Water coming in only during driving rain

If water only appears when rain is being driven against the house, the defect is almost never a hole in the roof covering. Wind is pushing water sideways into a vertical surface, or through a lap that only fails under pressure. Work out which elevation the rain was hitting and you have already halved the search.

The short version

A leak that waits for wind is telling you the water is travelling horizontally. Look at walls, joints and details, not at the tile field.
Note the wind direction and the elevation that got wet. That pairing rules out three quarters of the house.
Penetrating damp appears during driving rain and dries back over a dry fortnight. Damp that never switches off is a different problem.
Approved Document C maps the country into zones for exposure to driving rain, and an exposed elevation needs more of its details than a sheltered one.
Test with a garden hose from the bottom upwards, one band at a time. Never a pressure washer, which drives water where rain could not go.

The clue in the timing

What a storm-only leak rules out

You have already done the hard part by spotting the pattern. A leak fed by gravity does not care about the wind: water finds a gap, runs downhill and drips, in drizzle as happily as in a gale. If yours stays dry through ordinary vertical rain and only shows up when the rain comes in sideways, it is not being fed by gravity. It is being pushed.

Two mechanisms do that pushing, and both point away from the middle of the roof. The first is saturation: a vertical face takes so much water for so long that the wall wets right through, or a crack fills and carries water across. The second is pressure. Overlaps are designed to shed water running down them, so wind arriving at an angle drives water uphill through the lap. That is why a flashing or a verge that is perfectly sound in still rain can pass water in a storm. Government guidance puts walls first in its own definition, calling penetrating damp water that gets into the building from outside due to defects in the walls, roofs, windows or floors.

The dry-back test, which costs nothing

Pencil round the patch and write the date on the wall, then leave it alone. Penetrating damp darkens in driving rain and shrinks back over a dry fortnight. Damp that sits at the same size in a dry spell is not coming through from outside, and belongs with damp at the bottom of a wall. Damp that is worst in cold, still weather, with black spotting instead of a spreading stain, is condensation, the opposite weather to a storm, and in a loft that is the whole of telling loft condensation from a roof leak.

The question that narrows it

Which way does that wall face?

Ask two questions and write the answers down: which way does the wet wall face, and which way was the wind blowing when it got wet. If the leak follows one wind direction, the defect is on the elevation that wind hits, which narrows the search faster than any amount of poking about indoors.

The Met Office puts the national pattern plainly: the prevailing west to southwesterly winds across the UK lead to northern and western areas being typically more exposed than the south and east, with most hills and coasts windier than low-lying inland ground. Wet weather here arrives on a south westerly, so a south west facing gable takes far more driven water over its life than the north east face of the same house, and the two were built exactly the same way.

There is a map, and your wall is on it

Measured, not guessed. Approved Document C carries a diagram dividing the UK into zones for exposure to driving rain, then tells you to adjust it for your own site: add one where local conditions accentuate wind effects, such as an open hillside or a valley that funnels wind onto the wall, and subtract one where the wall does not face into the prevailing wind. The document is saying, in its own terms, that the elevation changes the answer.

For a proper site assessment the reference is BS 8104, which sets out two methods for assessing exposure of walls to wind-driven rain: a local spell index and a local annual index. The spell index is used for resistance to rain penetration, the annual index for durability, weathering and the growth of mould or mosses. Exposure is an engineering quantity, not an opinion.

An exposed elevation on the Dorset coast sits at a very different point on that map from a sheltered inland one twenty miles up the road, and a detail that is fine on the second is marginal on the first. In the most exposed zone the Approved Document expects checked rebates at every window and door reveal, with the frame set back behind the outer leaf. Your wall is not unlucky. It sits at the severe end of a national scale and was built for a gentler part of it.

The shortlist

Where the water is actually getting in

Water that only comes in during driving rain is often not getting through the roof at all, which is the part we would rather not write. We are roofers, and the honest shortlist for this symptom is dominated by walls, pointing, render and openings. Relaying a slope that was never letting water in is a wasted invoice, and the damp returns with the next south westerly.

What to look at behind a damp patch that only appears in driving rain
Could be	Where to look	What tells you it is this
Solid wall saturating	The outside face directly behind the internal patch	Damp that spreads and does not track, at any height on the elevation the rain hit, drying back in a dry week
Perished pointing	The joints on that elevation, low down and up at the top	Joints you can rake out with a car key, or mortar proud of eroded brick faces
Cracked or hollow render	Tap the render panel with your knuckles	A crack, or a dull hollow note where the render has parted from the wall
Bridged cavity	A cavity-built house, damp at one repeatable spot rather than spread out	Something carries water across the gap: mortar on a wall tie, or debris at the cavity base
Window or door detail	Reveals, the underside of the sill, the corners of the opening	Split mastic, a sill with no drip groove, or no sub-sill at all
Chimney, flashing or verge	Top of a wall, a roof junction, the mortar at the gable edge	One face of the stack taking the weather, or a lap that sheds vertical rain but not sideways

The roof belongs on that list in a specific way. Wind lifts the covering on an exposed slope and rain crosses the underlay lap behind it, so the loft is wet high up and only in storms. Flashings and soakers behave the same way: sound in still rain, permeable under pressure. Gutters come into it more often than people expect, because one overflowing down the wall wets the elevation the wind is already loading, which is why a gutter overflows in heavy rain. If what you have is a stain on the ceiling rather than damp on a wall, start with what a brown stain on the ceiling means: the entry point is almost never directly above it.

