A crack in the wall has a way of catching your eye every time you walk past. The good news is that most of them are harmless. Fine hairline cracks in plaster or paint come from everyday shrinkage and movement, and you can fill and repaint them in an afternoon. A few, though, are worth a closer look: wide, diagonal or stair-step cracks that keep growing can point to movement in the structure. This guide helps you tell an ordinary crack from one that needs a professional, walks through how to fix the everyday kind properly, and points you to the right filler so it does not come straight back.

Key takeaways

• Most wall cracks are cosmetic: fine hairline or shrinkage cracks that fill and repaint easily.
• Watch for wide, diagonal, stair-step or growing cracks, which can signal structural movement.
• Fix the cause first. A crack caused by damp will return until the moisture is dealt with.
• Rake out and fill the crack properly. Painting straight over it only hides it for a while.
• If a crack is wide, growing, or comes with sticking doors, have it checked before you repair.

What causes wall cracks?

Walls move a little all the time, and cracks are usually the sign of that everyday movement. A handful of causes account for most of them.

• Shrinkage. As fresh plaster and concrete cure and lose their moisture, they shrink slightly and fine cracks appear. This is the most common cause in newer walls.
• Temperature movement. Plaster, brick and timber expand in the heat and contract as they cool, and that constant flexing opens hairline cracks, often near joints.
• Settlement. In the first few years, a building settles under its own weight, which can leave fine cracks near the corners of doors and windows.
• Moisture. Damp can swell and weaken plaster, then crack it as it dries. These cracks tend to show near the floor or around plumbing.
• Structural movement. Less often, cracks point to something deeper, such as foundation movement. These are the ones to take seriously.

What are the different types of wall cracks?

Telling the types apart is the key to knowing what to do next. Here is a quick guide.

Most cracks are cosmetic. A few patterns are worth a professional’s eye before you repair.

Type	What it looks like	Usual cause	Serious?
Hairline	Very thin lines, under about 1 mm, in plaster or paint	Shrinkage, thermal movement	Cosmetic
Shrinkage / crazing	A network of fine cracks over an area	New plaster or render curing	Cosmetic
Settlement	Fine cracks near door and window corners	A new building settling	Usually cosmetic
Structural	Wide, diagonal, stair-step or horizontal; often growing	Foundation or structural movement	Get it checked

When should you worry about a wall crack?

Most cracks are nothing to lose sleep over. A few signs, though, are worth a professional’s opinion before you reach for the filler. Call in an expert if a crack is:

• wider than about 5 mm, or growing steadily over weeks and months;
• diagonal and running from the corner of a door or window;
• a stair-step pattern following the mortar joints in brickwork;
• horizontal across a wall, which can signal serious pressure;
• joined by other signs of movement, such as doors or windows that stick, uneven floors, or water coming through.

A crack like this is telling you about the building, and a filler will only hide it. A structural engineer or surveyor can find the cause and advise on the right repair.

How do I fix a wall crack?

For the ordinary hairline and shrinkage cracks that make up most cases, the repair is well within reach of a careful DIYer.

1. Rake out the crack. Use a scraper to open the crack slightly and clear out loose plaster and dust. A clean, slightly widened crack gives the filler something to hold onto.
2. Check for damp. If the crack is near a damp patch, deal with the moisture first, or the repair will crack again. Our guide to damp walls can help you trace it.
3. Fill the crack. Work a suitable filler such as Astral Crack Fill Paste firmly into the crack, pressing it right in so there are no gaps.
4. Level with putty. Once the filler is set, skim over it with a wall putty such as Astral Acrylic or Smooth Wall Putty to bring the patch flush with the wall.
5. Sand smooth. When everything is dry, sand the repair flat so it disappears under paint.
6. Prime and repaint. Seal the patch with a primer so the colour sits evenly, then repaint. Priming stops the repair showing through as a dull spot.

How do I stop cracks coming back?

• Fill properly, rather than painting straight over. A crack skimmed with paint alone will telegraph straight back through.
• Deal with damp at the source. Moisture-driven cracks return until the water is stopped.
• Let new walls settle and cure. Give fresh plaster time to dry before painting, so shrinkage happens before the topcoat, not after.
• Use a flexible filler on movement-prone spots. Joints and edges that flex hold a repair better with a filler that can move a little.

Frequently asked questions

Are hairline cracks in walls serious?

Usually not. Hairline cracks under about a millimetre are almost always cosmetic, caused by shrinkage or ordinary movement. They fill and repaint easily. The exception is a hairline crack that keeps returning in the same place and widening.

Can I just paint over a wall crack?

No. Paint bridges a crack for a short while, then splits along it again. Rake the crack out, fill it, level it with putty, and then prime and repaint for a repair that lasts.

When is a wall crack a structural problem?

Be cautious with cracks wider than about 5 mm, diagonal cracks from door or window corners, stair-step cracks in brickwork, and horizontal cracks, especially if they are growing or come with sticking doors and uneven floors. Have these assessed by a professional.

What should I use to fill cracks in walls?

For fine cracks, a crack-filling paste worked into the opening does the job, followed by a wall putty to level the surface. For wider or moving cracks, a flexible filler holds up better. Always prime before repainting.

Why do cracks keep coming back after I fill them?

Usually because the cause is still active. Ongoing damp, movement or a structural issue will reopen a filled crack. Fix the underlying reason, and the repair holds.

Patching up cracks? Astral Crack Fill Paste and our Acrylic and Smooth wall putties give you a smooth, sound base for a lasting finish. Explore the range, find your nearest Astral Paints dealer, or call our helpline on 1800 309 9393.