The short answer
A roof that leaks only in wind usually has a gap that ordinary rain cannot reach. Wind drives water sideways and upwards, so it gets under lifted or cracked tiles, past worn verges and ridges, behind loose flashings and through gaps around chimneys. A roofer checks these edges and joints first, because the main slope is rarely the cause.
How wind changes the way rain behaves
Roofs are designed for rain that falls downwards. Tiles and slates overlap, so each course sheds water onto the one below until it leaves at the eaves. Gales spoil that arrangement. Rain blown sideways can be pushed up under the lower edge of a tile, and a gust can lift a loose tile just enough for water to slip beneath it before it drops back.
Wind also creates pressure differences. Air is forced against one side of the house and pulled away on the other. Water is pushed through small openings that would stay dry in still air, and pulled upwards by suction on the leeward slope. That explains why a leak may appear on a particular wall or in one room whenever the gale arrives from a single quarter. Jot down the wind direction at the time, since it narrows the hunt, and compare it with the wall where the damp showed.
Gaps that wind can use include nail holes in worn tiles, cracks in aged mortar, and the thin joints beside chimneys and walls. They may be harmless in still weather and troublesome in a gale.
Verges and ridges: the exposed edges
The verge is where the tiles stop at a gable wall, and it is the part most open to the weather. No neighbouring slope shelters it. Edge tiles there are usually bedded in mortar or held by a dry-fix trim. Old mortar cracks and drops away, and wind then drives rain beneath the edge tiles.
Ridges are the top lines of the roof, where tiles are bedded or fixed over the join between two slopes. A cracked ridge bed, or a ridge tile that has crept out of line, lets wind blow water in at the highest point. Under BS 5534:2014, ridge and hip tiles are no longer trusted to mortar alone and need a mechanical fixing as well. A ridge that depends on mortar alone is a prime suspect.
Hips, the angled lines running down from the ridge on a hipped roof, behave the same way. Our page on bird spikes on roofs mentions that fittings on ridges also need care not to disturb the tiles.
Flashings, chimneys and lead
Flashings are lead or other metal dressed round chimneys, abutting walls and dormer cheeks. Lead can lift at its edges, and the mortar that holds it can crumble. In still rain the water runs over the top. In wind, it is driven up behind the lead and into the structure.
Chimneys are exposed on every side. Beyond the flashing, cracked flaunching, the mortar shoulder around the pots, lets rain run down inside the stack. A missing or cracked chimney cowl can have the same effect. A mark on the chimney breast indoors that shows only in certain winds is a strong sign.
Lead needs to be fixed so it can move with temperature. Fixed too tight, it cracks. Left loose, it lifts in gusts. Getting that balance right is part of the skill, and it is why we do not seal over lead with a bead of mastic as a repair.
Lifted, cracked and worn coverings
Older tiles lose their fixings. Nails corrode, clips fatigue, and a tile that once sat firmly may now rest loosely on its batten. A gust can lift it and set it down off line, leaving a gap. Slate roofs suffer from nail sickness, where iron nails rust through and slates slide.
Cracked tiles may stay shut in still air and open when the roof flexes in wind. Porous tiles that have lost their surface also take in more water when driven by gusts. Moss does not help, because it holds moisture against the surface.
Underlay is the last line of defence. Where it has torn, perished or been pushed out of place, wind-blown rain reaches the timbers. A few early roofs never had any. Indoors, what a ceiling water stain means helps you read the evidence.
What a roofer checks and what you can note
On site the roofer looks at the exposed edges first: verges, ridges, hips and the abutments to chimneys and walls. Then the slope facing the prevailing wind is checked tile by tile for slips, cracks and loose fixings. The loft inspection looks for marks beneath the tiles that trace the route the water took.
Where it is safe, a hose test can reproduce the leak: water is sprayed on one area at a time while someone indoors watches for the drip. It takes patience, but it finds the entry point with confidence.
You can help by noting the wind direction, how heavy the rain was, and where and when the water appeared. Dated photographs of the stain and of the loft are useful. Do not climb up to look, because wind and wet surfaces make that dangerous.
Putting it right, and keeping it right
The repair depends on the cause. A cracked verge is re-bedded and mechanically fixed. A loose ridge is rebuilt or fitted with a dry-fix system. Lead is renewed or re-dressed with proper fixings, and lost tiles are replaced with matching ones. If the underlay is the problem, it is renewed across the affected area.
A stop-gap may be needed first. Our page on temporary roof repairs sets out what a make-safe involves while the proper work is arranged. The permanent repair then follows once a free survey has produced photographs and a written fixed price, and it helps if you note the wind direction on the days the leak appears.
Wind-driven leaks often point to age, so we also look at what else on the roof is close to failing. For a broader view, see roof repairs in Bath, and ring 01225 285181 or use WhatsApp or the form to book a survey.
