Why does the crack in my plaster wall keep coming back?

Cracks are information. Where they run, how wide they are and whether they change with the seasons all point to a specific cause, and the correct repair follows from that rather than from the filler aisle.

What you'll need

• scrim or paper jointing tape
• bonding or jointing compound
• flexible caulk (optional)
• filling knives
• primer

Step by step

1. Map where the crack runs.Straight vertical or horizontal lines at regular spacings are plasterboard joints. Diagonal lines from the corners of doors and windows are stress cracks. Cracks following a change of material are movement joints.
2. Measure it and watch it.Mark the ends with a pencil and date them, or fit a simple crack monitor. A crack that stops moving is historic; one that continues opening needs professional assessment.
3. Take seriously the cracks that warrant it.Anything wider than about 5mm, cracks that pass through brickwork outside as well as plaster inside, doors and windows that have started sticking, or a crack that is visibly widening. Those want a structural engineer, not filler.
4. For a plasterboard joint, tape it.Rake out the failed filler, apply jointing compound, bed scrim or paper tape into it, and skim over. The tape bridges the joint so movement is spread rather than concentrated in one line.
5. For a junction between materials, use caulk.Where plaster meets timber, or an extension meets the original house, the two materials move differently and always will. A flexible caulk bead in a raked joint is the honest repair.
6. Rake out properly whatever the cause.Open the crack into a shallow V so there is a body of material rather than a hairline skim, and brush out all the dust.
7. Use the right filler for the width.Fine filler for hairlines, a bonding or deep-repair filler for anything substantial, applied in layers.
8. Consider lining the wall.For a wall covered in fine crazing or repeated hairlines, lining paper or a fibreglass matting system distributes the movement across the whole surface and is far more durable than repeated filling.
9. Prime and paint the whole wall.A repaired strip primed and touched in will show. Prime the repair and repaint corner to corner.
10. Address the underlying cause where you can.A loose ceiling board, a shrinking timber lintel, a leaking gutter soaking a wall — cracks are frequently the visible end of a cause you can actually fix.

Tips

• Scrim tape is what stops a plasterboard joint crack returning. Filler alone in that location will always fail.
• Date the ends of a crack with a pencil mark. Six months later you know whether it is live or historic.
• Fine crazing over a whole wall is a surface problem, not a structural one — lining paper solves it far more cheaply than replastering.

Common mistakes

• Filling a plasterboard joint crack without tape — The joint between two boards will always move slightly. Without tape to spread that movement, all of it goes into the filler line and the crack reappears in exactly the same place.
• Using rigid filler at a junction between materials — Timber and plaster, or old and new construction, move at different rates. A rigid repair cracks every season; a flexible one absorbs it.

If it doesn't work

Questions people ask