How do I level an uneven floor before laying anything?

Levelling is the step people skip, and it is the one that determines whether the finished floor survives. Every fault that appears later — clicking laminate, cracked tiles, lifting vinyl — usually traces back to it.

Safety

Step by step

1. Measure how far out it actually is.A two-metre straight edge or a long spirit level across the floor in several directions, with a tape measure in the gaps. Write the numbers down — the whole decision follows from them.
2. Tell dips from slope.A floor can be flat but sloping, or level overall but full of dips. Flooring cares about the dips, not the slope. A consistent slope in an old house is usually left alone.
3. Check the tolerance for what you are laying.Laminate and click flooring typically want no more than about 3mm over 2 metres, tiles less. Vinyl shows every imperfection through it. The manufacturer states the figure and it is a warranty condition.
4. Find out why it is uneven.Sagging joists, a failed screed, damp, or settlement. Levelling over a moving or wet floor is wasted work, and a bouncy floor needs the joists dealt with first.
5. Fix the substrate first.Screw down loose boards, replace anything rotten, fill large holes, and remove old adhesive and nails. Compound poured over a loose board will crack when it moves.
6. Use self-levelling compound for up to about 25mm.Mixed to a pourable consistency and spread with a trowel, it finds its own level. Read the depth range on the bag — most have a minimum as well as a maximum.
7. Prime before pouring.Acrylic primer on the substrate, always. It stops the base sucking the water out of the compound, which is what causes a powdery, weak surface that fails later.
8. Mix it properly and work fast.A paddle on a slow drill, exactly the water on the bag, mixed for the stated time. You have around fifteen to twenty minutes and there is no pausing halfway across a room.
9. Work from the far corner outwards.Pour in bands and help it with a trowel or a spiked roller. Plan the exit before starting — being stranded in a corner is a common and avoidable mistake.
10. Use plywood overlay on timber floors.For a boarded floor that is uneven rather than dipped, 6 to 12mm plywood screwed down at close centres gives a flat base. It is generally better than compound on timber and it is what most tilers want.
11. Pack or shim joists for large variation.More than 25mm usually means the structure. Sistering or packing joists is a bigger job and it is the right one — compound at that depth is expensive and heavy.
12. Let it dry fully before laying.Walkable in a few hours does not mean ready. Follow the stated drying time, which can be days for thicker pours, and test for moisture before laying anything impermeable.
13. Check it with the straight edge again.Before laying. Compound does not always cure perfectly flat, and a second thin pour or a little filler is far easier now than after the flooring is down.

Tips

• Write the measurements on the floor in pencil as you take them. It makes the pattern of dips obvious in a way a list does not.
• Buy more compound than the calculator suggests. Running out mid-pour leaves a permanent ridge and there is no way to blend it in.
• On a timber floor, plywood overlay is usually the better answer. Compound on boards is heavy and moves with them.

Common mistakes

• Pouring compound without priming — The substrate draws the water out and the compound cures weak and powdery. It then breaks up under the flooring, which is only discovered when the floor fails.
• Levelling over a bouncy or loose floor — The compound cracks as the floor moves. Deal with the joists and fix loose boards first, or the work is wasted.

If it doesn't work

Questions people ask