Why does it only look bad in certain light?

Because raking light exposes flatness. Light travelling almost parallel to a wall throws a shadow off every hollow and ridge, which is why a wall looks fine at midday and wrong beside a window in the evening. The wall has not changed.

Can I sand a wavy wall flat?

No. Sanding takes material off the high spots without filling the low ones, so the surface stays uneven and the shadows often get worse. Local hollows are filled with a smoothing compound; a wavy wall is corrected in the background and re-skimmed.

Does it matter if I am tiling over it?

It matters more. Tiles are rigid and cannot follow a hollow, so an out-of-tolerance wall shows up as lippage between tiles and as poor adhesive contact. Getting the plaster flat first is cheaper than a tiler working around it.

Low risk

Updated 30 August 2026

Wavy Or Uneven Plastered Walls

Q: How flat should a plastered wall be?

The trade test is a 2m straightedge on the surface. The Tile Association's guidelines for tile fixing use plus or minus 3mm under a 2m straightedge for finished work, and a wall that is going to be tiled needs to meet that kind of tolerance.

A newly plastered wall shows hollows, ridges or trowel marks, usually worst in low sunlight.

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What's happening?

Flatness is measured, not argued about. The trade test is a straightedge: a 2m rule laid on the surface, and the gap under it checked. The Tile Association's guidelines for tile fixing use plus or minus 3mm under a 2m straightedge as the tolerance for finished work, and a wall that is going to be tiled has to meet something like that or the tiles will not sit. A wall you are only painting is judged by eye, and the eye is brutal under raking light — light skimming across a surface at a shallow angle turns a 2mm hollow into a shadow you cannot stop looking at. That is why walls that pass a straightedge can still look wrong beside a window, and why the fix is usually about flatness of the background rather than the skill of the final trowel.

What usually causes it?

Background not dubbed out first, so a thin skim was asked to correct hollows it cannot fill
No beads or screeds set to work to, leaving the plasterer nothing to rule the surface flat against
Dot and dab boards fixed on uneven dabs, so the board face itself follows the wall behind it
An old wall that is out of plumb or bellied, skimmed to follow its shape rather than straightened
Skim applied too thick or over-trowelled, leaving ridges, ripples and burnished patches
Plaster allowed to go off before it was finished, so the last passes dragged rather than flattened
Existing wallpaper, textured coating or paint left on the wall, telegraphing through the new finish

Is it dangerous?

No. This is entirely a quality-of-finish fault, with no safety or structural consequence. It matters for two practical reasons instead. Tiling and heavy wall coverings will not lie on a wall that is out of tolerance, so an uneven wall costs money again at the next trade — the Tile Association's straightedge tolerance exists because a tiler cannot correct a plasterer's hollows with adhesive. And repeated attempts to fix it with filler and sanding usually make it worse, because sanding a hollow wall flattens the high spots and deepens the shadows rather than levelling the surface.

Can I fix it myself?

Assessing it, definitely, and it is worth doing before anyone starts arguing. Hold a 2m straightedge against the wall in several places and note where the gaps are and how big; shine a light along the surface at a shallow angle and photograph what shows. Small local hollows can be filled with a smoothing compound, sanded and painted. What does not work is trying to skim over a wavy wall yourself, sanding a whole wall to level it, or hanging heavy paper to hide it. A flatter matt paint reduces the shadowing considerably and is a legitimate short-term answer.

When should you call a plasterer?

Call the plasterer back where a straightedge shows gaps beyond the tolerance the next trade needs, where trowel ridges or burnished patches are visible in ordinary light, or where the wall is due to be tiled. Say what you measured rather than that it looks wrong, because that is the conversation that gets resolved. Call before tiling rather than after, since correcting the background is far cheaper than lifting new tiles off it.

What will the plasterer actually do?

Measure the surface with a straightedge and raking light to establish where it is out and by how much
Dub out hollows in a backing coat, and set beads or screeds so the finished face can be ruled flat
Straighten the wall as a background rather than trying to correct it in the finish coat
Re-skim in correct thicknesses, timing the trowel passes so the surface is flattened rather than dragged
Bring the wall to the flatness the next trade needs, tighter where tiling or a large-format finish is planned
Remove or seal old paper, paint or textured coatings that would otherwise telegraph through the new plaster

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Hairline Cracks In Newly Plastered WallsLow risk Cracks Along Plasterboard Joints And Nail PopsLow risk Uneven Tiles With Edges Standing ProudLow risk Tiles That Sound Hollow When TappedCaution

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