Cracks in plaster are one of the most common concerns for homeowners. Most are harmless and easily repaired, but some indicate a more serious underlying problem. Here is how to tell the difference.
Types of Plaster Cracks and What They Mean
| Crack type | Likely cause | Action needed |
|---|---|---|
| Hairline cracks | Plaster shrinkage as it dried | Fill and redecorate |
| Map cracking (crazing) | Plaster dried too fast or was too thick | Skim over or re-plaster |
| Cracks at corners / window frames | Thermal movement or slight settlement | Flexible filler, monitor |
| Diagonal cracks from door/window corners | Minor structural movement | Monitor; if widening, get survey |
| Large horizontal or vertical cracks | Potential structural issue | Get structural survey |
| Damp plaster / blown plaster | Damp penetration | Fix damp source, replaster |
How to Test if Plaster Has Blown
Tap the wall gently with your knuckles. Hollow-sounding areas indicate the plaster has separated from the wall behind — known as “blown” plaster. Blown plaster will eventually fall off and should be hacked back and replaced, not patched over.
When Are Cracks Serious?
Cracks wider than 5mm, cracks that reappear after filling, or cracks that are accompanied by sticking doors and windows may indicate structural movement. In these cases, get a structural engineer to assess before having any plastering done.
Can Cracks Be Patched or Does the Whole Wall Need Replastering?
Small isolated cracks can be filled with fine surface filler and re-decorated. Areas of blown plaster should be hacked off and replastered; patching over blown plaster will just fail again. If a room has widespread crazing or large areas of blown plaster, a full re-skim is usually the most cost-effective solution.
Getting Quotes for Crack Repair and Replastering
A good local plasterer will assess your walls honestly and recommend the right approach. Get up to 3 free quotes to compare.