The short answer
A roof leak follows the rain: stains appear during or soon after a downpour and sit under a particular spot on the roof. Condensation follows the temperature: it forms on cold mornings, on north-facing slopes and over a wide area, even through a dry week. Watching when the damp appears, and where, usually tells you which one you have.
Two sources of water that look alike
A leak is rainwater getting through the roof covering, a flashing or a gutter joint. Condensation is water vapour from inside the house turning back into liquid when it meets a cold surface. Kettles, showers, wet washing on radiators and ordinary breathing all load the air with moisture, and warm air lifts a good deal of it through the ceiling into the loft.
In a loft, the underside of the roof is the coldest surface around, so that vapour condenses on felt, rafters and the heads of nails. The drops then fall or run, wet the insulation and eventually stain the ceiling below. To the eye it can look exactly like a slow leak, which is why people so often call a roofer for something that no repair to the covering will cure.
How the weather gives it away
Timing is the best test. Keep a simple note of when the damp appears and what the weather has done in the previous day.
- Leak: the stain grows or drips during rain, or a few hours after it, and dries out slowly in a dry spell. A gale from the west may set it off while the same rain from the east leaves it dry.
- Condensation: the wetness is worst on cold, still mornings and after a cold night, and may ease in the afternoon as the loft warms. It can appear in dry weather and it does not need a storm.
A third pattern is a stain that appears after a long dry cold snap when the heating comes on. That is typical of condensation. If the stain only ever shows after heavy, sustained rain, suspect water coming in through the roof. Our page on a roof leaking in heavy rain describes what to look at in that case.
Pattern and place: what the damp is telling you
A leak tends to be local. You get one patch, often with a defined edge or a tide mark, beneath a chimney, valley, wall junction or damaged tile. The patch may be brown and may spread outward in rings as each wet spell leaves a new edge.
Condensation spreads thin and wide. Think of a dusting of tiny black mould spots along a north wall, on the cold corner of a bedroom ceiling, or across the underside of the roofing felt across the whole loft. Water drips from nail points rather than from a single hole, and the felt may look beaded like a cold window. Window frames and cold external corners will often be damp too, which a roof leak does not explain.
Wet insulation is a clue for both. In a leak the insulation is soaked under one spot, while condensation leaves it damp in patches along the eaves or across the loft. Our page on wet loft insulation goes through what that means.
Why condensation builds up in a loft
Lofts are designed to breathe. Air should move in at the eaves, cross the space and leave near the ridge or through the covering, carrying vapour away. BS 5250 is the British Standard that loft ventilation advice draws on, and Marley's guidance built on it describes continuous vents along the eaves.
That air path can be blocked. Insulation pushed hard into the eaves cuts off the airflow. An extractor that ends in the roof space rather than outdoors pours in more moisture. Gaps around a loft hatch let warm air in. Tightly sealed modern houses make this worse because less moisture escapes through the walls, and a family that dries washing indoors all winter adds a great deal more. The usual cure is opening up ventilation and stopping the vapour at source, not repairing the roof covering.
Checks to try before calling anyone
Try these in order. They cost nothing and each one narrows the field.
- Note the date, time and weather each time damp appears, for at least a fortnight.
- Take a torch up on a dry, cold morning. Frost or tiny droplets on the felt and nail tips suggest condensation.
- Check that bathroom and kitchen extractors vent outside, and that the hatch seals properly.
- See whether insulation blocks the eaves, and whether the damp sits there or on the wall.
- Look for daylight, staining or wet timber right under one tile or flashing, which points to a leak.
It is possible to have both at once, and a cold loft with a small leak is a common pairing: a small leak and a poorly ventilated loft. If the weather test and the pattern disagree, a survey is the quickest way to settle it. Whether a small leak can wait is another matter, covered in leaving a small roof leak.
What we do when we look
On a visit we check the covering, flashings, valleys and gutters from outside, then go into the loft to examine the underside of the felt, rafters and insulation. If the evidence says leak, we trace it and quote a repair. If it says condensation, we will say so and suggest ventilation and moisture fixes, because replacing tiles would be money wasted.
The quote you get in writing includes pictures from the loft and the roof, so you can judge the evidence yourself. Fault-finding of this type sits within our roof repairs work, and any repair we carry out has a 10-year workmanship guarantee in writing. A stain on a ceiling is another common worry, and ceiling water stains explains what each pattern tends to mean.