The short answer
A leak follows rain, usually after a delay, and leaves a stain with a clear edge below the spot where water entered. Condensation follows cold, still, humid conditions, and shows as beads of water, dark speckled mould or damp patches in cold corners, with no link to rainfall. Check the weather pattern first, then the loft, because the repairs are different.
What the weather pattern shows
Keep a note of when the damp appears. Rain-driven leaks are tied to rainfall: the patch grows during or soon after a downpour, then stops growing and slowly dries when the weather clears. A heavy shower after a dry spell often finds the fault first.
Condensation behaves differently. It builds up on cold mornings, in rooms where moist air collects, after cooking, bathing or drying washing indoors, and it can arrive on a dry, frosty day with not a drop of rain. The damp often returns every winter in the same places, and clears somewhat in warm weather.
Wind direction can help. A leak that appears only when rain is driven from one side points to a flashing, verge or tile on that side. Condensation does not care which way the wind blows.
The Met Office climate station for South Farnborough (1991 to 2020) records around 118 wet days a year and about 53 days of air frost, so both causes are plausible in this area, and a diary of two or three weeks usually separates them. Record the date, the conditions outside and what you see in each room.
If you cannot tell, a roofer's survey gives a firm answer, and the next sections explain what we look for.
Clues in the loft
The loft is where the diagnosis usually gets settled. Take a torch and look, without stepping off the joists or boards, at the underside of the roof covering and the timbers.
Signs of a leak include a dark, defined stain running down a rafter or batten, wet or stained insulation directly under one spot, rust around nails, and a faint tide mark where water has dried and left a line. Daylight showing through the covering is a gap that needs closing.
Signs of condensation are more widespread. Look for beads of moisture or frost on the underside of the felt or boards, nail points that look wet, a general dark speckling of mould across the timbers, and damp insulation along the eaves rather than in one patch. Blocked eaves vents, or quilting squashed right up to the roof edge, are frequent culprits.
Approved Document C wants a roof that keeps precipitation out and is not damaged by damp condensing inside the structure, and it asks for a proper draught seal on the loft hatch so warm, humid air does not drift upwards. A hatch that does not close well can feed condensation above a bathroom or landing.
Our page on signs of a roof leak in the loft goes through the same checks in more detail.
Fixing a leak
A leak is cured at the point where water gets in, and that point can be some distance from the stain. Typical causes are a slipped or cracked tile, flashing that has given up at a chimney stack or abutting wall, a blocked or damaged valley, a gutter that overflows behind the fascia and an old felt that has torn.
The roofer traces the water uphill from the stain, looks at everything above it, and checks inside and out. The repair is then targeted: a replaced tile, a re-dressed flashing, a cleared and re-lined valley, or a section of new underlay.
If the covering as a whole is worn out, a repair may only postpone the problem, and we flag the point at which a bigger job makes better sense. We never push a re-roof when a repair would hold.
A fresh leak is also worth stopping early. Water that reaches timber and insulation spreads damage quietly, and the cost of putting it right rises with every storm.
The find a roof leak page shows how the entry point is followed uphill, and roof repairs covers the work we do on a confirmed leak.
Fixing condensation
Condensation is treated by controlling moisture and airflow, and a new covering will not cure it. The first step is to cut the moisture entering the loft: seal the hatch, ensure extractor fans in bathrooms and kitchens vent outside rather than into the roof space, and check that no pipes or fittings leak warm damp air upwards.
The second is ventilation of the roof. The current moisture code is BS 5250:2021. Marley's advice for cold roofs laid with HR underlay, which does not let vapour through, is a 25 mm eaves gap at pitches up to 15 degrees and 10 mm on steeper ones. With breathable LR underlay and a well-sealed ceiling, ordinary eaves detailing can be enough, if the product's certification allows.
Third, keep insulation from blocking the eaves. Fitting a ventilation tray, so air can pass over the insulation at the roof edge, is a common remedy, and it is a task for a roofer or insulator who checks the pitch and the underlay first.
After the cause is fixed, clean off the mould and let the timber dry. Our page on mould from a roof leak covers the clean-up, and the roof repairs hub sets out how we survey and report.
