How do I flatten a wavy, bumpy wall?

Measuring how far out the wall actually is, then choosing between filling, skimming, battening and boarding, or lining paper — because the size of the deviation decides the method.

Difficulty: intermediate
Time: 4 hr
Read: 4 min

Short answer

Hold a two-metre straightedge against the wall in several directions and measure the biggest gap. Under about 3mm, fill and sand. Three to about 10mm, a skim coat over the whole wall. More than that, or where the wall leans, batten out and board — you cannot build 20mm of plaster onto a wall in one job and expect it to behave.

"Wavy" covers everything from a wall that needs a careful filling job to a wall that is 30mm out from top to bottom, and the methods do not overlap. Measuring first is not perfectionism — choosing a method for a deviation it cannot handle is how people end up filling a wall four times and still seeing the shadow.

What you'll need

A 2m straightedge or a clean length of aluminium
A long spirit level
Torch for raking light
Pencil

Step by step

Find the high points first.Run the straightedge across the wall horizontally, vertically and diagonally, and mark where it touches. The high points define the surface you are working to, because everything else has to come up to them.
Measure the largest gap.Slide a tape or a set of feeler gauges into the biggest hollow. This one number chooses the method, so measure it rather than estimating.
Check whether the wall is out of plumb as well as uneven.A long spirit level top to bottom. A wall that leans is a different problem from a wall that undulates, and it is fixed by battening or boarding rather than by any thickness of filler.
Under about 3mm: fill and sand.A broad blade and a fine surface filler worked across the hollows in thin passes, feathered well out. Prime before judging, because bare filler and bare wall look different regardless of flatness.
Three to about 10mm: skim the whole wall.This is what a skim coat is for. Prepare the background properly, and accept that a plasterer will follow the wall to some degree unless they are ruling to a datum, so agree what you are paying for.
Ten millimetres and up, or a leaning wall: batten and board.Timber battens fixed to the wall and packed out to a plumb, flat plane, then plasterboard over them. It is the only method that produces a genuinely flat wall independent of what is behind it, and it is carpentry rather than plastering.
Consider dot and dab as the middle route.Board bedded on adhesive dabs, tapped back to a line. It handles deviations up to the depth of the dabs, needs no framing, and is quicker than battening — but it is not easy to get truly flat without practice.
Set out from the worst point, not the best.Whichever method, the finished plane has to clear the highest point on the wall. Working out from the flattest area and hoping is what produces a wall with a bulge in one corner.
Check the consequences before committing.Battening and boarding brings the wall forward 20 to 40mm, which affects skirtings, architraves, window reveals, sockets, radiator pipes and door swings. Plan those before the first batten goes up.

Common mistakes

Trying to build out a large hollow with filler — Filler shrinks, cracks and is expensive in volume. Beyond a few millimetres it is the wrong material and the repair will show as a patch.
Assuming a skim will make the wall flat — Skim is a surface treatment 2 to 3mm thick. Unless the plasterer is deliberately floating the wall to a datum with a backing coat, it follows the shape underneath.
Battening without checking the plane first — Battens fixed straight to an uneven wall reproduce the unevenness exactly. The packing is the entire job.

Alternatives

Fill and sand: For deviations of a couple of millimetres. Cheap, undramatic, and the correct answer far more often than people expect. Needs good light and patience rather than skill.
Skim coat: Two to three millimetres of finish plaster over the whole wall. Refreshes the surface and takes out modest undulation. Will not correct a leaning or badly bowed wall.
Heavy lining paper: Hides texture and minor unevenness for very little money. Does nothing about shape, and raking light still shows the wall's form.
Batten and board: Timber framework packed to a flat plumb plane, then plasterboard. The reliable answer for a badly out-of-true wall, and it creates a service void as a side benefit.
Dot and dab board: Plasterboard on adhesive dabs tapped back to a line. Faster than battening, handles moderate deviation, and creates the fixing complications that dry lining always creates.

Questions people ask

How flat is flat enough?

For a painted wall in ordinary light, deviations under about 3mm over two metres are not noticeable. Under a wall light, in raking sunlight, or behind a large mirror or a gloss finish, the tolerance is much tighter.

Why do old walls wander so much?

They were built to a different standard, they have moved over a century or two, and the plaster was applied by hand over an uneven background. It is normal, and in a period property it is part of the character rather than a defect.

Will fitted furniture cope with a wavy wall?

Not well. Kitchen units, wardrobes and bookcases all need scribing to a wall that is out of true, and a very uneven wall may be the reason to batten and board that room specifically.