First of all: measure the height difference
Before you buy anything you need to know how much has to be corrected, because everything else follows from that.
Take a long aluminium straightedge (two metres at least) and a spirit level. Rest it at several points in the room, crossways and diagonally, and measure the largest gap between the straightedge and the floor with a tape. That gap is your height difference.
- Up to 5 mm: almost any floor covering will tolerate it. A thin coat of self-levelling compound is enough.
- 5 to 30 mm: self-levelling compound. This is the usual case.
- 30 mm to 10 cm: a mortar screed, or lightweight aggregate if weight matters.
- More than 10 cm: lightweight aggregate or boards, because mortar alone would weigh an enormous amount.
Self-levelling compound: the most common method
This is a very fluid mortar that spreads out on its own and finds its own level. It is used for small and medium height differences, and leaves a surface ready for boards, vinyl or tiles.
The order matters:
- Clean the floor thoroughly. It has to be free of dust, adhesive residue and grease. Self-levelling compound bonds badly to a dirty floor and later lifts in sheets.
- Apply a primer. This is not optional: it seals the surface so the floor does not draw the water out of the mix, and it improves adhesion. On very absorbent floors, two coats.
- Fit a foam edging strip where the floor meets the walls, so the screed can move without cracking.
- Mix with exactly the amount of water on the bag. Too much water is the most common mistake: the mix separates, loses strength and cracks as it dries.
- Pour it and help it along with a notched trowel or a spiked roller, which also releases trapped air.
You can walk on it after a few hours, but before laying the floor covering it has to dry for as long as the manufacturer says. Rushing this stage ruins whatever goes on top.
Lightweight aggregate: when you need height without weight
Expanded clay is a very light aggregate. It is used when you have to build up several centimetres and do not want to load the floor structure, which often happens in upper-floor refurbishments and in old houses.
It is laid dry or mixed with cement, compacted, and finished with a layer of mortar or self-levelling compound to give a firm surface. It has the added advantage of insulating, against both cold and noise.
Battens: levelling without mortar
If you are laying floating boards and want to avoid wet trades, you can level with timber battens packed up to the right height and lay the floor on top. It is quick and clean, and lets you run services underneath.
In exchange, it raises the floor more than a self-levelling compound would, and it is no use for ceramic tiles.
Outdoor floors and terraces
Outside there are two things you cannot forget. The first is that the floor should not end up perfectly flat: it needs a fall of between 1% and 2% towards the drain, or water will pond on it. The second is that the product has to be rated for external use, because an internal one breaks down with frost and sun.
Mistakes that cost you
- Skipping the primer. This is the single most common reason a screed ends up debonding.
- Adding more water than specified so it flows better.
- Leaving out the edging strip: the mortar cracks against the wall.
- Laying the floor covering before the screed has fully dried.
- Levelling over a floor that moves. If there are loose tiles or an unstable base, that gets fixed first; self-levelling compound does not hold down anything that is already loose.
What you will need
An aluminium straightedge, a spirit level, primer, edging strip, the right mortar or lightweight aggregate, a large bucket, a paddle mixer for a drill and a spiked roller.
We stock all of this at the yard, and if you tell us the area and the height difference we will work out the quantity and tell you which particular product suits your case best. Over-ordering is money thrown away, and running out halfway through is worse.