Do this first
- Electrics before water. Ceilings below bathrooms are full of downlights. If the wet area is near a fitting, switch that lighting circuit off at the consumer unit and leave it off until the ceiling is dry and has been checked.
- Stop using the shower. Not indefinitely, just until the test below has been done, because every use adds to the damage and muddles the timing evidence.
- Take the weight out of a bulging ceiling only under the conditions set out on water coming through a ceiling: modern plasterboard, circuit off, bucket underneath, stand to one side, one pierce at the lowest point. Never on lath and plaster, never on a pre-2000 textured coating, never while water is still arriving.
- Do the timing test. Dry the ceiling below and tape a sheet of paper to it. Then run the shower for ten minutes with the door shut and nobody standing in it. Check below. Then check again four hours later with the shower unused.
- Repeat with the tray filled but the shower off if you can plug the waste: standing water with no spray isolates the waste and the tray from the plumbing.
- Photograph everything with times before anyone mops or repairs.
Wet only when it runs, or wet regardless
That is the whole diagnosis, and the paper test above gives it to you in a few hours.
| When the ceiling wets | Where the leak is |
|---|---|
| Only while the shower runs, stops shortly after | The tray, the waste, the seal between tray and wall, or the grout and tanking behind the tiles |
| Only when the tray is standing full, no spray | The waste, its seal to the tray, or the trap below it |
| While spraying but not while standing full | The wall: failed grout, silicone or tanking letting water behind the tiles |
| Constantly, including when nobody has used the bathroom | The concealed mixer valve or the pipework feeding it inside the wall |
| Only when the basin or bath is used too | A shared waste branch rather than the shower |
The constant case is the one that needs detection rather than a bathroom fitter. A concealed thermostatic valve buried in a stud or masonry wall, or the hot and cold feeds running up to it, sit at full pressure all day, so they wet the ceiling at three in the morning as readily as at breakfast. That behaviour is diagnostic on its own, and it is the reason the timing test is worth doing before anyone starts stripping tiles. A worn concealed valve can also give a visible warning at the head, which keeps dripping with the shower off; shower head dripping explains what that drip means.
The wall-versus-tray distinction matters almost as much. Water getting behind tiling through failed grout or perished silicone soaks the board and runs down inside the wall, and re-grouting the visible line does nothing if the tanking behind it has failed. If the bath rather than the shower is the suspect, a bath leaking through the ceiling has its own standing test.
What the repair involves
Detection is staged around the tests above, with moisture mapping across the ceiling to find the wettest point, thermal imaging to follow the hot feed, and, where the fault is constant, isolation and pressure testing of the concealed pipework before anything is opened. On a wall leak the tiling is opened at one point rather than stripped, once the wet zone is mapped.
Repairs split the same way as the diagnosis: a new waste and seal, a rebedded tray, renewed tanking and tiling, or a repair to the concealed valve or the pipework behind it. Then the making good on both sides. The bathroom is retiled and resealed to match, and the ceiling below is cut back to sound board, dried, reboarded, skimmed and painted wall to wall. That is bathroom and shower leak repair done properly, and where an insurer is involved our report covers the trace and access element: see the insurance claims hub.
When it is not us
- The silicone around the tray has visibly failed and the ceiling only wets in use. Cutting the old sealant out properly, drying the joint and resealing is a job a careful homeowner can do, and it fixes a fair number of these.
- Water is escaping past the shower screen onto the floor. That is a screen seal or a spray pattern issue, not a plumbing leak.
- The extractor is not working and the ceiling has mould rather than stains. That is condensation and ventilation.
- Wet downlights. Once the leak is fixed and everything is dry, an electrician should inspect and replace anything that has been wet before it is re-energised.
- The flat above is the source. Then start with a leak from the flat above, because access and liability come before repair.
- You rent. Report it in writing. The landlord holds the repairing obligation for sanitation installations.