Why did my self-levelling compound crack, lift or stay powdery?

Almost every levelling compound failure is a bond failure at the bottom, and the causes are dust, no primer, wrong water ratio, or a floor that moves.

Difficulty: intermediate
Time: 45 min
Read: 5 min
Safety: caution

Short answer

Compound that lifts in sheets was never bonded — the slab was dusty or unprimed. Compound that cracks in lines is following movement below or was poured too thin. Compound that stays powdery had too much water or dried too fast. In every case the compound is reporting a preparation problem rather than being a bad product.

Levelling compound looks like the easy part of a floor: mix, pour, it finds its own level. In practice it is the most preparation-dependent material in the whole job, because it is a thin layer bonded to a surface that is usually dusty, sometimes damp, and occasionally moving — and it has no strength at all if it is not stuck down.

Safety

Cement-based levelling compounds are alkaline and burn skin on prolonged contact — gloves and knee protection, and rinse splashes off promptly. Mixing produces dust: an FFP3 mask while pouring the powder. Grinding or sanding a failed compound produces silica dust and needs extraction and proper respiratory protection.

What you'll need

the correct primer for the substrate
a vacuum, not a brush
a measuring jug and a bucket
a mixing paddle and a suitable drill
a smoothing trowel and a spiked roller (optional)
the product data sheet

Step by step

Diagnose from how it failed.Lifting in sheets with clean powder underneath is a bond failure. Cracks in lines are movement or thin pours. A surface that dusts off under a fingernail is a mix or curing problem.
Check what is under a lifted piece.Turn it over. Dust on the underside means the slab was not clean. A smooth glossy underside means it was sealed or unprimed. Damp means moisture from below.
For a bond failure, remove all of it.Anything that is not solidly bonded has to come up, and the boundary has to be found by tapping — the debonded area is usually larger than the visibly lifted part.
Vacuum, do not sweep.Sweeping redistributes fine dust and leaves a film. A slab that has been ground or scraped needs vacuuming several times, and the last pass should come up clean.
Prime with the compound manufacturer's primer, at the dilution they state.The primer does two things: it locks down residual dust and it controls the slab's suction so the compound does not lose its water instantly. Absorbent slabs often need two coats.
Mix by measured water, not by eye.The water ratio is the single biggest control on strength. Too much water gives a compound that flows beautifully and cures weak, dusty and prone to cracking.
Respect the minimum and maximum thickness.Every compound has both. Poured too thin it cracks and crazes; poured too thick it can crack from shrinkage or fail to cure through. Deep areas may need building up in stages or a different product.
Work fast and pour wet into wet.Flow time is short. Mix the next batch before the last has stiffened and pour into the live edge; a cold joint between batches is a line the floor will crack along.
Control the conditions while it cures.No draughts, no direct heat, no underfloor heating switched on. Drying too fast is a recognised cause of crazing and surface weakness.
Deal with movement below before repouring.Compound is not a structural material. Over a timber floor it needs a compound rated for the job with the deck properly fixed, and over a cracked slab a moving crack will telegraph straight through.

Tips

Do not walk on it. A single footprint in setting compound is a permanent dip that a floor will telegraph.
Prime the day before if the product allows, so the primer is fully cured and you are not fighting the clock.
Have everything ready before mixing the first bag — the working time is short and there is no pause button.
For a small dip, a repair mortar or a feather-finish product is often more appropriate than a whole floor pour.

Common mistakes

Pouring onto an unprimed, dusty slab — The compound bonds to the dust, and the dust bonds to nothing. It looks perfect for weeks, then lifts in sheets as soon as the floor above it is loaded or as soon as anything is stuck to it. This is by a wide margin the most common failure.
Adding extra water so it flows better — The flow is governed by the mix design, not by how much water you add. Extra water reduces the cement-to-water ratio, so the cured compound is weaker, dustier and more likely to crack and craze. It also increases shrinkage.
Pouring over a floor that is still giving up moisture — Moisture coming up through a slab pushes at the bond line and can carry salts with it. Both attack the bond. A polythene sheet test costs nothing and tells you whether the floor is ready.

If it doesn't work

Compound lifting in sheets with a dusty underside

Cause: No primer, or dust not removed — Fix: Remove all debonded material, vacuum thoroughly, prime properly, and repour. There is no repair that leaves a bad bond in place.

Fine crazing across the whole surface

Cause: Dried too fast, or over-watered mix — Fix: If it is sound and only surface crazing, it may be acceptable under a floating floor. Under a glued floor or tiles it needs a further correctly mixed and cured coat.

A straight crack across the poured area

Cause: A crack or joint in the slab below, moving, or a cold joint between batches — Fix: Determine which. A moving crack needs a movement joint carried up through the compound; a cold joint means working faster or in smaller bays next time.

Surface dusts off under a fingernail

Cause: Too much water, or the surface dried before curing — Fix: It has no strength as a bonding surface. Remove it, or in mild cases apply a consolidating primer — but a weak layer under a glued floor will fail.

Questions people ask

Can I use levelling compound over floorboards?

Only with a product specifically rated for timber and flexible floors, and usually with the boards screwed down, the gaps sealed and a primer or membrane appropriate to timber. A standard cement levelling compound over floorboards cracks along every board joint.

How thick can I pour?

Every product states a minimum and a maximum, and they vary widely between a feather-finish and a deep-fill compound. Going outside either end is a common cause of failure, so this is one of the numbers to take from the bag rather than from advice.

Do I still need to level if I am laying laminate?

You need the floor within the flatness tolerance the flooring manufacturer states, which is looser for a rigid laminate than for a thin flexible LVT but is not nothing. A floor laid over hollows clicks, creaks and eventually opens its joints.