Wind-driven leaks
Why does my roof only leak when it is windy?
The short answer
Because wind changes where the rain goes. In calm weather rain falls down the roof and drains away. In a gale it is shoved sideways and upwards, under tile laps, behind flashings, past a loose verge and into any gap that normally faces away from the weather. A roof that leaks only in wind nearly always has a small fault on the exposed side, and that tends to mean a repair, not a new roof.
Where wind-driven rain gets in, which parts of a roof give way first, and what to note down before a roofer visits.
How wind turns a dry roof into a leaking one
A roof is built to shed water downhill. Tiles and slates overlap so that rain running down one course passes over the top of the next. That works when rain drops from above. Wind-driven rain does not drop. It arrives at an angle, sometimes close to level, and it can be forced up the slope, beneath the lap of a tile and across the gap behind it.
Wind also lifts things. A tile that sits still in a light breeze can flutter in a gale, and the gap it opens lets rain onto the underlay. Pressure across a roof is uneven too. Marley points out that on slopes below 30 degrees the windward side can suffer severe suction, so a shallow bungalow slope or a porch roof can let rain in during gusts that a steeper roof ignores.
So a leak that only appears on blustery, wet days is a useful clue. The fault is small, it sits on the side taking the weather, and it needs a push from the wind to show itself.
The usual places the wind finds
Start with the edges, because wind attacks those first. The verge is the sloping edge running up the side of a gable end. If the tiles there are loose or the bedding has cracked, rain is blown in beneath them. The ridge, the level line along the very top, lets water in along its length if one tile has cracked or come adrift. A single slipped tile out in the field of the roof does the same job on a smaller scale.
Next come the junctions. Flashings are the metalwork, often lead, where the roof butts up to a wall or chimney stack. Once an edge lifts, or the pointing behind it fails, wind-driven rain goes behind it. Valleys (the internal angle between two slopes) and the joint round a roof window or vent pipe are other weak points. Our answer on why flashings leak in storms looks at those.
Do not forget the gutters. One that is clogged or drooping overflows when the wind throws water over its lip, and it can track back behind the fascia and into the eaves.
What to look at, and what to leave alone
Much can be learned from inside the house and from the ground. Jot down the date, the direction the wind is blowing and which room is affected. If a stain always appears on the same wall during a south-westerly, that points a roofer to one slope. Photograph the stain, and in daylight photograph the roof from the garden or the street with a zoom lens, not a ladder.
In the loft, use a torch to hunt for dark marks on the underlay or timbers, chinks of light, or damp insulation. Our guide to checking the loft for a roof leak covers what is safe to touch. Stand a bowl under any drip and shift electrical items out of the way.
Do not climb up to hunt for the fault yourself, and do not smear mastic over anything. Sealant can hide a gap for a week and then trap water behind it.
Why an intermittent leak is still worth fixing
When the roof is dry for weeks at a stretch it is tempting to wait. The trouble is that water still gets in each time wind and rain coincide, and timber, plasterboard and insulation stay damp long after the sky has cleared. You may see nothing until a stain has spread.
Waiting also makes the fault harder to find. A fresh leak leaves a clear trail from the stain back to the entry point. An old one has been soaked and dried so often that the marks run in several directions.
If water appears near a light fitting or socket, treat it as urgent and read our answer on water near lights after a roof leak. Otherwise a leak of this kind is rarely a middle-of-the-night emergency, but it belongs on the list for the next dry spell, and a temporary cover over a lifted tile can bridge the gap until then.
What we do about a wind-only leak
We examine the exposed slope first, photographing each fault, and then work back to the stain. Often it comes down to one or two items: a few loose or cracked tiles, a flashing edge that has lifted, a verge with failed bedding, or a ridge needing to be fixed down again. Where the fault is on an edge, fixing it mechanically, with clips or nails to BS 5534 and not mortar alone, is what stops it returning in the next gale.
If the covering is generally tired, with many slipped or cracked tiles across the slope, that is the verdict you get, with the photographs beside it. Where one repair will hold, that is the job we put forward.
Once the roof has been surveyed, the figure arrives in writing and covers the whole job. Our emergency roof repairs page describes our approach to a roof that is letting water in, and our answer on tiles rattling in the wind covers the warning sign that often comes before a leak.
Related questions
Wind and rain leak questions
Can wind alone make a roof leak without heavy rain?
Wind by itself does not let water in, but it can lift loose tiles, tear at flashings and crack old pointing, so the next shower finds a way through. Light rain driven hard against one slope can be enough to show up a small gap.
Is it safe to leave a wind-only leak until spring?
Not usually. Each wet gale puts more water into the timber and insulation, and a small gap tends to widen as tiles shift. A temporary repair can hold things while you wait for settled weather, but the fault should be found and fixed, not left.