Why Does My Roof Leak After Heavy Rain?

Wind-driven rain

Irish weather rarely delivers rain straight down. Driven at an angle, water can be pushed up under the tail of a slate, in behind a flashing upstand, past a verge, or into a junction that works perfectly well in vertical rain.

This is why so many homeowners notice a leak only in certain storms, and not others of similar intensity. The variable is usually direction, not volume, and it is genuinely useful information — if the damp always follows a south-westerly, that narrows down which elevation to examine.

Drainage that cannot keep up

In prolonged heavy rain, everything designed to carry water away is working at capacity. Anything partially restricted then stops coping.

Where water backs up

• Valleys holding leaves, moss or grit, so water backs up under the coverings — see roof valley repairs
• Gutters and downpipes overflowing behind the fascia rather than discharging — see guttering
• Flat-roof outlets and gutters restricted by debris — see flat roofing
• Box gutters and back gutters behind chimneys filling faster than they drain
• Ground-level drainage backing up and being mistaken for a roof problem

Defects that only show under load

Plenty of roof defects are small. A hairline split in a lead flashing, a slate with a crack in it, a nail hole that has opened up, a lap in a valley that has lifted a few millimetres — each is capable of passing a trickle of water in a heavy downpour and nothing at all in light rain.

Common culprits

• Cracked, slipped or missing slates and tiles — see slate and tile repairs
• Split, lifted or poorly formed lead flashing
• Chimney junctions and deteriorated pointing — see chimney repairs
• Perished or torn underlay below the covering
• Failed laps, blisters or splits in a flat roof covering
• Detailing around roof windows — see Velux and skylight repairs
• Ridge, hip and verge details loosened by wind

Why intermittent leaks still need investigating

It is tempting to leave a leak that only appears two or three times a winter. The problem is what happens between appearances: water that gets in is soaking timber, underlay and insulation each time, and a defect that lets water past under load rarely gets smaller.

Intermittent leaks are also harder to trace the longer they are left, because by the time they become constant there may be several wet areas rather than one clear trail.

Tracing a leak that will not perform on demand

A roofer arriving on a dry day cannot watch your leak happen, which is the honest difficulty with intermittent ingress. What we can do is read the evidence it leaves: staining and tracking on the underlay, watermarks on rafters, debris lines in valleys and gutters, and the condition of each detail on the elevation the weather comes from.

Where the picture still is not clear, we will say so rather than replace details speculatively. Sometimes the right advice is to check again after the next heavy spell.