How do I repair dot-and-dab plasterboard that has come loose?

A dry-lined wall that drums, flexes or has cracked along a board joint has lost some of its adhesive dabs. How to find the extent, refix it mechanically, and when the boards have to come off.

Difficulty: intermediate
Time: 2 hr
Read: 4 min

Short answer

Refix it mechanically rather than trying to re-glue it. Long plugs and screws through the board into the masonry behind, spaced across the loose area and pulled just tight enough to bring the board back without crushing it, will stabilise a debonded panel permanently. Filling the crack without refixing gets you the same crack next year.

Dot and dab relies on gypsum adhesive bonding to both the board and the masonry. It fails when the masonry was dusty, damp or painted when the dabs went on, when the wall has been wet, or when the board has taken a knock hard enough to break the bond. The board then moves independently of the wall, which shows up as drumming, a springy feel, cracked joints and fixings that will not stay tight.

What you'll need

Long plugs and screws, or dry-lining fixings sized for the void
Masonry drill and a bit long enough for board plus void plus embedment
Cable and pipe detector
Filler and jointing compound
A long straightedge

Step by step

Map the loose area by tapping.Loose dabbed board drums with a lower, longer note than board that is stuck. Work across the wall systematically and pencil the boundary. It is usually larger than the visible symptom.
Push gently and watch the joints.If the board moves and the joint line opens or closes as you push, the panel is free of the wall in that area. That is the confirmation.
Establish why it let go before you refix it.Impact damage is a local one-off. Water is not — a dry-lined wall that has been wet will have soft board and failed adhesive over a wide area, and it needs replacing rather than screwing back. Look for staining, softness and a musty smell.
Detect for services, and isolate.The dry-lining void carries cables. You are about to drill through the board and into the masonry, so scan first and stay clear of the zones around sockets and switches.
Drill through the board into the masonry.A bit long enough to pass through the board, cross the dab void and give at least 30 to 40mm of embedment in brick or block. Hammer action only once the bit is through the board, or the board face breaks out.
Fix at a sensible spacing.Roughly 300mm apart across the loose area, and a line of fixings close to any board edge or joint that has cracked. Countersink so the head sits just below the paper face.
Tighten just enough, and no more.The screw should pull the board back against the wall and stop. Over-tightening crushes the board core, tears the paper and leaves you with a fixing that holds nothing and a dimple that will not fill.
Check flatness with a straightedge as you go.The board was originally set to a plane by the dabs. Pulling it back too far in one place makes a hollow. Fix from the centre of the loose area outwards and check as you tighten.
Fill the fixings and make good the joints.Jointing compound over the screw heads in two thin coats. Rake out and re-tape any cracked joint rather than filling it, because the tape is what was holding it.
Replace the board where it is wet or crushed.Cut back to a joint or to a clean line, take the failed board off, clean the masonry back, and re-board — dabbed or, better, mechanically fixed to battens.

Tips

The void depth varies across a wall because the dabs were used to take up an uneven background. Probe in a couple of places and buy fixings for the deepest, not the average.
Where a fixing has to land close to a board edge, drill and fix at least 15mm in from the edge or you will simply break the corner off.
If a whole wall is loose, boarding over it with battens fixed through to the masonry is often faster and better than trying to refix a hundred points.

If it doesn't work

Board drums but is completely stable

Cause: Normal — that is the dab void — Fix: Nothing to do. Dry lining always sounds hollow between dabs.

Board flexes when pushed and joints crack repeatedly

Cause: Adhesive dabs have failed over an area — Fix: Mechanically refix through into the masonry, then re-tape the joints.

Screws will not hold and just spin

Cause: The plug is in the void rather than in the masonry — Fix: Longer fixing, deeper hole, and check the actual void depth with a probe before buying.

Board is soft, discoloured and smells

Cause: It has been wet — Fix: Replace it. Wet plasterboard does not recover, and refixing it seals a damp problem in.

A crack that follows a vertical line the height of the wall

Cause: A board joint that has moved because both boards are loose along it — Fix: Refix both sides of the joint, rake out, scrim and re-finish.

Questions people ask

Can I inject adhesive behind the board instead?

There are foam and grab adhesives sold for the purpose and they can work on a small area, but the results are unpredictable — the foam expands and pushes the board out of plane, which is worse than the original fault. Mechanical fixing is controllable.

Why does dot and dab fail?

Nearly always a preparation issue at the time it was built: dusty, painted or damp masonry, or too few dabs. Sometimes impact, and sometimes water at a later date. It is a workmanship failure far more often than a material one.

Is loose dry lining dangerous?

Not usually on a wall — the board is trapped at the floor, ceiling and edges. On a ceiling it is different, and a dabbed ceiling is not something that should exist in the first place.