Screed and levelling come up on almost every concrete floor we survey in Bexley, Dartford, Bromley and Eltham. Extensions meet original slabs at slightly different heights, old hearths leave humps, and service trenches get patched and forgotten. A thin floor covering shows all of it. This guide explains what a self-levelling pour is, when your floor needs one, and the checks that should happen before anything is poured.
What self-levelling compound actually is
It is a fluid latex or cement based mix that is poured over a primed concrete floor and flows out under its own weight into a smooth, flat plane. Once cured it gives a glue-down floor covering a surface it can bond to evenly, which is what makes a thin vinyl plank look poured rather than laid.
People use the words screed, latex and smoothing compound fairly loosely. As a rule of thumb, a smoothing compound deals with a floor that is rough, while a self-levelling screed is for a floor that is genuinely out of level. Your fitter should tell you which you need at the survey rather than discovering it on the day.
How to tell if your floor needs it
The standard we work to for glue-down LVT is no more than 3mm of deviation under a 2 metre straightedge. Rigid click floors cope with around 5mm, and carpet on a good underlay hides gentle undulation entirely, although a sharp step or ridge will still show and wear through the backing in time.
You can check this yourself. Lay a long level or a straight length of timber across the room in several directions and look for daylight. If a pound coin slides under it anywhere, the floor is likely to need levelling before a thin covering goes down.
- Glue-down LVT and sheet vinyl: 3mm over 2m, levelling common on older slabs
- Click SPC and laminate: about 5mm over 2m, levelling sometimes needed
- Carpet: most forgiving, but ridges and steps still need removing
Checks before anything is poured
Damp comes first. Many ground floor slabs built before the 1960s have no working damp-proof membrane, so we test with a hygrometer. Readings above 75% relative humidity mean a liquid membrane goes down before levelling, otherwise the moisture is sealed under an impermeable floor and the adhesive fails later.
Old adhesive is the second check. Black bitumen residue needs a bitumen-tolerant primer, because a standard latex will not grip it and will lift away. Loose or hollow sounding tiles have to be taken up, not poured over, and deep hollows are filled in stages or with a repair mortar, because one very thick pour tends to crack.
Drying time and finished height
A thin levelling pour cures in a few hours, which is why preparation and fitting can often happen close together. Deeper pours, damp membranes and fresh sand and cement screeds take longer, and a screed that is still wet when flooring goes down will show up later as failure.
Every millimetre of levelling adds to the finished height, on top of the floor covering itself. Doors, thresholds, kitchen plinths and the bottom stair riser are all checked against that height before work starts, and a door that will catch is trimmed beforehand rather than afterwards.
What it costs and how to read a quote
Our published starting price for subfloor preparation is 12 pounds per square metre. Across typical jobs, latex self-levelling runs about 10 to 18 pounds per square metre including primer and labour, more where the depth is significant, and a liquid damp-proof membrane adds around 8 to 14 pounds.
Levelling is the item most often missing from a cheap quote, which is why two prices for the same room can be hundreds of pounds apart. Ask each fitter what preparation is included and what happens if the floor needs more once the old covering is up. We state our preparation allowance in writing so there are no surprises on fitting day.
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Frequently asked questions
Do I need self-levelling compound under LVT?
On concrete, usually yes unless the slab is already within 3mm over 2 metres. Thin glue-down LVT shows every ridge, so most older floors need at least a smoothing coat.
How long does self-levelling compound take to dry?
A thin pour cures in a few hours. Deeper pours and damp-proof membranes take longer, and the fitter should confirm the base is dry before flooring goes down.
Can you level over old tiles?
Only if they are firmly bonded. Tiles that sound hollow when tapped are debonded and must be lifted first, or they will move under the new floor.
How much does floor levelling cost?
Our subfloor preparation starts from 12 pounds per square metre. Latex levelling typically runs 10 to 18 pounds per square metre including primer, more for deep pours.