The short answer
Because wind pushes rain sideways and upwards, into places a roof sheds water from in calm weather. Gaps at the verge, the ridge, the flashings and under lifted or slipped tiles let that rain reach the underlay and the loft. A roof that stays dry in a steady shower but leaks in a gale usually has a small detail failing at an edge or a joint.
If your roof only leaks when the wind blows, the covering may be sound and the weak point may be at an edge, a joint or a single lifted tile.
How wind drives rain past a roof
Roofs are built to shed water downhill. Tiles and slates overlap, so rain running down one course is carried over the top of the next. That works well for rain falling straight down. In a gale the rain is thrown sideways, and the wind can push it up the slope, under the lower edge of a tile and across the lap between courses.
Why does this show up only in some storms?
The direction matters. A gap that faces away from the wind on the day may never leak, and the same gap facing into the wind can let in a cupful in each squall. That is why a leak can seem to come and go without any change to the roof. It also explains why the stain on a ceiling can appear a day after a storm, because water first soaks into the underlay, the sheet beneath the tiles, and then works its way through at a low point.
An older roof with worn underlay or slipped tiles lets in more. A newer roof with a poor detail at one edge can do the same, as our answer on a new roof that is leaking explains.
Verges, ridges and flashings
The weak spots in wind are at the edges. Look first at the verge, where the roof edge meets a gable wall. Older verges are sealed with a mortar fillet, a strip of cement under the tiles, and when it cracks or crumbles, rain blown at the gable gets under the tiles. Since the 2014 revision of BS 5534, Marley says, mortar alone cannot be relied upon to hold ridge and hip tiles, which is why dry fix systems tested to BS 8612 are now common.
Where does water get in at the ridge and the flashings?
The ridge runs along the apex, capped with ridge tiles bedded on mortar or held with dry fix clips. Cracked bedding lets wind lift the ridge tile or blow rain under it. Flashings, the strips of lead or metal sealing the roof against a chimney or wall, can lift at an edge or crack at a fold, and a gust of wind finds the gap straight away.
Stand in the garden with binoculars, if you own a pair, and look for tiles sitting at an angle, bits of mortar on the ground near the gable and visible gaps in the ridge line. Do not climb up to check. A roofer can check from a roof ladder, which hooks over the ridge, and photograph what is found.
Lifted and slipped tiles
In strong wind, air pressure changes across a roof, and it can lift a tile at its edge. A tile that is no longer lying flat, or that has slipped a few centimetres down its course, leaves a slot through which rain can be driven in. One slipped tile on a slope is often a sign of others, and a hole in the pattern of a slate roof is the first thing that wind-driven rain finds.
Why do tiles work loose?
Nails corrode, the timbers under the tiles can rot, and clips can fail. A tile that was once held firmly can loosen over many winters and then be shifted by a single gust. Our answer on roof tiles blown off covers what to do about the pieces that come down.
If a few tiles are lifting, a roofer can usually refix or replace them, and check the surrounding ones for the same fault. If many are loose, the better value may be to refix a whole slope. The survey will show which one you have.
Wind that has not done any visible harm can still leave a roof weaker than it looks, and it is worth having a roofer look after a bad storm, even when nothing has yet come through the ceiling.
What to do if it leaks in a gale
Stay off the roof. Wet slopes and strong gusts are a bad combination, and the leak can be dealt with from inside until conditions ease.
How do I limit the damage meanwhile?
Stand a bucket under the drip, clear the furniture and any electrical items from the room, and note where the water is coming through. If it is close to a light or socket, switch the circuit off at the consumer unit and ring an electrician. Our page on repairing a roof in high winds explains why the repair itself has to wait for safer conditions.
Photograph the stain, and write down the wind direction and the time. These details help a roofer match the leak to a weak spot on the roof. Because wind-driven rain is intermittent, a note of when it happened is often more use than a long description.
The temporary fix goes on first, with a survey and a plan for the repair after it. Send your photographs and a survey follows, with shots of the damage and a single total in writing. Our emergency roof repairs page explains how it works.
More on wind and leaks
Why does my loft look dry on days when the ceiling is stained?
Water often travels along rafters and the underlay before it drops, so it can appear a long way from the entry point and a day later. The loft may look dry because the insulation has absorbed the water. Check along the timbers with a torch, from the hatch, for dark trails.
Can wind-driven rain get through a roof that is in good order?
Rarely. A sound roof with correct laps sheds even driven rain. When a good roof leaks in a gale, the cause is usually a specific weak point such as a verge, a flashing or a lifted tile. That is good news, because a local repair usually fixes it.
The full picture: Emergency and general roof repairs in Taunton ยท More short answers