Why have small round bumps and cracks appeared over my plasterboard fixings?

A row of small circular bumps or a ring of cracked filler on a wall or ceiling is one of the most common defects in a house of any age, and it is almost always a timber problem rather than a plaster problem. Understanding that explains why filling them repeatedly never works.

What you'll need

• Plasterboard screws, 38 to 50mm
• A drill/driver with a depth stop or a dimpler bit
• A nail punch, or pliers
• Jointing compound or fine filler, and a broad knife
• Fine abrasive and a sanding block

Step by step

1. Understand what has happened.The board is fixed to timber. As the timber dries and shrinks it moves back very slightly, but the fixing head stays where it is — so the head ends up proud of the board face, pushing the skim and the filler off in a small circle.
2. Recognise the pattern.Regular round bumps in a line up a stud or across a ceiling means fixings. Random cracks that do not line up are something else — settlement, a board joint, or a moisture problem — and need a different investigation.
3. Look for the underlying cause if it is widespread.A whole ceiling popping at once usually means the timber has dried out substantially, often in the first heating season of a new build or an extension, or after a leak. New-build first-winter popping is normal and expected.
4. Fix a new screw 50mm away, into the same timber.This is the step that makes the repair permanent. The new screw pulls the board back tight against the timber, so the old fixing is no longer holding anything.
5. Drive the new screw to just below the surface, not through the paper.A slight dimple, with the paper face intact. A screw driven through the paper has no grip at all and is a future pop of its own. A dimpler bit or a depth stop makes this repeatable.
6. Deal with the old fixing.A nail: punch it well below the surface, or grip and pull it if it comes easily. A screw: drive it deeper, or back it out entirely. Either way it must not be standing proud of the board.
7. Fill in thin coats.Jointing compound or fine filler, applied in two or three thin coats over a wider area than the defect, each feathered out and allowed to dry. One thick blob shrinks, cracks and shows as a bump under paint.
8. Sand, prime and paint.Sand flat with fine abrasive, prime the filler so it does not flash through the topcoat, and paint. On a large area it is usually easier to paint the whole wall or ceiling than to patch.

Tips

• In a new house, wait until the end of the first full heating season before repairing pops. Fixing them once the timber has finished shrinking means doing it once.
• Screws pop far less than nails, which is why plasterboard is now screw-fixed. If a ceiling is nail-fixed and popping badly, adding screws across the whole area is a worthwhile job while it is being decorated.
• If pops appear along a line that is not a stud, look at whether it is a board joint. A cracking joint is a different repair — it needs scrim tape, not more filler.

Common mistakes

• Filling over the pop without adding a new fixing — The board is still loose against the timber and the proud head is still pushing outwards. It will crack again within a year.
• Driving the new screw hard until it breaks the paper — The paper face carries most of the board's strength around a fixing. Break it and the screw has almost no holding power, so you have created the next pop.

If it doesn't work