Why Does My Rooflight Leak Only in Driving Rain?

A rooflight that leaks only in driving rain is almost always failing at the flashing laps or at a perished seal, not at the glass. Wind pushes water sideways and upwards, so it travels under laps designed to shed water downwards and past gaskets that have gone hard. In still rain, gravity keeps the water on the outside. In a gale, it does not.

That is why the leak feels random. It is not random at all. It is a specific weather combination, usually rain arriving with wind from one particular direction, and once you match the leak to the wind you are most of the way to knowing what has failed.

A leaking rooflight is genuinely urgent, because water sitting on a trimmer or in the insulation does damage long before it shows on a ceiling. So deal with it. But you do not need to deal with it in the next twenty minutes, and nothing about the diagnosis requires anyone to go outside and climb anything.

What driving rain actually does to a rooflight

A pitched roof is a drainage system that assumes water falls. Every lap in a flashing kit, every tile overlap, every soaker, is sized so that water landing above runs over the joint and continues downhill. The lap dimension is a calculation, not a guess, and it assumes the water has weight and direction on its side.

Wind changes the arithmetic. Sustained wind against a roof slope creates a pressure difference between the outside face of the covering and the ventilated space beneath it. Water sitting in a joint is then pushed, not pulled. It moves uphill under a head flashing, sideways under a soaker, and along the underside of a lap by capillary action once the gap is narrow enough. A 100mm lap that is comfortably adequate on a calm November afternoon is being asked to do something entirely different when a north-easterly is driving rain flat at the same slope for six hours.

On the Suffolk and north Essex coast this is not an occasional event. Weather here arrives predominantly from the east and north-east off the North Sea, so an east-facing slope at Felixstowe, Harwich, Frinton or Aldeburgh takes driven rain close to square on several times a winter. Inland at Stowmarket, Needham Market or Hadleigh the same failure exists, it just gets exercised less often, which is exactly why it takes longer to be noticed there.

The three failures that only show in wind

Flashing laps that are short, lifted or bridging

The commonest cause. Either the laps were short from the day it was fitted, or the head flashing is bridging across the tile course above rather than dressing down into its profile, or the metalwork has lifted slightly over years of thermal movement. In vertical rain the geometry still just about works. Under pressure it does not. We cover the mechanics of this in more detail on our pitched rooflight flashing leak page.

Perished gaskets and seals around the sash

The rubber gasket that seals the sash against the frame has a service life. EPDM gaskets go hard, lose their memory and stop returning to shape after being compressed, typically somewhere past the fifteen year mark and sooner on a slope that gets full weather. A hardened gasket still touches the frame, so the window looks shut and feels shut. It simply no longer forms a seal that can resist positive pressure. Push water at it with a 40mph wind behind it and it passes.

Failed sealant at the glass to sash joint

The perimeter seal holding the glazing unit into the sash is a structural and weathering seal at once. When it perishes, water tracks between the glass edge and the sash frame and appears inside at the bottom corner of the glass. Again, quiet in still rain, active in wind. This one often arrives together with a blown, misted double glazed unit, because the same seal failure lets moisture into the cavity.

How to work out what is happening, from inside

The evidence that actually narrows this down is all internal, and it is worth writing down as it happens because the pattern is the diagnosis.

• Note the wind direction each time it leaks. If it leaks in a north-easterly and stays dry in a south-westerly, that tells us which side of the unit is failing and roughly why.
• Note where the water first appears. Top of the reveal points at the head flashing. One corner of the glass points at the glazing seal. Evenly around all four reveals in cold weather, with no rain involved, is condensation rather than a leak, and that is a different problem covered on our draughty loft rooflight page.
• Note the colour. Brown or tea-coloured staining means the water has run through timber or old felt before it reached you. Clear water on the glass alone in cold weather is usually condensation.
• Look at the timber inside the loft in daylight. Dark staining, a tide line on a trimmer, or a musty smell around the opening tells us this has been going on longer than the ceiling suggests.

Take a photograph each time. Two or three dated photographs of the same stain growing is far better evidence than one description from memory.

Why the answer is replacement

We do not patch rooflights, and it is fair to explain why rather than just state it.

By the time a unit leaks in driving rain, two things are usually true at once. The flashing kit has been on the roof long enough to have moved, and the sash seals have aged in the same weather over the same period. Chasing one of those leaves the other. A fresh bead of sealant over a lifted lap sheds vertical rain perfectly well, which is why it appears to work, and then the first proper easterly finds the same path again. That is why so many people report a leak that was fixed twice and came back twice.

There is also the part nobody sees. Water that has been getting in for a few winters has been sitting on trimmers, in the insulation collar if there is one, and against the plasterboard. Taking the unit out is the only way to see that timber, dry it, replace anything soft, reinstate the underfelt collar properly and put a new insulation collar and vapour seal back. A surface treatment cannot reach any of it.

Whole life cost lands the same way. A replacement unit with the correct flashing kit for your covering, fitted with both collars, carries a 10-year workmanship guarantee and resets the clock on the seals, the glazing and the metalwork together. Repeated visits to the same lap do not. Our flashing kit installation page sets out what the correct kit for slate, plain tile, interlocking concrete and Suffolk pantile actually looks like.

What the survey settles

The survey confirms the covering type and profile depth, so the right flashing kit is ordered rather than the one that comes in the box. It checks the frame depth relative to the tile batten, which is what makes the flashing geometry work at all. It gets into the loft to look at the underlay, the trimmers and the insulation around the opening, because the extent of the wet timber decides the scope. And it establishes whether the existing opening size can be reused or wants adjusting.

You then get one written fixed price. Related reading: what to do when water is coming through a loft rooflight, storm damaged rooflight, and the parent guide to pitched roof skylight installation.

Get it looked at

Tell us which direction the rain was coming from, where the water appears, and roughly how old the unit is. We cover Ipswich, Suffolk and north Essex, about 30 miles out. Request a fixed quote and we will survey it properly, from inside the loft and from the roof with our own access, and tell you what has failed and what it costs to put right permanently.