Short answer
Self-leveling compound is a pourable cement-based underlayment that flows out to a flat, smooth surface. It's used when a floor isn't flat enough for the new flooring: large tile, vinyl plank and laminate all have flatness limits. The steps are: clean the floor, prime it, dam off openings, mix exactly, pour and spread within the working time, then wait the required time before installing flooring. Small dips are often fixed faster with a patching compound instead.
Flat vs level
A floor can be level (not tilted) but still not flat (it has humps and dips), and flooring mostly cares about flat. A slight overall slope rarely matters. A 6 mm dip under a tile or a vinyl plank joint does.
How flat does the floor need to be?
| Flooring | Typical flatness requirement |
|---|---|
| Tile, all sides under 15″ | No more than ¼″ variation in 10 ft (and 1/16″ in 1 ft) |
| Large-format tile (any side 15″+) | No more than ⅛″ in 10 ft (and 1/16″ in 2 ft) |
| Vinyl plank, laminate, engineered wood | Commonly around 3/16″ in 10 ft; check the product's instructions |
| Glue-down vinyl | Smooth as well as flat; every bump can show through |
Check with a long straightedge or level: lay it in several directions and measure the gaps underneath.
Self-leveler or patch?
- Patching compound (troweled on): small low spots, seams, cracks and feathering edges.
- Self-leveling compound (poured): whole rooms or large areas with many dips, or old floors with a wavy surface.
- Grinding: high spots on concrete are ground down rather than filling around them.
How it's done
- Clean and prepare. Remove old adhesive, dust and anything loose. Over concrete, the surface must be sound. Over wood, the subfloor must be stiff and screwed down.
- Prime. Primer is not optional. It helps the leveler bond, and stops the floor sucking water out of the mix, which causes pinholes and weak spots.
- Dam and seal. Block doorways and floor openings and seal gaps, because self-leveler runs like pancake batter.
- Over wood subfloors, use a product rated for wood, usually with metal lath or reinforcement, and the minimum thickness it specifies.
- Mix exactly. Measure the water precisely. Too much water weakens it and makes the surface dusty.
- Pour and spread quickly with a gauge rake, then go over it with a spiked roller to release air. Working time is short, often 10–20 minutes, so larger floors need a crew working fast.
- Let it dry. Many levelers take foot traffic in a few hours and tile the same or next day. Moisture-sensitive flooring such as vinyl, laminate and wood needs it drier, which can take days depending on thickness. Follow the product.
Common problems
- Cracking or not bonding: no primer, a dirty floor, or a flexing wood subfloor.
- Pinholes and bubbles: no primer on porous concrete.
- Soft, dusty surface: too much water in the mix.
- Flooring problems later: flooring installed before the leveler was dry enough.
Is it a DIY job?
A small bathroom can be manageable. Larger areas are a race against the working time, and a bad pour is hard to fix: it usually means grinding it down or pouring again. If you're getting new flooring installed, floor preparation should be part of the quote. Ask how flatness will be checked and corrected (see preparing for flooring installation).
