The short answer
A roof that leaks only in heavy rain usually has a detail that copes with light rain but not with volume or wind. The usual suspects are a valley or gutter that overflows, a flashing with a small gap, loose or missing tiles that wind-driven rain gets under, and a clogged drain outlet on a flat roof. The fault is real even if it appears rarely, and each downpour adds to the damage.
What heavy rain tests that light rain does not
Light rain runs gently down a roof and drains away. Heavy rain behaves differently in two ways: there is far more water, so channels that were never full suddenly are, and it often arrives with wind, which pushes drops sideways and uphill under the lap of a tile or slate.
A roof can therefore be sound in ordinary weather and still have a marginal detail. A gap in a flashing that drizzle never reaches becomes an open door in a gale. A valley lining that carries a normal flow can overtop when a storm sends everything down one narrow channel. A rare leak is not a small leak. It is a fault that shows only when conditions are severe, and it works on the timber behind it each time.
Wind-driven rain under tiles and slates
A pitched covering sheds water by overlap. Each tile or slate sits partly over the one below, and the amount of overlap, the headlap, is what stops a sideways gust pushing water over the top edge. When tiles slip, crack or have been laid with short laps, the margin is gone. Rain driven by the wind then gets onto the underlay, and a failing or torn underlay lets it through to the timbers.
Exposed gable ends, the verge, are common places for this. Mortar under the verge tiles perishes, the tiles loosen, and wind lifts the edge. Open ridges are another. On a roof with a modern dry-fix ridge, a broken or missing cap can let rain in at the apex.
The evidence is often in the loft rather than on the ceiling: darkened timber, a damp patch on the underlay, or a tide-line mark on a rafter near the verge. Photographs from the survey show these details, and we recommend a repair that deals with the specific weakness rather than recovering a whole slope that is otherwise sound.
Valleys, gutters and the overflow that looks like a leak
The valley is the crease between two slopes, and it carries the water of both. If moss, leaves or grit block it, or the lining has cracked, heavy rain overtops the edge and runs under the tiles beside it. Our page on roof valley repair cost explains what a valley repair involves.
Gutters play the same trick. A full or sagging gutter sends water over the front and back against the fascia and wall, and some is drawn into the eaves. Brett Martin advises laying gutters to a slight fall towards the downpipe, so one that has sagged holds water instead. Look for staining on the wall below and water falling in sheets over the edge.
The leak may therefore start some distance from where it shows, and the heavy-rain pattern hints at overflow instead of a holed covering.
Flashings, chimneys and flat roof outlets
Metal flashings close the junctions with walls, chimneys and roof windows. A lifted edge or failed seal is no trouble until water arrives under pressure. A chimney is the classic case: the back gutter behind it collects what runs down the slope, and if the lead has split or the pointing has failed, heavy rain gets behind the lead and down the stack.
Flat roofs behave differently. A clogged outlet or a dipped deck makes a shallow pond in a downpour, putting every seam and upstand under water. Clearing the outlet fixes the immediate problem, but a roof that holds water regularly needs its falls looked at.
Our page on condensation or leak will help if the damp you see could be either. Dampness that appears in cold, dry weather as well is more likely condensation, and the heavy-rain pattern points the other way.
What to do while the leak is still intermittent
Waiting for the next storm is tempting. Gathering evidence now is better. Note the date and wind direction, where water showed, and whether the drip stopped soon after the rain or carried on for hours. A drip that stops with the rain is usually near its source, while one that runs on has travelled along timber or insulation.
Look from the ground at the ridge, verges, valleys and chimney, through binoculars if you own a pair, and watch the gutters and downpipes in the rain itself. Do not climb up, because a wet roof is slippery. In the loft, put a bucket under any drip. Our page on whether a small roof leak can wait sets out why a leak that comes and goes still deserves attention, and our daylight through the roof page covers gaps you can see from the loft.
How we trace it and what happens next
We begin with the pattern you describe, then check the likeliest details: exposed edges, valleys, flashings, ridge and gutters. For a leak in an awkward spot, a look in the loft during rain can help. Your own photographs arrive with the written quote, which names the repair that cures the cause.
It might be a few refixed tiles, a new length of flashing or a cleared and relined valley. Our roof repair process page describes the steps from first call to cleared site, and the roof repairs hub gathers the rest. The work comes with a written 10-year workmanship guarantee. A survey can be booked on 01793 626096, on WhatsApp or through the form below.