How to Find Where a Roof Is Leaking: A Practical Guide

Start in the loft, not on the roof

The safest and most reliable place to begin is inside the loft with a good torch. Go up during or shortly after heavy rain if you can, because an active drip is far easier to trace than a dried stain. Look for glistening wet timber, dark staining on the rafters, drips on the underfelt, or daylight showing through where it should not.

When you find wet timber, follow the moisture uphill. Water almost always runs down the slope of a rafter or along the top of the felt before dropping off, so the actual entry point is usually higher up the roof than the wet patch. Mark the spot with chalk or tape, then measure its position from the gable wall and the ridge. Those measurements let you find the same spot from outside without guesswork.

• Wet or stained rafters and roof timbers
• Drips or tide marks on the underfelt or membrane
• Daylight visible around chimneys, vents or between tiles
• Damp, compressed or mouldy loft insulation
• A musty smell, which often points to a slow, long-standing leak

Check the usual suspects from ground level

Most leaks come from a handful of predictable weak points rather than the main field of tiles. With binoculars from the garden or street, look for slipped, cracked or missing tiles or slates, especially after a storm. In South Wales the prevailing wind drives rain in from the south west, so the exposed face of the roof and any junctions on that side deserve the closest look.

Pay particular attention to the lead flashing around chimneys, which cracks and lifts with age, and to valleys where two roof slopes meet, as these carry a huge volume of water and fail when debris builds up or the lining wears through. Blocked gutters are another common culprit: when water overflows behind the gutter it can soak the fascia and track into the wall head, mimicking a roof leak. Ridge and hip tiles with crumbling mortar let water in along the very top of the roof, and on older terraced houses the mortar fillet where the roof meets a neighbouring wall is a frequent failure point.

Match the symptoms to the source

The pattern of the leak tells you a lot. A drip that only appears during wind-driven rain, but not in steady drizzle, often points to flashing, ridge mortar or rain being forced under tiles on the exposed elevation. A leak that appears hours after rain stops suggests water is pooling somewhere, commonly a blocked valley or a sagging section of felt, before slowly finding its way through.

Condensation is the great impostor. If the loft feels damp all over, the felt is beaded with moisture on cold mornings and there is no staining in one clear spot, you may have a ventilation problem rather than a hole in the roof. Similarly, a stain around a ceiling light or near an internal wall can come from pipework or a bathroom above rather than the roof, so rule out plumbing before assuming the worst.

When to stop investigating and call a roofer

There is no shame in handing over at this point. Working on a roof without scaffolding, proper footwear and experience is genuinely dangerous, and walking on tiles often cracks more than it fixes. A reputable local roofer can usually inspect from a ladder, a drone or a tower and give you a clear diagnosis, often within half an hour on site.

Repair costs vary with access and the fault itself. Refixing a few slipped tiles might sit in the low hundreds, while renewing chimney flashing or relining a valley costs more because of the labour and access involved. Be wary of anyone who diagnoses a full replacement from the pavement without getting up for a proper look; most leaks are repairable, and an honest roofer will tell you when a patch repair is the sensible option and when the roof is genuinely at the end of its life.