The short answer
Keep people out from under the wet patch, switch off the lights and circuits in that room at the fuse board if water is near any fitting, and put a bucket under every drip. If the ceiling bulges, let the water out through one small hole at the lowest point. Photograph everything, tell your insurer, and have a roofer trace the real entry point.
How to make a wet ceiling safe, what to do about a bulge and the electrics, and how the source on the roof is found afterwards.
Making the room safe
Water and electrics are a dangerous pair, and a ceiling holds light fittings, cables and often an extractor fan. Where water is coming from or close to a light, a fan or a socket, turn that circuit off at the fuse board or consumer unit, and leave that fitting alone until an electrician has checked it. Do not stand under a wet patch to do this, and do not touch wet fittings.
Next, move people and furniture out from under the stain. Plasterboard soaks up water and gets heavy, and a wet section can drop without warning. Cover floors and anything you cannot move, and put buckets or bowls under drips, and lay a folded towel in the base to stop splashing.
If the stain is near a light fitting in particular, our answer on a roof leak near a light fitting covers the electrical side in more detail. Write down what you switched off so you can tell an electrician and your insurer.
It is also worth opening a window or two in the room once things are safe. Moving air starts drying the area, which cuts the risk of mould taking hold while you arrange the repair.
A bulging ceiling: pierce and bucket
A ceiling that sags or bulges is holding a pool of water behind the plasterboard. Left alone, it can burst and bring the lot down at once, together with the insulation above. The usual advice is to release the water in a controlled way. Put a large bucket underneath, with plastic sheeting around it, and stand to one side rather than directly beneath. Then make one small hole with a screwdriver where the bulge hangs lowest, and let it drain. You may need to do this in a few places if there is more than one bulge.
It feels wrong to damage your own ceiling, but a small hole is simple to patch, whereas a collapsed ceiling is not. The water has to go somewhere, and it is better that it goes into a bucket.
Do not try to hold up a bulging ceiling or push it back. If the whole ceiling is sagging, or you cannot reach it safely, keep out of the room and have a roofer or builder make it safe. Once the flow slows, keep checking the bucket, since water often keeps coming for some time after rain has stopped.
Tracing the true point of entry
The stain is rarely directly under the hole. Water travels along rafters, felt and pipework before it drops, and it can run several metres from where it entered. The cause may be a slipped or cracked tile, a worn flashing at a chimney or wall, a blocked valley, an overflowing gutter, a failed flat roof covering, or a loose ridge. It can also be something that is not a roof leak at all. A burst or weeping pipe in the loft, or condensation running down a cold surface, can look identical, and our answer on condensation or roof leak helps you decide.
The way to trace it is to work upwards and outwards from the stain. A roofer will look in the loft first, with a torch, for wet timbers and tracks of water, and then examine the covering above that point. Our order is to make the leak safe, then show you photos of what the survey found, then price the lasting repair. A good first look rarely needs the whole roof opened up.
Our emergency roof repairs page gives the wider picture.
Records, your insurer and drying out
Before you clean anything up, photograph the stain, the bulge, the buckets and anything damaged. Pictures carry the weight in any claim, a point the Institution of Structural Engineers makes about storm damage. Tell your insurer first, keep receipts for temporary work, and ask what the policy covers before agreeing to anything larger. No roofer controls whether a claim is paid. Disputes over a claim can be taken to the Financial Ombudsman Service.
The ceiling will need time to dry once the leak is stopped. A repair that seals the roof but leaves wet insulation and soaked plasterboard behind can leave mould and a stain that keeps returning. Our page on a damp ceiling after a roof leak covers drying and redecorating.
Ask your roofer to show you photos of the source and the repair, so you can compare the position of the entry point with the stain. If wires or fittings were wet, have an electrician test them before the circuit goes back on, and keep the report with your other records.
More on this in Emergency & General Roof Repairs in Crawley
Two ceiling questions
Is it safe to sleep in a room with a wet ceiling?
Not if the ceiling is bulging or dripping near any electrical fitting. Plasterboard can fall when it is saturated, and wet wiring is a hazard. Move to another room until the ceiling has been released or made safe and the circuit has been checked. A small dry-edged stain is a lower worry, but still needs a proper look.
Can I ignore a small brown stain that is not dripping?
It is better not to. A stain means water has reached the ceiling at least once, and it may be getting in slowly or only in certain winds. Mark its edge with a pencil and date it. If it grows, or the plaster feels soft, have the roof checked before the damage spreads.