What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thingself-levelling compound
also latex, levelling screed, floor levelling compound, SLC
Called LATEX from the polymer additive in older two-part products, which survives as a name for compounds containing no latex at all. Distinct from a SCREED, which is thicker and can be structural.
Australia
Close, not identicalself-levelling compound
also floor leveller, levelling compound, topping
As the UK, with topping used where the layer is thick enough to be a wearing surface in its own right.
United Statesyours
Same word, different thingself-leveling underlayment
also floor leveler, SLU, gypsum underlayment, topping
UNDERLAYMENT is heavily overloaded: it also means the membrane under a roof covering, the acoustic mat under a floating floor, and the sheet ply over a subfloor. Three different products, one word.
Canada
Same word, different thingself-leveling underlayment
also floor leveler, gypcrete, topping
As the United States, with a brand name used generically for gypsum-based pours in the way Hoover is for vacuum cleaners.
The governing standard, by market
United Kingdom
BS EN 13813 / BS 8204-1
Screed material and floor screeds: properties and requirements, and code of practice for concrete bases and cement sand levelling screeds.
United States
ASTM F710 / ASTM F2170
Preparing concrete floors to receive resilient flooring, and determining relative humidity in concrete slabs using in-situ probes.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Is it the same as a screed?
- No, and treating one as the other is a common and expensive substitution. A SCREED is a thicker layer — commonly tens of millimetres — laid and worked to a level, and depending on its type it may be structural, may carry underfloor heating, and may be a wearing surface. A SELF-LEVELLING COMPOUND is a thin correction layer, usually a few millimetres, that flows out under its own weight to remove small deviations before a covering goes down. It has a maximum thickness beyond which it cracks or debonds, it is not designed to bridge a void, and it is not a wearing surface. Using it to correct a floor that is out of tolerance by twenty millimetres means either applying it far thicker than its specification allows or several passes with the cost of each, and the correct answer is usually a screed.
- Why does it need a primer when it is mostly water anyway?
- Because of what the substrate does to that water. A porous base — an old concrete slab, a sand-cement screed, a plywood deck — pulls water out of the compound as it is poured, and a compound that loses its water before it flows and hydrates does two things: it stops flowing, so it does not level, and it cures weak and dusty at the interface, so it debonds. The primer seals the surface enough to control that suction while still leaving a key. The reverse problem exists on a non-porous base — a power-floated slab, an old adhesive residue, a tiled floor — where nothing absorbs and nothing keys, and the primer there is a different product providing grip rather than control. Using the wrong primer, or none, is the single most common cause of a levelling layer lifting in sheets.
- Why does moisture in the slab matter so much before pouring?
- Because the compound and everything above it is a lid. A concrete slab dries slowly from the surface, and a floor covering with an adhesive is a moisture-sensitive assembly sitting directly on it — so a slab still releasing water will push it into the compound, the adhesive and the covering, and the failure appears as lifting vinyl, blown timber or a mouldy smell months later. That is why the American standards here are moisture tests rather than product specifications: measuring relative humidity inside the slab with in-situ probes, or the calcium chloride emission rate. It is also why a levelling compound is not a remedy for a damp slab: it will bond to a damp substrate and then trap the water, which is worse than the uneven floor it was correcting.