How do I deal with a stain on the ceiling?

A ceiling stain is a symptom. Painting it without finding the cause simply means painting it again in a few months.

Safety

What you'll need

• Stain-blocking primer, shellac or oil-based
• A moisture meter if you have one
• Dust sheets and a torch

Step by step

1. Check whether it is still active.Feel it and mark the outline in pencil with the date. If the stain grows or feels damp, water is still coming in and decorating is pointless.
2. Deal with an unsafe ceiling first.A bulging, heavy area may collapse. Stay clear, and pierce it deliberately over a bucket from the side to release the water safely.
3. Isolate electrics if water is near a fitting.Water tracks along cables and into fittings. Turn off the lighting circuit at the consumer unit before investigating, and have it checked before switching back on.
4. Look directly above.A bathroom, a radiator, a pipe run, a flat roof. Water tracks along joists though, so the source can be some distance from where it shows.
5. Check the obvious suspects in order.Overflowing shower tray or bath seal, a leaking waste, a radiator valve, a failed roof tile, or condensation in the loft. Each leaves a different pattern.
6. Distinguish a leak from condensation.A leak gives a defined tea-coloured ring that grows. Condensation gives diffuse patches, often with mould, usually near cold spots and external walls.
7. Check the loft on a wet day.A torch during or just after heavy rain shows exactly where water is entering, which is far harder to find when everything is dry.
8. Let it dry completely.Weeks rather than days for a saturated ceiling. A dehumidifier speeds it up. Sealing over damp plaster traps moisture and causes it to fail again.
9. Seal with a stain blocker, not emulsion.Shellac-based or oil-based stain blocker. Water stains are soluble and bleed straight through water-based paint however many coats go on.
10. Apply two coats of blocker where the stain is strong.Nicotine and heavy water marks often need two. Check it is fully covered before topcoating.
11. Repaint the whole ceiling.A patch of fresh paint on an older ceiling shows as a bright square. Take the new paint to the edges of the room.

Tips

• Pencil the outline and date it. If it grows, the leak is still live.
• Emulsion never covers a water stain. You need a shellac or oil-based blocker.
• Check the loft during heavy rain — the source is far easier to find when water is actually coming in.

Common mistakes

• Painting over the stain before finding the cause — The leak continues and the stain returns within weeks, usually larger. The paint also traps moisture in the plaster, which makes the eventual repair worse.
• Using emulsion or an all-in-one paint to cover it — Water stains are water-soluble and bleed through water-based paint. Even five coats will not hold them back — a solvent-based blocker is the only thing that works.

If it doesn't work

Questions people ask