Almost every plastered home develops hairline cracks eventually. They are part of the natural life of any building. Understanding which cracks are cosmetic and which warrant investigation saves unnecessary worry — and unnecessary expenditure.
What Causes Hairline Cracks in Plaster?
The most common cause is shrinkage as the plaster dried after application. All plaster shrinks slightly as it cures, particularly at the surface, producing a network of very fine cracks. This is entirely normal and does not indicate a structural problem.
Other causes include:
- Seasonal thermal movement as the building expands and contracts
- Vibration from traffic or nearby construction
- Age — old plaster becomes more brittle and prone to surface cracking
- New plaster applied too thickly or dried too quickly
Are Hairline Cracks a Problem?
In most cases, no. A hairline crack is by definition less than 0.2mm wide — barely visible except in raking light. These are a cosmetic issue, not a structural one.
Cracks worth investigating further are:
- Wider than 5mm
- Accompanied by sticking doors or windows
- Diagonal cracks running from the corners of door or window frames
- Cracks that suddenly appear or grow rapidly
- Any crack accompanied by damp
How to Fill Hairline Cracks
For cosmetic hairline cracks, fine-surface decorator’s filler applied with a flexible knife, sanded when dry, and then painted over is entirely adequate. For slightly wider cracks, use a flexible paintable sealant rather than rigid filler, as it will move with the building rather than cracking again.
When to Call a Plasterer
If cracks have grown beyond cosmetic, if they keep reappearing after filling, or if you want a room professionally skimmed to address widespread fine cracking, get a quote from a local plasterer.