bridging them and the tolerances are tighter than for sheet finishes.
How dry does a floor need to be before laying LVT?
Relative humidity should be at or below 75 per cent, measured with a hygrometer left in place to reach equilibrium under BS 8203 rather than with a hand-held surface meter. Above that threshold the choice is to wait for the slab to dry or to apply a surface damp proof membrane, which on many systems cures fast enough to let the programme continue.
Do I need a DPM before laying LVT?
You need one where the relative humidity reading is above the threshold, where the slab has no structural damp proof membrane beneath it and often as a precaution on a tight construction programme where the slab has not had time to dry. An epoxy DPM is applied in one or two coats and is inexpensive against the cost of taking up a finished floor.
Why is my LVT showing lines and marks from underneath?
That is telegraphing and it means the substrate was not flat or uniform enough. Trowel ridges, joints, patch repairs, nail heads and changes in porosity all read through a thin bonded tile. It is usually caused by patch-smoothing rather than full-cover smoothing, or by a compound applied too thin and it cannot be corrected without lifting the floor.
What is surface regularity SR1?
BS 8204 defines surface regularity as the maximum gap permitted under a two metre straightedge and SR1 is the tightest classification while SR3 is the most relaxed. The requirement for a thin bonded finish such as LVT comes from BS 8203 and the manufacturer’s instructions rather than from BS 8204 itself, though in practice it is normally met by SR1 or a tight SR2 and specifying it as a number makes the standard measurable before the floor is laid rather than arguable afterwards.
Can LVT be laid over an existing floor?
Sometimes. A well-bonded ceramic tile floor can make a good base once joints are filled and the surface is prepared. An existing vinyl or an old adhesive bed is riskier because the new floor is only as sound as the old bond beneath it and any bitumen-containing adhesive needs proper assessment before it is disturbed.
Can you lay commercial LVT on a timber floor?
Yes, over a suitable overlay. Boarded floors move and have joints that will telegraph, so the usual approach is flooring grade plywood screwed at close centres with staggered filled joints, then primed and smoothed. Ventilation to any suspended void must be maintained, because sealing the surface above an unventilated void causes damp problems.