The short answer
Because wind-driven rain travels sideways and upwards, reaching places that fall-from-above rain never touches. Gaps under tile laps, cracked verge pointing, failed flashings and torn underlay all stay dry on a calm wet day, then let water in when a gale pushes it uphill. The leak is real, it is only waiting for the right wind direction to show itself.
Why some leaks appear only when the wind is behind the rain, and how a roofer works out which side of the house is letting it in.
How wind changes where rain goes
On a still day, rain runs down the tiles and off the eaves. Tiles are designed to work that way, with each course lapping over the one beneath so the water has no way inward. In a gale the path changes. Wind drives drops against the face of the roof and also pushes them up under the lower edge of each tile, through the small gap between courses called the lap.
On a gentle slope the effect is worse, because rain has less distance to fall before it reaches a joint and wind finds a smaller gap to push through. Each tile range has a lowest safe pitch in the maker's data sheet, and a roof laid below it, or one whose pitch has changed over time as the structure settled, is more likely to let wind-driven rain in.
That is why a leak that appears only in storms from one direction is a clue. The wind from the south-west may find a failing verge on the gable end, while a north-easterly will not. Write down when drips began, the weather and the wind direction. A pattern across three or four episodes can save a roofer an hour of searching and a lot of guesswork.
The usual weak spots
Several details are exposed to sideways rain. A verge is the sloping finish at a gable end, where the tiles stop short of the wall below. If the mortar there has cracked or fallen away, the underlay beneath is exposed and wind can drive water into the roof void. Dry verge systems, tested to BS 8612, fix the edge mechanically instead, though they too can slip or lose a cover.
Ridge and hip tiles cap the roof's top and angles. Bedding mortar cracks with age, and a gap under a ridge can pull spray inside. Chimney flashings, the lead or metal sheet that meets the stack, can lift or split, letting water run behind them. Valleys can overflow in heavy rain if they are blocked or if their lining has failed. And tile laps can fill with leaves, which hold water against the gap.
Underlay faults add to all of this. If the sheet beneath is torn or sagging, as described in our answer on torn roof underlay, nothing stops spray reaching the battens once it passes the tile. A weak spot and a torn sheet together produce the typical storm-only drip.
How a roofer traces a storm-only leak
The work starts indoors. A good roofer looks at the stained ceiling and the loft above it, because water seldom comes in where it shows. It travels down a rafter or across the underlay before it drops. Marks in the insulation, dark batten ends and drip stains on the underside of the roof show the line of travel, and that leads upslope to the entry point.
On the roof, the next step is to check the details on the windward side: verge pointing, ridge bedding, flashings, valley linings, cracked or lifted tiles and the lap spacing. If the cause is not clear, the roofer can wet the roof in sections with a hose, working from the lowest point upwards while someone watches the loft. That test is slow, but it shows which patch is the culprit without opening the whole roof.
Photos from the survey of each suspect detail and the evidence beneath it are shared with you, so you see what we found and why. Where a leak runs through the roof edge, our answers on roof tiles lifting in wind and mortar valley cracking may be relevant.
Repair, then check it was the right one
Once the entry point is found, the repair is usually local: renewing verge pointing or fitting a dry-fix system, re-bedding or clipping the ridge, replacing a flashing, relining a valley, or relaying tiles over fresh underlay. A fix that treats only the stain on the ceiling will fail in the next gale.
Be wary of the quick answer. Smearing sealant over a joint might buy a short reprieve, yet it cannot cure a gap that moves with the weather. Ask what was found, where, and why that explains your pattern of leaks. If you have noted the wind direction, the explanation should match it. A leak on the west gable after a south-westerly and a fix on the east gable would not add up.
After the repair, keep watching through the next two or three storms and tell the roofer if anything returns. Our pitched roofing page covers the wider work. To get started, send us the date, wind direction and a photo of the ceiling mark, plus a loft picture from the hatch, and we start our survey from there.
More on this in Pitched Roofing Services in Crawley
Two more wind-driven rain questions
Could a leak that only shows in storms be condensation?
It is possible. Condensation forms on cold surfaces in a loft and can drip, but it follows damp, cold spells more than wind, and it tends to show as spotting on rafters or insulation. A leak that matches wind direction and heavy rain is more likely a storm entry. A survey separates them, since the fixes are very different.
Can wind-driven rain get through a good roof?
Spray can reach the underlay even on a sound roof, which is why the underlay exists. What should not happen is water reaching the ceiling. If it does, something beneath the tiles has failed or a detail has opened up. Age is not the only cause here; a poorly fitted verge or flashing can fail early.