Why Is My Plaster Cracking?
Most plaster cracks are cosmetic and the trick is spotting the few that need a closer look.
The short answer
Plaster cracks mainly because it shrinks as it dries, especially when heating, sun or draughts dry it too fast. Thick coats that debond, damp, poor mixing, painting too soon and small movements in the building also cause cracks. Cracks under 5mm wide can be filled and painted yourself, while wider or growing cracks need a plasterer or structural engineer.
Cracks in plaster turn up in brand-new houses and old ones alike, and most of them look far worse than they are. They range from fine lines running across a freshly skimmed ceiling to wider splits above a door frame where the house has moved a little over the years.
Width is the thing to look at first. A fine crack under 5mm wide is something you can fill and paint over yourself, while anything wider is worth a second opinion. Below we run through ten common causes, how to tell them apart and how to repair the small ones with a good wall filler and a bit of redecorating.
Hairline Cracks Versus Wider Cracks
Hairline cracks are the thin, wandering lines you spot when the light hits a wall or ceiling from the side. They come from the plaster itself, either shrinking as it dries or reacting to heat and moisture in the room, and they rarely get much worse once the plaster has settled.
Wider cracks, and especially ones that run diagonally from the corner of a window or door, tell you more about what's happening behind the plaster. These are the ones to measure, mark and keep an eye on before you reach for the filler.
Why Does Plaster Crack?
1. Shrinkage As The Plaster Dries
Fresh plaster loses a lot of water in its first few hours on the wall, and if it gives that water up too quickly the surface shrinks and splits. Plasterers call this plastic shrinkage, and it shows up as a web of fine lines that's among the easiest cracking to fix.
Plaster keeps losing moisture for a while after it feels dry to the touch, so a second round of fine cracks can appear weeks later. This dry shrinkage is just as common and nothing to worry about, as long as the cracks stay thin.
2. Debonding Between Coats
Debonding happens when the inner layer of plaster shrinks at a different rate to the outer layer, and a coat laid on too thickly is the usual culprit. Tap the wall near the crack and you'll hear it sound hollow, which tells you there's a gap behind the surface.
Small hollow patches can be left alone if the plaster is sound around them. Larger ones need cutting back and patching, as filler on its own won't stop the loose section moving.
3. Movement In The Building
Structural cracks sound alarming, but the slightest settling in the foundations, shrinkage in the brickwork or expansion in a roof slab is enough to cause them. They form vertical or diagonal lines across walls and ceilings and they start at the weak points, such as the corners of windows and doors.
Fill these only once you're sure they've stopped growing. A pencil mark at each end of the crack with the date written beside it is a simple way to check over a few weeks.
4. Grinning Mortar Joints
Grinning is when the pattern of the mortar joints shows through the plaster as faint lines or cracks. It comes from the difference in suction between the cement mortar and the bricks, so the plaster dries unevenly over the two. It looks untidy but it won't make existing cracks worse, and filler and a fresh coat of paint sort it out in most rooms.
5. A Poor Plaster Mix
A badly mixed batch causes all sorts of trouble. Too little binder leaves the plaster soft once it sets. Contaminant particles in the mix are the other problem, as they react with moisture and expand after the plaster has gone on, pushing out small craters that plasterers call popping. Clean water, clean tools and good quality plaster from a reputable supplier keep this to a minimum.
6. Expansion From Damp
Wet wall coverings, high humidity and poor air circulation can make plaster swell and soften as well as crack. Bathrooms, kitchens and rooms where washing is dried indoors are the usual trouble spots, and the crack keeps coming back until the damp is dealt with.
7. Drying Too Quickly
New plaster needs time, and rushing it is one of the commonest reasons for cracks on a fresh wall. Our guide on how long plaster takes to dry covers the timings for skim coats and backing plaster. Cranking up the central heating or putting a powerful dehumidifier in a small room straight after the plasterer leaves pulls the water out too fast, so let the room dry naturally with a window open for the first stretch.
8. Wind And Direct Sun
Plaster left open to the weather while it dries is up against two things. A constant draught from an open doorway or a missing window dries the surface faster than the plaster underneath, and strong sun through the glass does the same. Both leave the surface shrinking ahead of the rest of the coat, which is what pulls it apart.
9. Painting Too Soon
Bare plaster isn't much to look at, but paint it before it has fully dried and the moisture gets trapped behind the paint film. That can lead to cracking, flaking and patches that never quite look right. Our guide to painting new plaster with a mist coat explains how to tell when a wall is ready.
