The short answer
Stay out from under the bulge and move anything valuable away. When drips land near a light or socket, cut that circuit at the fuse board before anything else. Put a bucket and a sheet down, take photographs, and ring a roofer to keep the rain out. A bulge is a ceiling holding water, and it can come down without warning.
A bulging ceiling means water is pooling above it. The weight can bring plasterboard down, so the first minutes are about safety, not tidiness.
Safety first: keep clear of the bulge
A bulge in a plasterboard ceiling is water trapped between the board and the joists, the timbers it is fixed to. Water is heavy, and the board can split or drop at any moment, taking the water, wet plaster and sometimes loft insulation down with it. So stand to one side of the bulge, not beneath it, and shut the door on the room so nobody wanders in, especially small children.
Should you use the room at all?
Not until the water is controlled. Lift rugs and move furniture, bedding and anything electrical well away, and cover what is too big to shift with a plastic sheet. If the ceiling is directly above the stairs or a hall you need to cross, use another route if there is one.
Do not poke at the bulge with your hand or lean ladders against it. A damp ceiling can look sound for the length of one more drip and then give way. Think about where the water is likely to land and put yourself, and your belongings, somewhere else.
If the plasterboard has split or hangs low, treat the whole room as unsafe until it has been dealt with.
Electrics and water do not mix
Water finds light fittings, because the cable runs through the same space and the hole for the fitting is the easiest way out. A downlight or pendant with water running into it is a shock and fire risk. If the bulge is near a light, a ceiling fan, a smoke alarm or a socket on the wall below, treat the circuit as unsafe.
How do you make electrics safe?
Go to the consumer unit, the fuse board, and isolate the circuit serving that room. If you are not sure which switch it is, or the floor is wet around the board, turn off the main switch and keep away from standing water. Do not flick lights on to see better. Use a torch.
Leave any wet fitting alone afterwards. A qualified electrician should check the wiring and fittings before they are used again, even if everything looks dry on the surface, because damp can sit in a junction box for weeks. Our answer on a roof leak near a light fitting goes into more detail.
If you have any doubt about whether the board is safe to reach, leave it to a professional to isolate.
Containing the water you can reach
Stand a bucket or a deep tray beneath the sagging belly of the ceiling, with a sheet or towels round it to catch splashes. Check it often, because a steady drip fills a bucket faster than you would think, and an overflowing bucket makes a second problem.
Is it safe to pierce the bulge?
Some people release the water on purpose, so that it drains into a bucket rather than falling all at once. That is a judgement call. If the board is clearly about to fail and the electrics are off, a small hole in the lowest part, made with a screwdriver held at arm's length from the side, eye protection on and a bucket beneath, lets the water out in a controlled stream. If you are unsure, leave it and stay clear. Once the hole is made you cannot undo it.
Photograph the stain, the bulge and the water before you touch anything. The pictures help a roofer trace the entry point, and they matter if you claim on your home insurance. Keep receipts for buckets, sheets or dehumidifiers too. Do not wait for the stain to dry before you ring someone, since drying hides nothing and tells you nothing about the roof.
When to call and what to say
Call a roofer as soon as the room is safe. A ceiling that holds water is telling you the roof is open somewhere above, and the next rain will add to it. If the leak is heavy, or you cannot slow the water, say so when you ring.
What should you tell the roofer?
Describe where the bulge is, which way the roof slopes above it, and whether it started in rain, wind or snow. Mention any loose tiles, blocked gutters or earlier repairs you know about. Send photographs of the stain and of the outside of the house. That lets a roofer plan the right access and materials before arriving.
Getting the water to stop is the priority, and the permanent repair is planned for when weather and access allow. Our guide on who to call for a roof leak helps if you are unsure where to start, and the page on temporary roof repairs shows what a stop-gap can and cannot do. Everything else sits under emergency roof repairs.
Tell your home insurer as well, and keep a note of the time you rang each person.
More on a bulging ceiling
What should I do while I wait?
Keep out from under the bulge, isolate the circuit when drips are close to a light or socket, and put a bucket and sheet down. Move furniture and valuables away, and take dated photographs. Tell your home insurer too, and keep every receipt for temporary protection.
Will the ceiling need replacing afterwards?
Often the board that bulged has to be cut out, because wet plasterboard loses its strength and can stain through any paint. The roof is repaired first so the new ceiling stays dry. Let the timbers and insulation dry out, then have the ceiling reinstated and decorated.
The full picture: Emergency and general roof repairs in Taunton ยท More short answers