How is a rooflight made watertight?

Lapped, not sealed

The distinction matters because it explains why bad installations fail and good ones do not. A sealed joint depends on a material staying stuck to two surfaces that move at different rates in temperature swings of fifty degrees across a year. It has a service life measured in a handful of years.

A lapped joint depends on geometry. Water arriving at the joint is on the outside of the upper component and lands on the outside of the lower one. Nothing has to stay stuck for that to keep working. The lap can be dirty, the lead can be a hundred years old, and the water still goes where it is told.

So the correct question about any rooflight perimeter is not what it is sealed with. It is which component laps over which, and by how much.

Two lines of defence, and what each does

Every modern pitched roof works in two layers. The covering, the tiles or slates, is the primary barrier and it sheds the overwhelming majority of the water. Beneath it, the underlay or sarking is the secondary barrier, catching the small amounts that get past under wind pressure or through capillary action and carrying them down to the eaves.

A rooflight interrupts both layers, so both have to be reinstated around it. The flashing kit deals with the primary layer. A drainage collar, gutter and apron system deals with the secondary one, above and below the frame respectively. Installations that address only the visible layer are the ones that produce a wet mark on the ceiling in the first wet autumn.

The order the perimeter goes together

Sequence is not a preference. The assembly can only be built in one direction, and it is always from the bottom of the slope upwards:

• Underlay cut, dressed and lapped so water on it is diverted around the frame.
• Battens cut back and new short battens fixed to support flashing at the sides and head.
• Frame set, squared, packed and fixed to the sub-frame.
• Insulation collar fitted around the frame, then the drainage gutter above the head.
• Apron at the foot, laid over the covering below.
• Side sections next, lapping over the apron.
• Head flashing last, lapping over the side sections and tucked under the course above.
• Covering reinstated, working upward, the topmost course going back last.

Anyone building that in a different order is committing to a joint that faces the wrong way.

The apron and the sides

The apron at the foot is the easiest component and the one that fails least, because everything it deals with is already heading away from the opening. Its job is to carry water from the bottom edge of the frame out onto whole tiles or slates below, and its lap onto them wants to be generous.

The sides carry water down past the frame. On a profiled tile that is a pressed section folded into the trough. On slate it is individual soakers interleaved with each course, one per course, which is why slate takes longer. Our page on soakers, aprons and head flashings takes those components one at a time.

The head is where installations fail

Everything coming down the slope arrives at the head of the unit at once, and it has to be split and sent around both sides without any of it getting behind the frame.

Three things have to be right. The gutter above the head has to be formed with a fall and enough depth to hold a surge. The head flashing has to tuck under a full course above, far enough that wind-driven water reaching under that course still lands on top of the flashing. And the covering above has to terminate cleanly on the flashing rather than being cut short and made up with mortar.

Where a rooflight shows water indoors, the head is the first place to look, and overlong lead dressed too far down the slope, so that it forms a channel rather than a shed, is the classic cause.

The underlay collar, and where the water is meant to end up

Water that gets past the covering has to have somewhere to go, and where it goes is back out onto the covering below the unit. The underlay is cut, folded and dressed so it runs from above the opening, around the sides, and out over the apron zone at the foot.

That route has to remain open. Filling the void between the frame and the rafters with expanding foam, which is a common shortcut, blocks the route and holds any water that arrives against the timber. The correct filler is the manufacturer’s insulation collar, which insulates without closing the drainage path.

The unit’s own gaskets and channels

The window itself is a designed drainage system. Between sash and frame there are compression gaskets, and behind them channels that collect anything that gets past and take it down to weep points at the bottom of the frame.

That is why a small amount of water inside the frame is normal rather than a fault, and why the weep points must not be filled or painted over. It also explains a common misdiagnosis: water appearing at the bottom inside corner of a rooflight is far more often condensation collecting and draining as designed than it is a failure of the perimeter.

Where sealant belongs, and where it does not

There is a legitimate place for sealant on a rooflight, and it is small: bedding a flashing edge onto a very irregular tile where the geometry alone leaves a gap wind could work at, and at certain proprietary junctions where the manufacturer specifies it.

It does not belong along the top edge of a flashing as a substitute for tucking under a course. It does not belong around the frame internally. And a perimeter with a visible bead of mastic running around it is telling you that something upstream was not built to lap properly. Mastic is a component in a system, never the system itself.

What watertight is tested against, and what we guarantee

Manufacturers test units and their flashings together under simultaneous wind pressure and water spray, and publish a pitch range and a weathertightness class for the pair. That test result belongs to the system, not to the window on its own, which is why fitting the right kit for your covering matters as much as choosing the right unit.

On our side, four details decide whether it stays dry: how the underlay is dressed, how the battens are set, how far the laps run, and where the covering terminates against the flashing. None of them is visible once the job is done. That is exactly why the workmanship carries a ten-year guarantee, and why the roof is opened and closed on the same day and never left open overnight.