Why Roofs Leak: The Ten Most Common Causes

The junction between the roof and a chimney, wall, dormer or parapet is the number one leak location. Lead flashings should be dressed and chased into the masonry, but on a great many houses they were replaced at some point with a sand and cement fillet.

Cement cracks because the two surfaces move at different rates. Once it cracks, wind driven rain runs straight behind it and down inside the wall. Repointing the crack lasts one winter.

The fix is proper lead: apron, soakers, stepped flashing and a back gutter where needed. Done once, it is good for decades.

The obvious cause and the one everybody assumes. On a roof with sound underlay, a single missing tile rarely produces an indoor leak straight away because the membrane beneath catches the water. On an old roof with no underlay, or perished underlay, it produces one immediately.

Nail fatigue is the usual reason slates slip. The nails corrode, the slate drops, and neighbouring ones follow. When a roof starts shedding slates in ones and twos every winter, that is nail sickness and the roof is telling you it is near the end.

Cracked tiles are often frost damage or foot traffic from somebody working on the roof, an aerial fitter being the classic culprit.

Failed bedding mortar at the ridge lets water in right at the top of the roof, from where it can travel a very long way before it appears inside. Gaps visible from the ground mean water is getting in.

Verges are similar. Once the mortar at the gable edge crumbles, wind lifts the edge tiles and drives rain underneath.

Both are relatively cheap to put right and both cause expensive damage if ignored, because the water goes into the wall head and the top of the ceiling where it is hard to spot.

A valley carries water from two slopes and takes more flow than any other part of the roof. Old lead valleys crack at the fold, and mortar valleys should never have been mortar in the first place. Debris collecting in a valley dams the water and pushes it sideways under the tiles.

Blocked gutters overflow backwards as well as forwards. Water backing up over the fascia gets into the eaves and shows as damp at the top of an upstairs external wall, which almost everyone misdiagnoses as a wall problem.

Undersized or badly falling gutters do the same thing in heavy rain even when they are clean.

Bitumen underlay from the fifties to the seventies goes brittle and tears, especially over the rafters and at the eaves. Once it has gone, the roof has no second line of defence at all and every small fault above becomes a leak.

On flat roofs, look at the upstands, the outlets and any penetration before you look at the field. Blocked outlets and low upstands cause most flat roof failures.

Penetrations generally: soil vent pipes, extractor terminals, satellite dish brackets drilled through a covering, and solar panel fixings all break the surface and all need a proper flashing rather than a bead of sealant.

Condensation accounts for a surprising share of the damp we get called to. Warm moist air rises into a poorly ventilated loft, hits the cold underlay, condenses, and drips onto the ceiling below. It gets worse in cold snaps and better in mild wet weather, which is the opposite of a real leak.

The tells are droplets across the whole underside of the underlay, black mould on the timbers, and damp that appears without rain. The fix is ventilation at the eaves and ridge, not new tiles.

Plumbing overflows, failed sealant around a bathroom, and chimney flues carrying rain all masquerade as roof leaks too. A good roofer eliminates these before quoting for roofing work.