Can a rooflight be fitted to a flat roof?

Why the unit sits above the roof, not in it

On a pitched roof, gravity does most of the work. Water meets a flashing dressed into the courses around the unit and carries on down the slope past it. Nothing has time to sit anywhere.

A flat roof has no such help. Water crosses it slowly, and after heavy rain it lingers. So the arrangement is different: a raised timber perimeter, called a kerb or an upstand, is built off the roof structure around the opening. The waterproof covering is dressed up the outside face of that kerb and terminated near the top, and the rooflight frame closes over the termination.

What that achieves explains almost every decision that follows. The point at which the covering stops is lifted clear of the roof surface, so the vulnerable line where waterproofing turns from horizontal to vertical is never sitting in water. Get it wrong and no quality of glass or frame compensates.

Height is set by water, not by looks

The height of that kerb is not a number lifted from a catalogue. It follows from the roof: how the falls run, what the covering is, how exposed the roof is, and where it drains.

The kerb has to clear the highest level water will ever reach, and that is not the level water reaches in ordinary rain. It is the level it reaches when the outlet is half blocked with leaves and there is no overflow. Standing water on a flat roof is normal rather than a defect, and a well-designed roof still holds water briefly after a downpour while it drains.

Two things push the requirement upwards. The fall means the roof upslope of the unit sits higher than the surface downslope, so a kerb measured from the low side is shorter than it looks where it matters. And a kerb standing in the path of running water makes it spread sideways and gather at the two upslope corners, which is where flat roof units fail. That is designed out with tapered insulation cut to divert the flow round the kerb, or on a wider unit a gutter behind the upslope face.

Insulating the kerb is not optional

This is the detail most often left out, and it produces a complaint that sounds like a leak and is not one.

A modern flat roof is a warm roof: insulation above the deck, below the covering, with a vapour control layer beneath it, so the structure stays on the warm side. A rooflight kerb is a hole cut through all of that. If the kerb is treated as carpentry rather than as part of the roof build-up, it becomes a loop of solid timber connecting the warm inside of the building directly to the cold outside, all the way round the unit.

In cold weather the inner face of that loop runs colder than the ceiling around it. Warm damp air from the room reaches it, and water forms in a neat line around the rooflight, on the reveal and at the bottom of the frame. It appears in still cold weather rather than during rain, and it improves in spring.

The answer is in the original detail: insulate the outer face of the kerb as a continuation of the deck insulation, and carry the vapour control layer up the inner face so the frame closes over it.

Flat glass or a raised lantern

Two shapes suit a flat roof, and the room and the sightlines choose between them rather than one being better.

A flat unit is quiet. It sits close to the roof plane, reads as a panel of glass in a surface, and is the right answer where the roof is low, closely overlooked from upper windows next door, or in a setting where the roofscape has to stay legible. It is also the option available where the ceiling below has no height to spare.

A lantern is a raised glazed structure with panels running up to a ridge. Because the glass sits on several planes it catches the sky at different angles through the day and carries light further into a room than a flat unit of the same footprint. The pitched form also sheds water and debris, where a level pane holds both. Against that, it is a larger object standing above the roof line and visible from anything overlooking it.

Our roof lantern installation page covers the sizing and the bar arrangement.

Glass somebody can stand on

If any part of the roof is walked on, that changes the specification from the outset rather than at the end.

Ordinary flat roof glazing is made to be looked at and rained on. It is not made to carry a person, and treating it as though it is has an obvious consequence. Walk-on units are a separate category: a thicker laminated build-up, a slip-resistant top surface, and a frame designed for the point loads that come with somebody standing on glass. They are specified against an imposed load, not only against wind and snow.

The situation that catches people out is not a terrace, which is obvious, but a lower extension roof reached through a first-floor door. Where anybody will ever stand on the unit, say so at the first conversation. Upgrading afterwards means replacing the glass, the frame and usually the kerb with them.

The covering is the waterproofing

One principle sits under everything above. The waterproof covering of the roof, whether that is single ply membrane, torch-applied felt or lead, is what keeps water out. The rooflight frame is a weathering layer sitting above it. Any detail that reverses those roles, where a bead of sealant at the frame is the only thing holding water out, will let go within a few winters.

That is why our survey of a flat roof looks at the structure, the deck, the covering, where the insulation sits and which way the roof falls before it looks at a product. The flat roof skylight installation page sets out how each covering terminates against an upstand, how falls are formed, and what a cold roof build-up means for a new unit.

We install new units and full replacements across Oxford and up to twenty five miles by road, we have been installing rooflights for fifteen years, the Building Control notification is handled as part of the job, and our workmanship carries a ten-year guarantee.