How to repair cracks in plaster walls — and when to call a pro
Honest advice on fixing hairline cracks, spider cracks and cracks in ceiling plasterboard — what you can sort yourself and when it needs a proper re-skim.
Cracks in plaster are one of the most common things I get asked about. The good news: most small cracks are cosmetic and easy to deal with yourself. The bad news: some cracks are a sign the plaster has failed underneath, and filling those is just papering over the problem.
Here's how to tell what you're looking at, what you can fix with a tube of filler on a Saturday afternoon, and when it's worth getting a plasterer in to do it properly.
How do I fix hairline cracks in plaster walls?
Hairline cracks — the thin, single lines you can barely fit a fingernail into — are almost always cosmetic. They're caused by the plaster shrinking slightly as it ages, by normal seasonal movement, or by the building settling over the years. They don't mean the wall is failing.
To fix one: rake the crack open very slightly with the corner of a scraper or filler knife (this gives the filler something to grip), dust it out, then push a flexible decorator's caulk or a fine-surface filler into it with your finger or a small filler knife. Smooth off, let it dry, lightly sand, then touch up with paint. Job done in 20 minutes.
For a wall covered in lots of hairline cracks, a lining paper before painting often gives a much better finish than trying to fill them all individually.
What are spider cracks and how do I fix them?
Spider cracks (sometimes called crazing or map cracking) are clusters of fine cracks that branch out in a web pattern. They normally show up across a whole wall or ceiling, not just one spot.
They're usually caused by the skim coat being applied too thick, dried too fast (heater blasted at it, hot summer day), or laid over an unsuitable surface. The cracks themselves are surface-deep — the plaster behind is generally still sound.
Small areas you can fill with fine-surface filler as above. But if a whole wall or ceiling is crazed, the proper fix is a fresh skim coat over the top. Filler-and-paint will hide it for a few months, but the pattern usually telegraphs back through. A 2–3mm skim seals it in for good.
How do I repair cracks in a ceiling plasterboard?
Cracks in ceiling plasterboard are normally one of two things: a cracked joint between two boards (often along a straight line), or a crack across the face of a board (usually shorter and more random).
A cracked joint is a sign the original scrim tape has failed or wasn't put in. Fix: rake out the crack, push in fresh fibreglass scrim tape, then a couple of thin coats of jointing compound over the top, feathered out. Sand smooth and paint.
A crack across the face of a board is normally cosmetic and a flexible filler will sort it. But if the crack keeps coming back in the same spot, something's moving — usually a loose board or a joist below that's flexing. That one needs looking at properly before you fill it again.
One important warning: if you see a crack with a sag, a brown stain, or daylight through it — stop. That's water damage or a structural issue and needs investigating before any plastering work.
When should I call a plasterer instead?
Honest answer — call someone in when:
• The same crack keeps reappearing after you've filled it twice. Something underneath is moving or the plaster has lost its bond.
• The crack is wider than about 3mm, or you can push a knife blade in and feel it go through to the brick or board behind.
• Tapping the wall either side of the crack sounds hollow or drum-like — that's blown plaster and filler won't save it.
• The whole wall is crazed with spider cracks. A fresh skim coat is the right fix, not endless filling.
• It's a ceiling crack with sag, staining or movement. Get that looked at before it comes down. See our ceiling repairs page for what we do.
If you're not sure which camp your crack falls into, send me a photo on WhatsApp and I'll tell you honestly whether it's a DIY job or worth a proper quote.
The bottom line
Small cosmetic cracks: filler, sand, paint, move on with your life. Recurring cracks, hollow-sounding plaster, or a whole wall of crazing: get it skimmed properly. Filling failed plaster is the number one way people waste money on the same wall twice.
Frequently asked
- What's the best filler for hairline cracks in plaster?
- A fine-surface filler (Polycell Fine Surface or similar) for very thin cracks, or a flexible decorator's caulk for cracks that open and close with seasonal movement. Avoid stiff powder fillers in hairline cracks — they don't push in properly.
- Why do cracks keep coming back in the same place?
- Either there's movement in the structure (a flexing joist, a settling lintel), or the plaster around the crack has lost its bond with the wall behind and is moving as a sheet. Either way, filling it again won't fix it — it needs investigating.
- Can you skim over a wall with lots of cracks?
- Usually yes, as long as the underlying plaster is sound. We rake out and fill the cracks first, sometimes bed scrim tape over the bigger ones, then skim the whole wall for a flat, crack-free finish.
- Do you charge to come and look at a cracked wall?
- No. Quotes are always free. If it's a 5-minute DIY job I'll tell you that too — I'd rather be straight with you than sell you work you don't need.