What causes cracks in plaster?

Almost every house has a crack or two in the plaster. Most are cosmetic and easy to fix, but some are worth taking more seriously. The trick is knowing the difference. Where a crack is, how wide it is, what shape it takes and whether it comes back after filling all tell you something about what's causing it. Here's a practical guide to the common types and what to do about them.

Are hairline cracks in plaster normal?

Hairline cracks are usually nothing to worry about. They happen as plaster dries and shrinks slightly, and as the house naturally moves with changes in temperature and humidity through the seasons. Newer homes often show a crop of them in the first year or two as timber dries out and the building settles. A flexible filler and a touch-up of paint usually sorts them. If the same fine crack keeps reappearing in the same place, bridging it with a flexible repair tape before filling can help stop it showing through again.

What causes cracks in new plaster?

If you're seeing a lot of cracks in fresh plaster within weeks of the job, something has usually gone wrong. Common causes include:

• Plaster applied too thick in one go
• Drying too fast, often from heaters or dehumidifiers on full blast early on
• Plastering onto an unsuitable or poorly prepared surface
• Board joints that weren't taped with scrim, or boards that weren't fixed securely
• Backing coats that weren't left long enough before the skim went on

The odd fine shrinkage crack can still appear on a good job, particularly around the corners of windows and doors. Widespread cracking or crazing across a whole wall is different, and it isn't something you should have to live with.

Why does plaster crack where the wall meets the ceiling?

Cracks at the wall and ceiling joint are very common. Ceilings and walls move slightly differently, especially where timber joists meet masonry walls, and that line is where the movement shows. It's usually solved by fitting coving, which covers the joint and looks smart, or by running a flexible decorator's caulk along the joint before painting. Cracks in plasterboard ceilings that follow the board edges are also common, usually because the joints weren't taped properly or the boards weren't fixed firmly enough to the joists.

When is a crack in plaster a sign of something serious?

Larger cracks are a different matter. Watch out for:

• Cracks wider than a few millimetres
• Stepped cracks that follow the brickwork in a staircase pattern
• Diagonal cracks spreading from the corners of windows and doors that are getting wider
• Cracks that keep opening up after repair, or are wider at one end than the other
• Doors and windows that have started sticking around the same time

These can point to movement in the structure, a problem with a lintel, or damp. If you see them, get the wall checked by a structural engineer or surveyor before any plastering is done. Filling and skimming over active movement only hides the problem for a while, and the crack will usually come back through the new finish.

Can damp cause plaster to crack?

Yes. Damp weakens plaster and can cause it to crack, bubble, crumble or come away from the wall. You'll often see staining, flaking paint or a white powdery residue, which is salts brought to the surface as the moisture dries out. The cause of the damp has to be found and fixed first, whether it's a leak, a blocked gutter, condensation or something wrong with the wall. Plaster that's been contaminated with salts usually needs removing and replacing, as it will keep drawing in moisture and spoiling any new finish.

How do you repair cracked plaster?

The right repair depends on the cause. For fine cracks, rake them out slightly, brush out the dust, fill with a flexible filler, sand smooth and repaint. Wider cosmetic cracks can be cut back, taped and filled, or the area patched with fresh plaster. Where a wall is covered in cracks, or the plaster has blown behind them, a skim coat or a re-plaster of the area gives a longer-lasting result than endless filling. On ceilings, badly cracked or sagging plaster may need over-boarding or replacing.

If you've got cracks in walls or ceilings in Leeds or around Yorkshire, Darren can take a look and tell you whether it's a simple repair or something worth getting checked first. Get in touch for a free quote.