Why does my roof only leak in heavy rain?

Light rain falls and drains. Heavy rain with a gale behind it travels sideways and upwards, and it is forced under the overlap between one slate or tile and the next. A covering with a small gap or a worn headlap, the overlap between courses, may stay dry for years and only let water in during a single storm blowing from one direction.

The clue is orientation. If the stain appears only after rain from the south-west, look at the slope and the walls facing that way: the chimney flashing on that side, the verge, a roof window on that elevation. A leak that follows one wind direction is rarely a general failure of the whole roof. It is a joint that faces the weather.

A valley is the lead or tiled channel where two slopes meet, and it gathers run-off from both. In a downpour it handles a heavy flow, so a blockage of leaves or moss, or a crack in the lead, makes the water bank up and spill over the edge into the roof space. Leaks that appear only in heavy rain are very often valley faults for this reason.

Gutters behave similarly at the eaves. Brett Martin guidance gives a fall of 1 mm for every 600 mm of run, with supports at no more than 900 mm apart. A gutter that has dropped between brackets, or filled with debris, overflows behind the fascia and wets the rafter feet and the ceiling at the wall. Overflow at the front is easy to see; overflow behind it is not, which is why the stain tends to appear along an external wall.

Each tile and slate type has a pitch below which it cannot reliably shed water. Marley's data sets clay plain tiles at 30 to 40 degrees depending on type, and concrete tiles anywhere from 12.5 to 35 degrees. For natural slate, BS 5534 indicates 20 degrees at moderate exposure with a 500 mm slate and 115 mm of overlap. Below those figures water sits in the laps, and a heavy storm pushes it through.

This matters where a roof has been altered. A porch, extension or dormer roof laid flatter than its covering suits will usually leak in storms first. If that sounds like your house, a survey should measure the pitch and compare it with the product's own data sheet instead of guessing.

A flat roof or a shallow lean-to leaks in heavy rain for a different reason: the water cannot leave fast enough. Rockwool, summarising BS 6229, asks for a 1:40 design fall that yields 1:80 once the roof is finished, and IKO gives 1:80 as the recommended minimum. Where the deck has dipped or the outlet has silted up, water ponds, finds a seam and works through.

Look for a pool that stays on the surface a day after rain. That standing water is usually where the covering is weakest. Short of resurfacing, the usual fixes are clearing the outlet, patching the seam and, where the fall is wrong, relaying the deck.

Water indoors after heavy rain does not always come through the covering. A cracked rendered wall can soak up rain and show damp on the inside face. A failed window seal, a blocked cavity or a downpipe emptying onto the masonry can do the same. A condensing loft may also look worse in wet weather, when the air indoors is already heavy with moisture.

The tell is the pattern. Roof water tends to appear on ceilings and at the top of walls, and it often drips from a fixed point. Wall damp tends to spread outwards from skirting height or from around a window. If you are not sure which you have, the photographs from your note-taking will help a roofer decide before anyone gets up on a ladder.

Write down the date, the wind direction if you know it, how long the rain had been falling and where inside the water appeared. Photograph the stain, then photograph it again a day later. Check the loft with a torch for wet timber, a dripping nail head or a dark patch on the underlay.

That record is worth more than a description. It narrows the search before anyone climbs up. Our guide to finding a roof leak sets out how we trace the water to its entry point, which is often a long way from the stain.

Let a stained ceiling dry before it is painted over, and fix the source first. Fresh paint over a damp patch only hides the next leak.

Never climb up in the rain. If the leak is serious, a roofer can sheet the slope or fit a temporary repair; our answer on tarpaulins on a leaking roof explains what that involves. We make a leaking roof safe before anything else, and price the permanent repair afterwards, when the slopes have dried enough for a proper look.

Emergency work is costed differently from planned work, and our page on what an emergency roofer costs sets out the factors. The permanent repair follows a written quote, which covers the make-safe, the repair and any scaffold.

Our roof repairs page covers the wider range of repairs around Stroud.