Why are my tiles uneven, with one edge standing proud of the next?

Lippage is the fault people notice with a bare foot before they notice it with their eyes, and it is the one that most often gets blamed on the tiler when the substrate was the problem. Working out which of the three causes you have matters, because two of them cannot be fixed with a bag of levelling clips.

What you'll need

• long straight edge or spirit level
• tile levelling clips and wedges (optional)
• rubber mallet (optional)
• a torch to rake light across the surface

Step by step

1. Rake a light across the finished surface.A torch or lamp held low and close to the surface throws a shadow off every raised edge. This shows you the pattern of the problem, which is what identifies the cause.
2. Read the pattern. Ridges over a broad area mean the substrate.If the lippage follows a gentle hump or dip across several tiles, the wall or floor is not flat and the tiles are simply following it. Adhesive is not a levelling material.
3. Ridges at the middle of the long edge mean bowed tiles.Large format tiles, especially rectified porcelain, are often slightly curved along their length. With a half offset the crown of one tile meets the end of the next, which is where the ridge appears.
4. Random single high tiles mean uneven pressure.One tile pressed harder, or bedded on a thinner patch of adhesive, or dropped into place rather than pressed and slid.
5. Check with a straight edge, not by eye.Lay a long straight edge across the tiles in several directions and look for light underneath. It tells you whether the surface as a whole is out or whether individual tiles are.
6. If the adhesive is still soft, correct it now.Levelling clips and wedges clamp adjacent tile faces flush while it sets. A tile that is low can be lifted, more adhesive added and re-bedded; a tile that is high can be tapped down. After the adhesive sets, none of this is available.
7. If it has set, grind rather than replace, if the tile allows.A small ridge can be taken off with a diamond pad, wet, on ceramic or porcelain — it dulls the glaze, so it is a compromise and not always acceptable. Wet working also keeps the silica dust down, which is not optional.
8. Prevent it next time by fixing the substrate.Check the wall or floor with a straight edge before any tiling. Fill or float a wall, use levelling compound on a floor, and accept that the flatter the substrate needs to be, the larger the tile.

Tips

• Reduce the offset from a half to about a third on long tiles. It is the single most effective change for bowed large format tiles and costs nothing.
• Levelling clips help hugely with big tiles, but they do not make a bad substrate flat and they do not straighten a bowed tile — they clamp faces flush at the joint only.
• Lay the first few tiles, then stop and check with a straight edge before committing to the whole wall. It is the last cheap moment.
• Wider grout joints tolerate more lippage than narrow ones, because the eye reads the joint as a deliberate step rather than an error.

Common mistakes

• Using extra adhesive to bury an uneven substrate — A thick soft bed shrinks unevenly as it cures and does not support the tile properly. The tile ends up lippy anyway, and often hollow or debonded as well. Flatten the substrate first with the right material for the job.
• Buying large format tiles for an unprepared wall — The bigger the tile, the less it can follow a curve and the more any deviation shows at the joint. A 600mm tile on a wall that is 5mm out over a metre will always show lippage; a 100mm tile on the same wall will not.
• Checking only at the end — Lippage is adjustable while the adhesive is soft and effectively permanent afterwards. Checking each row as it goes is what makes it fixable.

If it doesn't work

Questions people ask