Cracks appearing a month after plastering: what they mean

2 days ago
3 min read

Introduction

A month after a room was skimmed, the customer sends a photograph of a hairline crack running diagonally from a door head. They have decorated since, so the crack is now visible in fresh paint, and they would like to know why a new plaster job has cracked.

Most cracks in new plaster are not plaster faults. They are the wall, the ceiling, the timber or the building moving, and the plaster is simply the surface where that movement becomes visible. But explaining that to somebody who has just paid to have a room made good requires more than an assertion. It requires showing them.

1. Cracks appearing a month after plastering usually tell you where the movement is

The location and shape are the diagnosis. Read the crack before you speak.

Cracks at openings and junctions

Diagonally from door and window heads, along wall-to-ceiling junctions, at changes of material. These follow the structure and indicate movement rather than a plastering defect. Point at the line.

Cracks that follow the background

A line tracing a plasterboard joint, a lath, or an old bonding patch is the background telling you what is behind it. Explain what sits behind.

2. Know which ones are yours

Be honest, because some of them are. Own those quickly.

Shrinkage and drying cracks

Applied too thick, dried too fast, a heater aimed at a wall, or a poorly prepared bond. These are workmanship and they should be repaired without argument. Go back and sort them.

Debonding and hollow areas

If the plaster sounds hollow or is lifting, that is adhesion and it is yours. Tap it and be honest about what you hear. Cut it out and redo it.

3. Go and look, and take it seriously

Photographs decide nothing. Go and see it yourself.

Visit and inspect properly

Look at both sides of the wall, the loft if relevant, and whether the crack has opened further. Attending promptly is most of the goodwill. Offer within the week.

Ask what happened in the month

Heating turned on for the first time, a hot spell, work elsewhere in the house, a new floor fitted. Buildings move in response to events. The answer often explains everything.

4. Explain it so it is believable

An explanation without evidence sounds like an excuse. Bring the evidence.

Show them the pattern

Point at the diagonal from the lintel, or the line following the board joint. Once somebody sees the geometry they generally accept the cause. Use your hand to trace it.

Say plainly when it needs somebody else

Structural movement, a failing lintel, subsidence. Recommending a proper opinion is the right advice and it protects you. Suggest a surveyor.

5. Repair it well regardless of fault

The commercial answer is usually to fix it. Arguing costs more than filler.

Fill and repair minor cracking as goodwill

A hairline crack takes very little time. Doing it without a debate about liability costs less than the conversation would.

Explain that it may recur if the movement continues

Be clear in writing. A repair to a moving crack is cosmetic, and the customer should know that before you fill it rather than after it reappears.

Conclusion

Most cracks in new plaster are the building moving rather than a plastering fault, and the location tells you which: diagonals from door and window heads follow the structure, while a line tracing a board joint or a lath is the background showing through.

Some are genuinely yours — shrinkage from applying too thick or drying too fast, and anything hollow or debonding — and those should be repaired without argument. Attend rather than diagnosing from a photograph, ask what happened in the month, and show the customer the geometry, because once they see the pattern they usually accept the cause. Then repair minor cracking as goodwill, and say in writing that it may recur if the movement continues.