Proving it

How to test it with a hose, in the right order

A hose test settles it, but only in the right order, and most people do it backwards. Start at the top and you have wetted every surface below, so when the damp appears you have learned nothing. Start at the bottom.

Get somebody inside watching the patch while you work outside.
Set the hose to a gentle spread, not a jet. Rain does not arrive at mains pressure.
Wet the lowest suspected band only, for ten to fifteen minutes. That is longer than feels necessary, and cutting it short is why most home tests fail.
Nothing inside? Move up one band, and wet only that for the same time.
Keep going up: wall face, sill and reveal, head of the opening, abutment, verge. Stop the moment the patch darkens. That band is your answer.
Note which band did it and how long it took. A slow appearance points at saturation through a wall, a quick one at an open route such as a crack or failed joint.

Two rules before you start. Never use a pressure washer for this. It pushes water through joints rain could never reach, so you get a false positive on a sound wall, and it strips pointing, cuts render and drives water deep into masonry while it does it. That damage is the whole argument in soft washing against pressure washing. Do all of this from the ground, from an upstairs window and from inside the loft. Nothing here needs you on the roof, and the HSE rules on work at height exist because a wet roof in wind is where people get badly hurt. If the damp is near a socket, switch or light fitting, isolate that circuit first.

The wrong repair

The coating that usually makes it worse

The remedy most often sold for this is a water repellent coating over the whole elevation, and it is the wrong answer twice over. A coating bridges a crack rather than filling it, so water still gets in behind, and having made the face less permeable it then slows the wall down when it tries to dry.

The same water stays in the wall for longer. On a solid pre-1919 wall built to absorb and release moisture, sealing the outside face is a known cause of harm, not a fix.

The right answer is nearly always smaller and duller than a coating contract. Repoint the elevation taking the weather. Cut out and make good the failed render panel instead of skimming over it. Fit a sub-sill, cut a drip, reseal one opening, repair one flashing. Which mortar goes back in matters more than people think, and on an older wall it is the difference between the joint weathering and the brick weathering: lime mortar against cement mortar for repointing. If the elevation is rendered, the choice of system decides how the wall dries afterwards, which is silicone render against sand and cement. Re-rendering or recladding a whole elevation can also need permission where the look of the house changes materially, so ring the council before anyone orders scaffold.

And replastering the inside before the outside is fixed will fail. New plaster over a wall still being wetted blows, stains and comes off, and you pay twice. Get the outside right, let the wall dry, then put the inside back.

Tracing a driving-rain leak means noting which elevation gets wet and the wind each time, looking at the outside face behind the patch, and watching the gutter and downpipe in the next downpour. Work through it in this order and stop when something matches.

Note the elevation that gets wet and the wind direction each time. Two or three storms of notes beat any single visit.
Look at the outside face directly behind the patch, from the ground and from an upstairs window. Photograph it and zoom in. Do not climb up.
Watch the gutter and downpipe on that elevation during the next downpour. A joint spraying the wall is free to fix and accounts for a fair share of these.
Do the hose test, lowest band first, one band at a time, with somebody watching inside.
Only then get a price, for the specific defect you found rather than for a coating over the whole wall.

If it turns out to be pointing or masonry, that is repointing and brickwork. If it is render, or the inside needs putting back once the wall has dried, that is plastering and rendering. If it really is a flashing, a verge, a stack or an underlay lap, that is roofing, and usually a morning's work, not a re-roof.

Thomas comes out and looks at it himself, at no charge for the visit, and puts in writing what he found and what it would take to put right, itemised. If it is a gutter joint spraying the wall and you do not need us, he will say so. Ask for a written quote, or get in touch and describe which wall goes wet and in which wind.

Questions

Common questions

Why does my roof only leak in heavy wind and rain?

Because the water is not falling into a hole, it is being pushed sideways. Wind puts a pressure difference across laps and joints that shed vertical rain perfectly well, and it wets vertical faces hard enough for water to cross a wall. Look at walls, flashings, verges and openings, not the tile field.

Which wall is most likely to be the problem?

The one facing the wind during the storm. The Met Office notes that the prevailing west to southwesterly winds make western and northern faces more exposed, and Approved Document C tells you to subtract a zone where a wall does not face into the prevailing wind. Note the direction each time it happens.

Is a water repellent coating the answer?

Usually not, a coating bridges a crack without filling it, so water still gets in, and it then slows the wall down when it tries to dry. On an older solid wall that traps moisture in the masonry. Fixing the specific defect, normally pointing, render or one opening, is smaller and works.

How do I test a wall with a hose without getting a false result?

Start at the lowest suspected point and work upwards, wetting one horizontal band at a time for ten to fifteen minutes with someone watching inside. Never use a pressure washer: it forces water through joints that rain could not reach and damages pointing and render while it does it.

Could it be condensation rather than a leak?

Not if it follows storms. Condensation is worst in cold, still weather and shows as black spotting on cold surfaces. Penetrating damp appears during driving rain and dries back over a dry fortnight. If the damp never changes with the weather at all, it is something else again.

Related advice

The guides below pick up where this one leaves off, and the roofing page covers the work itself and the places we cover, if you would rather have the job looked at.

Roofing, and where we cover All advice articles

Who wrote this

Thomas Hannen

Thomas owns Bournemouth Roofing & Building and prices every job himself instead of sending somebody else. The company is Bournemouth Roofing & Building Ltd, number 15471719, registered in England and Wales. If you want to talk something through before you commit to anything, the site visit and the written itemised quote are both free.

Water coming in only during driving rain: what that tells you