The first time I used self-levelling compound I did not prime the concrete floor first. I mixed the SLC, poured it, and watched it go off within about eight minutes - nearly double the speed the instructions had suggested. The surface cured with a sandy, weak texture rather than the hard smooth finish it should have had. The porous concrete had drawn moisture out of the mix too fast. I had to grind it off and start again.
Self-levelling compound is a product that rewards doing the preparation properly. The pouring step takes about fifteen minutes. The preparation takes most of the morning.
What you need
- Self-levelling compound (calculate bags using the Self-Levelling Compound Calculator)
- Primer appropriate for your substrate
- Bucket and mixing paddle/drill
- Spreader or gauge rake
- Spiked roller (de-aeration)
- Tape or foam to seal edges/gaps
- PPE: dust mask when mixing, gloves
Step 1: Assess and prepare the substrate
Self-levelling compound will not fix a substrate that is not sound. Before you start:
Check for:
- Hollow areas (tap with a hammer - a hollow sound means delamination)
- Cracks wider than 1mm
- Contamination: oil, paint, adhesive residue, dust
Fix before applying SLC:
- Hollow areas: either remove and reinstall, or inject epoxy resin repair
- Cracks: fill with repair mortar or epoxy crack filler and allow to cure fully
- Contamination: degrease, scrape or grind as appropriate
- Dust and debris: vacuum thoroughly - this is more important than it sounds
Moisture check: If applying over an existing screed or slab, check the moisture content. For most SLC products, the substrate should be below 75% relative humidity. If moisture is high, the SLC may not bond correctly or may cause problems with the floor covering above it.
Step 2: Seal any low spots or drainage holes
SLC flows - that is its whole purpose. Before pouring, seal:
- Gaps at floor/wall junctions (foam strip or tape)
- Drain covers (tape over with duct tape)
- Doorways (a bead of silicone or strip of foam set into the wet primer)
- Holes in the substrate
If you skip this step and SLC escapes under a wall or through a crack, you cannot get it back.
Step 3: Apply the primer
This is the most important step.
Read the product data sheet for your specific SLC - it will specify which primer to use and the dilution ratio. Some primers go on undiluted; some are diluted 1:5 with water for a first coat on very porous substrates.
Apply with a brush or roller and work it in. On very porous substrates, you may need two coats - one diluted to penetrate, one full strength to seal. Let the primer become touch-dry (typically 30-60 minutes) but do not let it fully dry before pouring - slightly tacky is ideal.
Do not skip the primer. This causes:
- Too-fast set (porous substrate absorbs water from the mix)
- Delamination (SLC lifts from the substrate after curing)
- Weak surface that dusts or crumbles
Step 4: Mix the SLC
Most SLC products require the powder to be added to the water (not water to powder). Follow the product ratio exactly - the water content is critical. Too much water weakens the finished surface; too little affects the flow.
Mixing tips:
- Use a drill with a mixing paddle (whisk) rather than hand mixing - it is much faster and more consistent
- Mix for the full time specified (usually 2-3 minutes) until lump-free
- Let the mix stand for 1-2 minutes after mixing, then re-mix briefly (this eliminates air trapped during mixing)
- Work quickly - most products have a pot life of 15-30 minutes
Step 5: Pour and spread
Pour the SLC starting from the far corner of the area and work towards the door. Do not pour it all at once into one spot - move along and pour in strips or zones.
Use a gauge rake or spreader to guide the product to the correct depth. On a flat floor, SLC will self-level within its limits - but if your floor is significantly uneven, you need to help it reach all the low spots.
Immediately follow with a spiked roller. This de-aerates the surface - it pops any air bubbles in the mix and leaves a smooth, hole-free finish. Roll in two directions.
Work in sections if the area is large, but blend the sections quickly before the first pour sets.
Step 6: Protect and allow to cure
Most SLC products allow foot traffic after 1-4 hours and are ready for tiling within 12-24 hours. Check the specific product data sheet.
During curing:
- Do not allow draughts or direct sunlight - these cause uneven drying and surface cracking
- Minimum temperature is usually 5°C - do not apply in cold conditions
- Do not walk on the surface until the specified time has passed
Common mistakes
| Mistake | Consequence |
|---|---|
| No primer | Fast set, weak surface, delamination |
| Wrong primer for substrate | Poor adhesion |
| Gaps not sealed | SLC escapes, level drops |
| Mixing too thin | Weak surface |
| Not using spiked roller | Air bubbles, pinholes in surface |
| Working in cold conditions | Delayed set, weak surface |
| Hollow substrate not repaired | Delamination |
My tips on self-levelling compound
Read the TDS before you buy the primer. Different SLC products specify different primers - in some cases, a specific primer from the same manufacturer. Using the wrong primer can be as bad as no primer at all.
Pour more than you think you need for the low areas. SLC will self-level, but if you underestimate the depth needed in a low spot, it will level to the wrong datum. Err slightly on the side of too much rather than too little.
Combine sections quickly. If you pour section one and then spend five minutes mixing section two, the seam between them will crack. Mix section two before pouring section one finishes, or use a large enough team to keep up.
Test flatness before committing to tiling. A long straight edge and feeler gauges will tell you if the SLC result is within flatness tolerance for your floor covering. Most standard tiles need 3mm under 3m; large format tiles need 2mm under 3m.
Use the Self-Levelling Compound Calculator to work out how many bags you need before you start.