Leak causes
What are the most common causes of roof leaks?
The short answer
Roof leaks usually begin at a joint or a fixing, instead of across the open middle of a healthy covering. The usual culprits are slipped or cracked tiles, worn flashings around chimneys and walls, valleys, ridges that have lost their hold, seals around pipes, splits and ponding on flat roofs, and blocked gutters. Pinning down which one you have takes a survey, because water can travel a long way inside a roof before it shows.
Where roofs really let water in, from loose tiles and flashings to flat roof splits, and why the damp mark seldom sits under the fault.
Tiles and slates that have slipped or cracked
A pitched roof sheds water by overlap, each tile or slate lying across the joint of the row below. Once one slips, cracks or goes, that overlap breaks and rain reaches the underlay. Wind-driven rain off the sea makes matters worse on exposed elevations, because the water is shoved sideways and upwards beneath the laps instead of simply running down the slope.
Tiles move for several reasons: age, frost, a knock from a branch or a ladder, or fixings that have given up. A single gap is a modest repair, and one missing roof tile is covered on its own page. A pattern of slips across a whole slope, though, points at the fixings, not the tiles. Look at where the loose ones sit, because the pattern helps: scattered near the edges suggests wind, while a cluster mid-slope suggests the nails have let go.
Flashings, valleys and chimneys: the roof's joints
A flashing is the metal strip bridging the gap wherever a roof butts against a chimney, a gable end or a dormer cheek. A valley, the dip where two slopes join, gathers the water shed by both. These are places where a small failure admits a lot of water.
Lead can crack where it was dressed too tight or has been lifted by movement, and the mortar used to seal a flashing can shrink and drop out. A chimney has several joints at once, so a mark beside a chimney breast may come from the flashing, the stack itself or the pot. Our page on roof valley repair covers one joint in depth, and chimney repairs deals with the rest. A neat, properly lapped lead finish is what keeps these joints dry for years.
Nail sickness, ridges and the pipes that pass through
Nail sickness is the corrosion or loosening of the nails that hold tiles or slates, letting them slide out of line. It builds slowly, so the first sign is often one tile that has dropped a few millimetres. The ridge along the top can fail too. BS 5534 was revised in 2014 so that ridge tiles are held down by fixings as well as mortar, which means a ridge that was only bedded may loosen over the years.
Anything that passes through the roof is a weak point as well: soil vent pipes, extractor ducts, flues and aerial brackets. The seal around each can perish or lift. Our page on a roof leak around a vent pipe shows what to look for, and the page on nail sickness explains the slow version. Both can run for a long while before a stain finally arrives indoors.
Flat roofs: ponding, splits and failed edges
A flat roof is meant to drain, and water lying on it for days is a warning. BS 6229:2025, which replaced the 2018 edition in December 2025, sets a minimum finished fall of 1:80; designers normally aim steeper, at about 1:40, to be sure the completed roof sheds its water. A roof laid too flat, or one that has sagged, holds a pool that works at the membrane until it gives.
Splits at the upstand against a wall, and along the edge trims, are the usual entry points. Blisters in felt and gaps around a rooflight kerb are others. These leaks can be hard to trace because water follows the deck and emerges metres from where it went in. Our flat roofing service explains the coverings and how each is repaired, and why the details at the edges matter more than the open surface.
Gutters, moss and the stain that is not under the fault
Blocked or sagging gutters send water back beneath the bottom row of tiles and into the fascia and wall. Heavy moss can hold moisture against a covering and lift tiles slightly, as our page on whether moss can cause roof leaks explains. Dormers carry their own set of cheeks and joints, covered under a leaking dormer.
Water rarely drops straight down. It tracks down a rafter or across the felt before it reaches a ceiling, so the damp mark and the fault can be a few metres apart, and a ceiling can show a stain long after the rain that caused it. Our guide to finding a roof leak sets out how that tracing is done, and the wider service is under roof repairs. If the stain turns up in dry weather, the cause may not be the roof at all.
Related questions
Leak cause questions
Why does the stain appear far from where the water gets in?
Water follows timber and underlay downhill, often along a rafter or the top of a wall, until it finds a way through the ceiling. The place where it shows can be some distance from the entry point. Surveying roof and loft together traces the path, which saves guessing where to start.
Can a flat roof leak when the membrane looks sound?
Yes. Water can enter at a joint, an upstand or an edge trim that looks intact from above, then travel along the deck. Ponding that stays for days is another warning. A roofer checks the details around the membrane as well as the open surface, and photographs each one.