How flat does a wall need to be before tiling?
Tile follows the wall. Every hollow and hump appears in the finished surface as lippage, and adhesive is not the material to correct it with.
- Difficulty
- intermediate
- Time
- 40 min
- Read
- 2 min
Short answer
Check with a long straightedge in several directions and note the size of the worst deviation. Small deviations can be taken up within the adhesive bed; anything larger has to be corrected in the wall first, by taking humps down or by skimming or boarding. The larger the tile, the tighter the requirement.
This is the step most often skipped, and the one that produces the fault most often complained about afterwards. A tiled surface is rigid and flat by nature, so it cannot follow an undulating wall gently — it steps at each joint instead, which is what lippage is.
Step by step
- Check the wall with a long straightedge, not a short level.Hold a two-metre straightedge against the wall horizontally, vertically and diagonally, and look for daylight underneath. A short level reports local slope and misses broad hollows entirely.
- Measure the worst gap rather than eyeballing it.Slide something of known thickness into the largest gap. That measurement is what you compare against the tile manufacturer's flatness requirement.
- Understand that the tolerance tightens with tile size.A small tile spans a short distance and steps a little. A large tile spans further, so the same hollow produces more lippage. This is why big tiles feel so much less forgiving.
- Take humps down before filling hollows.Grinding or scraping a proud area is quicker than building everything around it up. Re-check afterwards, because removing the high spots often brings the wall inside tolerance on its own.
- Correct remaining deviation in the wall, not the adhesive.Skimming, a suitable levelling render or overboarding are the tools. A thick adhesive bed under one edge cures unevenly, shrinks more and is the classic cause of a tile that later sounds hollow.
- Deal with a bowed or leaning wall as a separate question.A wall out of plumb but flat is tileable. A wall that bows in the middle is not, and battening and boarding over it is usually easier than trying to build it out with plaster.
- Check again after any preparation.Two minutes with the straightedge before the first tile is the last cheap moment.
Common mistakes
- Using adhesive to make up an uneven wall. — Adhesives have a maximum bed thickness for good reason. Beyond it they shrink, cure unevenly and lose bond, and the tile ends up hollow at the thick end.
- Checking with a spirit level only. — That tests plumb, not flatness. A wall can be perfectly plumb and badly bowed.
Questions people ask
What is the actual tolerance?
It comes from the tile manufacturer and is stated as a deviation over a length. It varies by tile size and format, which is exactly why no figure is quoted here.
Will levelling clips fix an uneven wall?
No. They align adjacent tiles to each other across a joint. A wall that deviates over a metre is still deviating, and the clips just spread the step over more joints.