How do I repair a plasterboard joint where the tape has lifted?

A tape that has lifted was either bedded into too little compound, applied over a dusty joint, or is sitting on a joint that moves. The repair is straightforward; getting it flat again is the skill.

What you'll need

• A sharp knife
• Paper joint tape or self-adhesive scrim
• Joint compound or finishing plaster
• A 150mm and a 250mm taping knife
• Fine abrasive paper or a sanding sponge
• A dusting brush

Step by step

1. Find the extent of the failure.Press along the tape. Where it gives or sounds hollow, it has lifted. Mark the ends of the loose section.
2. Work out why.A blister in the middle of a wall usually means dry taping — the tape was pressed onto too little compound. Cracking along a corner or a ceiling line usually means movement.
3. Cut out only the loose part.A sharp knife along both edges of the tape, and lift the failed section out. Cut back to where the tape is genuinely well bonded, plus 50mm.
4. Scrape the joint clean.Remove loose compound down to the board, and dust the area. Compound over dust fails again in the same place.
5. Refill the joint itself.Where the board edges leave a recess, fill it first with jointing compound and let it set before taping.
6. Choose the tape.Paper tape is stronger and better at resisting movement, but must be bedded into wet compound. Self-adhesive scrim is easier and is fine for a straight, stable joint.
7. Bed paper tape properly.A generous bed of compound, tape pressed into it, then squeeze the excess out from behind with a taping knife. Too little compound behind the tape is the original cause of most failures.
8. Do not squeeze it dry.You want a thin, even layer left behind the tape. Pressing it flat against the board leaves nothing to bond with.
9. Build up in widening coats.First coat over the tape with a 150mm knife, second coat wider with a 250mm knife, third wider still. The width is what makes the build-up invisible.
10. Let each coat dry.Fully. Compound over damp compound cracks and shrinks in ways that are very hard to correct.
11. Sand lightly and only at the end.A sanding sponge or fine paper on a block. Over-sanding cuts through into the tape and you start again.
12. Prime before painting.New compound is far more absorbent than the surrounding wall and will flash otherwise.

Tips

• The widening-coats principle is the whole trick. A repair 300mm wide with a barely perceptible slope is invisible; one 100mm wide with the same thickness is an obvious ridge.
• Check the finished joint with a torch held flat against the wall. It shows every ridge and hollow before you paint rather than after.
• If a joint has failed repeatedly, it is moving. Consider whether it needs a proper movement joint or a flexible finish rather than another repair.

Common mistakes

• Skimming over lifted tape — The new compound is only as sound as what it sits on. Loose tape underneath continues to move and blister, and the crack reappears through the fresh finish within months.
• Building the repair up in one thick coat — Thick jointing compound shrinks and cracks as it dries, and a narrow thick band leaves a visible hump that catches every raking light. Several thin coats, each wider than the last, is the only way to make it disappear.

If it doesn't work

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