Why Does My Roof Only Leak in Heavy Wind?

Why wind changes the way water behaves

Roofs are designed to shed rain that falls from above. Tiles overlap, so water runs down the slope and off the bottom. In strong wind, that picture changes. Rain arrives horizontally and can be blown up the slope, under the laps and into the gaps between tiles. Air pressure also changes around the roof, and the suction on the downwind side can pull moisture through small openings. A roof with no obvious damage may therefore be fine for months and then leak badly once a gale from one direction blows against one face of the house.

That is why the leak so often appears on a particular wall or ceiling, and only when the wind is from a certain quarter. Make a note of the wind direction on the day. It narrows the search a good deal.

The usual places

• The verge. The sloping edge of a gable roof is sealed with mortar or a dry-fix system. If the mortar has cracked or the fixings have failed, wind-driven rain gets under the end tiles.
• The ridge. Ridge tiles bedded on old mortar can crack or lift, so water blows in through the gaps beneath.
• Chimney flashings. The lead where the stack meets the roof can lift at its edges, and cracked pointing lets wind-driven rain behind it.
• Tiles and slates. Slipped, cracked or porous ones allow rain through at the laps. Some old roofs have no underlay beneath.
• Roof windows and flat roof upstands. Poor flashing around a window can let in water at the sides when the wind blows against it.
• Walls and windows. Not every leak comes from the roof. Cracked render, failed window seals and open pointing in a gable wall can all let wind-driven rain in.

For more on how leaks are traced, see our guide to finding and fixing a roof leak.

Underlay and ventilation

The membrane under the tiles is a second line of defence. It catches the spray that blows past the tile laps and carries it to the gutter. Old roofs may have none, or have felt that has torn, perished or been pierced by nails. In that case, the wind-blown spray falls straight onto the insulation. Eaves details matter as well. A gap at the eaves, where the underlay should lap into the gutter, can send water back into the wall plate.

Loft vents, tile vents and ridge ventilators are another source. If they are poorly fitted or the wrong type, wind can push rain in through them.

What you can do

Note the date, the wind direction and where the water appears. Take photographs and look in the loft with a torch during or just after the wind, keeping your weight on the joists. Wet timbers on one slope, or a dark streak running down a rafter, will show where it comes in. Do not go up on the roof in wind. Checking roof damage safely explains why. For wind damage in general, see what wind speed damages a roof.

How a roofer traces it

We start with the pattern: which wall, which wind, how much. Then we look at the windward slope close up, checking tiles, ridge, verge, flashings and vents, and the underlay from inside. Sometimes a water test is used, running a hose from the lowest point upwards on one section at a time, to find where it enters. We photograph the fault and explain it to you, so you can see what is wrong.

When the roof is old

On an older roof, several of these faults can exist together. Mortar has weathered at the verge, the ridge bedding has cracked, the felt has perished and a few tiles have slipped, and the wind finds whichever is nearest. In that case, repairing one fault may only move the leak to the next weak spot. We say so at the survey, and show you the photographs, so you can compare repairing the roof in stages with replacing the covering.

The repair

The fix depends on the fault. It might be re-bedding or replacing ridge tiles, renewing a verge, lifting and re-dressing a flashing, replacing cracked tiles, or patching torn underlay. Our job is to repair what is needed, not to cover it with sealant. The price is written and fixed after the survey, and our emergency and general roof repairs hub explains the type of work we do.