Is a damp patch on the ceiling a roof leak?

A stain overhead tells you water has been there, not where it came from. A roof leak lets rain through the covering. Condensation forms when warm, damp household air meets a cold surface, and the moisture runs down or soaks into the plaster. A plumbing fault sends water from a pipe, a cistern or a radiator joint, and the weather outside has no bearing on it.

Treating the wrong cause wastes money. A roofer sent to a condensation problem will find a sound roof, and no ventilation fix will stop a leaking valley. Sorting out which of the three you have is the whole job at this stage, and it usually takes less effort than you might expect. A notebook, a torch and a few dry-weather and wet-weather observations are the main tools, and none of them costs anything. If you can, record the date, the weather and how large the patch was each time you look.

Keep a note of when the patch gets worse. One that darkens during or just after rain, and stays dry in settled weather, points to the roof. If the stain is wettest on frosty mornings and eases through the afternoon, condensation is more likely. A patch that grows whatever the weather, or one beneath a bathroom, kitchen or water tank, suggests plumbing.

Wind direction can matter, though only as a pattern in your own notes. A leak that appears just when rain is driven hard against one side of the house can be a flashing, a vent or a gap in a single slope, because water only gets in from that direction. Our answer on finding a roof leak shows how to build that record, and a week of notes can save a visit. Take the same photograph from the same spot each time, so that changes in size are easy to see.

A roof leak tends to leave a tea-coloured ring with a darker rim, because dirt dissolved in the loft is carried down with the water. Paint blisters, and the ceiling may sag if water has pooled above it. Marks near a chimney breast, a dormer or an outside wall often trace back to flashing or gutters.

Condensation looks different. It shows as grey or black speckled mould in corners, behind wardrobes and along cold external walls, and the plaster feels cool to the touch. It often follows a change, such as sealed windows, a new shower or washing dried indoors. Plumbing leaks tend to leave a stain with a clear centre directly under the fitting. None of these rules is absolute, so treat them as clues rather than proof, and expect a mixed picture when two causes overlap.

If the patch is bulging, put a bucket beneath it and keep the light fittings nearby switched off until it has been checked. Then climb into the loft on a dry day, torch in hand, staying on the joists. Look above the stain for wet or dark insulation, damp timber, drips on a rafter and glimpses of daylight. Check the cold tank and pipes for drips, and the underside of the roof for beads of moisture.

From the garden, look for slipped tiles above the mark, lifted lead at a chimney and gutters that overflow. Stay off the roof itself. Take dated photographs of whatever you find. Our guide to the common causes of roof leaks lists the places water gets in, so you know where to look. Write down which side of the house the weather came from the last time the patch grew, and whether the loft felt cold or humid when you went up. A stuffy, damp loft with no leak at all is a clue in itself.

A survey looks at the roof, the loft and the ceiling together, with pictures of each. If the roof is at fault, you see the exact point and receive a written quote at one total for the least work that cures it. If it turns out to be condensation, you are told that, along with where ventilation at the eaves and in the loft forms part of the answer. A repair is never sold for a problem the roof does not have, and a clean bill of health for the covering is a useful result in its own right.

Hold off painting over the stain until the cause is found and the plaster has dried, or the mark will return through the paint. For more, see roof inspections and surveys, the guide to the signs a roof needs repairing and the main roof repairs service.