Plaster Repair After A Water Leak

A brown stain on a ceiling gets painted over. Three weeks later it is back, slightly larger.

The patch was never the problem. The water was still arriving.

The sequence that works

Four steps, in order, and skipping any of them means doing the whole thing twice.

Find the source. Which is harder than it sounds, because water travels. A stain in the middle of a ceiling can come from a pipe metres away, a roof penetration on the other side of the room, or a bathroom on the floor above. Tracing it is its own job and it is covered properly under [water coming through a ceiling](/blog/water-coming-through-a-ceiling-in-an-apartment).

Fix the source. Nothing else happens until this is done and proven.

Dry the cavity. Not the surface, the cavity. Insulation holds water for weeks, timber framing holds it longer, and a ceiling that feels dry to the hand can have a soaked batt sitting on top of it.

Then repair. Which is now a straightforward job rather than a gamble.

Why wet plaster cannot be patched

Plasterboard that has been wet has lost its strength. The paper face delaminates from the core and the core itself crumbles.

Which means a patch applied over it is bonded to something that is already failing. It looks right for a few weeks and then the joint cracks or the surface bubbles, because the material behind it is still moving.

Cornice is worse again. A wet plaster cornice carries water along its length by capillary action, so the damage extends well past the visible stain.

What gets cut out

More than the stain.

The wet material comes out back to sound board, which usually means cutting to the nearest framing member on each side so the new sheet has something to fix to. A patch floating in the middle of a sheet with no support behind it is a crack waiting to form.

Insulation in the affected area is removed rather than dried in place, because wet batts stay wet and they hold moisture against the new board.

The framing is checked while it is open, which is the only time anybody will see it. Water that sat in a ceiling for months can leave timber that needs more than a new sheet.

Making the repair disappear

This is the difference between a repair and a visible patch, and it is almost entirely about the setting and sanding rather than the sheeting.

The joints are set in stages with the compound feathered wide, so the build-up is spread over a long distance rather than being a ridge at the edge of the patch. Three coats, each one wider than the last, each one fully dry before the next.

The surface is matched to what is around it. A square set ceiling, a trowelled finish, a textured or stippled ceiling and a sprayed finish all need a different approach, and a smooth patch in a textured ceiling is as obvious as no patch at all.

Then the paint. A patch primed and spot painted will flash, meaning it reads as a different sheen under raking light even when the colour matches exactly. The fix is to paint the full ceiling or the full wall to a corner, not the patch, which is the same principle as [the finish a tenant sees first](/blog/the-finish-a-tenant-sees-first-paint-and-flooring-between-tenancies).

Mould

If the cavity was wet for more than a couple of days, assume there is mould behind the board.

Painting over it does nothing. Mould-resistant paint resists new growth on a clean surface, it does not kill what is already there.

The affected material comes out, the framing is cleaned and treated, and the cavity is dried before it is closed. Where the original cause was condensation rather than a leak, the ventilation has to change as well or the mould comes back on the new sheet, which is the whole point of [bathroom condensation, mould and the exhaust fan](/blog/bathroom-condensation-mould-and-the-exhaust-fan).

The cracks that are not water

Not every ceiling or wall crack is a leak.

Seasonal movement, a cornice crack that opens in summer and closes in winter, a hairline along a sheet joint, and settlement cracking all have their own causes and their own repairs. Patching a movement crack with rigid compound means it reopens in the same place, and that distinction is set out in [ceiling and wall cracks, what they mean](/blog/ceiling-and-wall-cracks-what-they-mean).

Ample traces the leak, fixes it, dries the cavity and repairs the plaster to match, with the plumbing, waterproofing and painting run by one team so the ceiling is opened once and closed once.