10. Poor Preparation
Plaster is only as good as what's underneath it. Skimming over loose paint, dust or leftover scraps of paper gives the new coat nothing solid to hold on to, so strip every bit of old wallpaper and scrape back anything flaky before the plasterer arrives.
How To Repair Cracks In Plaster
From our own experience, cracks under 5mm wide are a straightforward fill and redecorate. Anything over 5mm is worth showing to a plasterer, and once a crack goes past 15mm wide you need a structural engineer to look at it before anyone patches it up.
For those smaller cracks in walls and ceilings, the repair goes like this:
- Rake out any loose plaster along the crack with the corner of a scraper, then vacuum out the dust.
- Press a fine plaster filler for cracks into the crack with a flexible filling knife.
- Draw the knife across the crack to leave the filler smooth and level with the wall.
- Add a second layer once the first has set if the filler has sunk back.
- Leave the filler to dry fully, following the time on the tub.
- Sand any proud filler flush with fine sandpaper and wipe off the dust.
- Spot prime the repair, then paint the whole wall or ceiling so the patch doesn't show.
If a filled crack keeps opening up in the same spot, the wall is still moving. A flexible decorator's caulk copes better with small seasonal movement along a ceiling joint, but a crack that keeps widening needs the cause finding first. Stains around a crack are another sign of damp, and a stain block paint stops them bleeding through the new paint once the leak is fixed.
When A Plaster Crack Needs A Professional
Most plaster cracks are cosmetic and nothing to lose sleep over. Call in a plasterer or a structural engineer if a crack is wider than 5mm, runs diagonally from a window or door corner, is wider at one end than the other or keeps growing after you've marked it. Doors and windows that start to stick at the same time point the same way.
Watch Us Do It
Repairing Cracks Has Never Been So Easy With The Latest Wall Fillers! Watch Us Test This Filler!
Quick Questions And Answers
Are cracks in new plaster normal?
Yes, fine cracks in new plaster are normal and come from the plaster shrinking as it dries out. They appear in the first few weeks, especially if the heating was on high while the plaster was drying. Leave the plaster to dry fully, fill the cracks with a fine surface filler and they won't come back once the wall has settled.
Should I be worried about cracks in my ceiling?
Thin ceiling cracks along plasterboard joints or around light fittings are rarely a worry and can be filled and painted. Look harder at a ceiling crack that is wider than 5mm, sags, has a brown water stain or keeps getting longer. Those point to a leak or movement above, so get a plasterer or builder to check it before you redecorate.
What is the best filler for cracks in plaster?
A fine ready-mixed surface filler is the easiest choice for hairline cracks in plaster because it spreads thinly and sands smooth. Deeper cracks take a powder filler mixed to a stiff paste and built up in layers. For cracks along ceiling edges that open and close with the seasons, a flexible decorator's caulk holds up better than a rigid filler.
Can you paint over cracked plaster?
You can paint over cracked plaster once the cracks are filled and sanded flush, but painting straight over an open crack won't hide it. Paint sinks into the gap and the line shows through within weeks. Fill the crack, let the filler dry, spot prime the repair so it doesn't flash through, then paint the whole wall for an even finish.
How do I tell if a plaster crack is still growing?
Mark it and wait. A pencil line across each end with the date beside it gives you something to measure against, and a few weeks of checking tells you whether it's still on the move. If it creeps past your marks or opens wider at one end, hold off with the filler and get a plasterer or structural engineer to look at it.
Should I turn the heating up to dry new plaster faster?
No. It's tempting when you want to get on with decorating, but high heat or a powerful dehumidifier in a small room draws the water out of the surface faster than the plaster behind it can keep up, and the skin cracks. A window left open and a bit of patience does a better job for the first stretch.
Why does a filled crack keep coming back?
Something behind it is still going on. The first suspect is movement, and along a ceiling joint that opens and closes with the seasons, flexible decorator's caulk will last where rigid filler won't. Damp is the other cause, and no amount of filling helps until the source of the water is fixed. A crack that keeps getting wider needs its cause tracked down first.
Does a hollow sound when I tap the wall mean the plaster has failed?
Not always. It means the coats have come apart and left a gap, known as debonding. A small hollow patch surrounded by sound plaster is fine to leave, but a larger area wants cutting out and patching, because filling the crack on its own does nothing to hold the loose section in place.
Dave, or Demolition Dave as the team calls him, worked as a builder for over 10 years before joining DIY Works. He now concentrates on his own properties and writes most of our construction content, along with tools, carpentry and plenty more.