Where the patch is and when it appears
A damp patch on an internal wall has four usual causes: a leaking pipe or fitting, condensation, damp rising from the ground, and rain finding its way in. Two questions sort most of them out. Where on the wall is the patch, and when does it get worse? A patch that keeps growing in dry weather, with a wet centre near a pipe route, is most likely a leak.
An internal wall is one inside the house, such as the wall between a hall and a lounge, rather than one facing the weather. Inside walls are warmer and sheltered from rain, so condensation and rain are less likely causes there than on outside walls. A damp patch halfway up an inside wall, well clear of the floor, deserves a closer look for pipes.
Low down, fading out at an even height: the ground or the floor
Round, with the wettest part in the middle: often a pipe or fitting
In cold corners or behind wardrobes, with black mould spots: condensation
Worse after rain, drier in a settled spell: water from outside
Worse after a bath or shower upstairs: the bathroom above
What causes a damp patch on an internal wall
Condensation forms when warm, moist air from cooking, showers and drying clothes meets a cold surface and turns back into water. It shows on windows, outside walls and cold corners, and behind furniture where air cannot move. It comes and goes with the weather and with how the house is used, and it seldom leaves a neat patch in the middle of a warm inside wall.
Rising damp is water from the ground soaking up through brick or stone. It happens where the damp-proof course, a waterproof layer near the bottom of the wall, has failed or was never put in, which is the case in many older flint and brick cottages across Norfolk and Suffolk. It stays low, often below about a metre. Rain usually gets in through outside walls, but it can reach an inside wall down a chimney breast.
A leak behaves differently. A supply or heating pipe stays full of water day and night, so a leak on one usually keeps going and the patch spreads whatever the weather. A waste pipe, the pipe that carries water away from a basin, bath or washing machine, usually only leaks while that fitting is in use. So a patch that grows after a bath upstairs and then dries out points to a waste pipe or a seal.
What moisture readings show
A simple moisture meter can help, as long as you compare readings rather than trust one number. Take readings in a line from dry wall into the patch. Numbers that climb towards one spot suggest water arriving there. Numbers that peak at floor level and stay even along the wall suggest the ground. Salts left by old damp can make dry plaster read high, so use the meter as a guide.
A square of kitchen foil helps separate condensation from damp coming through the wall. Tape all four edges tightly over the patch and leave it for a day or two. If the room side of the foil is wet, moisture is settling out of the air, which points to condensation. If the side against the wall is wet, the water is coming from inside the wall.
Pipes in the wall
Pipes run inside more walls than people expect. The water supply often rises from the stop tap through a kitchen wall. Hot and cold pipes reach a bathroom through the wall behind the basin or bath. In many houses, heating pipes drop down inside walls to feed the radiators. Newer homes often have plastic pipe in hollow plasterboard walls, where a slow leak can soak the board before it shows.
Work out what sits on the other side of the wall and directly above it. A patch below a bathroom, beside a radiator, behind a kitchen sink or next to an airing cupboard is a strong hint. Two quick checks help. If the boiler has a pressure gauge, watch it for a few days: a steady fall means the heating is losing water. And if the water meter keeps moving with every tap shut, water is escaping from the supply pipes somewhere.
Who to call
Who to call depends on the cause. For condensation, the answer is usually better airflow and steadier heating: extractor fans, trickle vents and warmer rooms. For rising damp, a damp surveyor can check the walls and the damp-proof course. For rain, a builder can look at gutters, downpipes, the mortar between the bricks and the chimney. For a leaking pipe, the first job is finding it.
If you rent, tell your landlord or letting agent in writing and send photographs, because the landlord usually arranges repairs to pipes and the building. If you are a landlord, ask the tenant when the patch appeared and whether it changes after showers or rain. A plumber can mend a pipe they can see. When the pipe is buried in the wall, we find the leak first, so the plaster comes off in one place.
How a leak in a wall is traced
Our engineer Charlie starts by mapping the damp with a moisture meter, marking where the wet area ends and where it is wettest. Next comes the test that suits the pipe. Thermal imaging, a camera that shows heat, follows hot water and heating pipes under the plaster. Pressure testing shows whether a section of pipe is losing water. Each fitting nearby, such as a shower or a bath, is run in turn to see whether the damp changes.
For a cold pipe, tracer gas can do the finding: a hydrogen and nitrogen mix goes into the pipe, seeps out through the hole and is detected on the face of the wall. We only mark the plaster when two methods point to the same place. The mark is photographed, and once the visit is paid in full you receive a written report with the location, methods, readings and photographs. Any repair to the pipe is quoted separately.
What to do while you wait
Take a photograph with a tape measure held against the wall, and draw lightly round the edge of the patch in pencil. Do it again every few days, so you can see whether it is growing. Hold off painting over it: paint hides the evidence and can trap moisture. If the damp is close to a socket or a light switch, switch off that circuit at the fuse box, then ask an electrician to check it before it is used again.