Why Is My Roof Leaking After Heavy Rain?

A roof is built to shed water quickly, and most of the time a small defect never matters. A hairline gap in a flashing, the metal strip that seals a roof where it butts a wall or chimney stack, passes little water in drizzle. In a downpour, the same gap meets a sheet of water moving fast, and the amount getting through is enough to drip inside.

Volume also changes the paths. Water that normally runs neatly down the tiles can pile up where something narrows or blocks it, such as a clogged valley or a gutter full of silt. Once it backs up, it can climb under the tiles or spill over a lip that is normally dry.

Wind makes it worse. Rain driven sideways reaches places that vertical rain never touches, including the underside of the lowest tiles and the joint where a window meets the roof. That is why a leak can appear on one gable after a westerly storm and never on the other. Write down the wind direction and the room affected, because the pattern often points at the culprit before anyone is on a ladder.

Watch what the gutters do in heavy rain, from a window or under an umbrella at a safe distance. A gutter that overflows at the front or sheets water down the wall behind it is either blocked, sagging or too small for the roof area feeding it. Water that misses the gutter can run back along the fascia board and into the eaves, where it soaks the rafter ends and the ceiling beneath.

A valley is the trough where two roof slopes meet, lined with lead or a plastic or concrete channel. Leaves, moss and tile grit collect there, and in heavy rain the water rises over the edge of the channel and under the tiles. Because a valley drains a large area of roof, it carries far more water than a single tile slope, and a small blockage there becomes a big problem within minutes of a downpour starting.

Old concrete gutters of the Finlock type are a known trouble spot. RICS's defects record says leaking joints have seriously damaged many of them, though it adds that removal should not automatically be assumed. Our blog on replacing concrete Finlock gutters looks at when they can be saved and when they cannot.

Flashing is where most heavy-rain leaks start. Around a chimney, against a gable wall or beneath a dormer window, a metal strip, usually lead, is pushed into a groove in the brickwork and folded down across the tiles. If the mortar holding it has crumbled, the lead has lifted or the sealant has split, water slips behind it. The stain on the ceiling often appears a short distance away from the real gap.

Tiles themselves matter too. A cracked tile, a slipped one or a row with failed nails lets driven rain reach the underlay, the sheet beneath. Underlay copes with some water, though if it is torn or old, or if the batten is wet, the moisture finds its way down.

Flat roofs and porch roofs also show up in storms. Standing water on a felt or rubber roof, a blocked outlet or a split at an edge can drip into a room only when the rain is heavy. If your leak sits under a flat section, our page on an emergency flat roof repair explains the usual causes and what a roofer checks first.

Before anyone visits, collect what you can safely see. Note the room and the exact spot, whether the drip is steady or only in gusts, and how long after the rain started it began. Shine a torch round the loft for damp timber, wet insulation or a dark trail along a rafter, and photograph the gutters and downpipes while the rain is still falling, from a window or the doorstep.

A roofer then works backwards from the stain, looking at the tiles, flashings, valleys and gutters above it, and sometimes testing with a hose, one area at a time, to reproduce the leak. Photos of what turns up go to you, and you receive a single written price once the survey is done. You can read more about tracing in our guide on how to find where a roof is leaking.

Should water be pouring through a ceiling right now, our answer on water coming through the ceiling sets out what to do first. The overview on emergency roof repairs shows how we approach urgent work.