Can a Flat Rooflight Be Fitted to a Dormer Roof?

A dormer box has three parts: the vertical front wall and its window, the two cheeks that tie it back into the main roof slope, and the flat, or very gently pitched, deck on top that closes the box off from the weather. That deck is built the same way as any other small flat roof, with joists spanning between the cheek walls, boarded over and covered in felt, GRP or EPDM. Because it already behaves like a flat roof, it can generally take a flat rooflight in the same way as a flat-roofed extension, once the joists, the fall and the covering have all been checked at survey.

The one thing that differs from a large flat roof is scale. A dormer built to serve one bedroom window is rarely more than two or three metres wide and often less deep, so the glass has to be planned around a deck that is already small, not squeezed into whatever space happens to be left once pipes, vents or a soil stack have been routed across it.

On a dormer roof the size of the glass is set by the joist spacing and the width of the deck, not by an ideal proportion for the room below, so the survey starts with what the timber will allow.

Where the existing joists already fall in a useful position, a single pane can sometimes sit between the joists without cutting into the structure at all. Where the opening needs to be wider than the joist spacing allows, a trimmed opening is formed the same way it would be in any loft floor or flat roof, with timber doubled up either side to carry the load the cut joists were taking. On a dormer this trimming has to clear the cheek walls as well, so there is usually less room to move the opening around than on a full-size flat roof. The same reasoning that governs sizing a flat rooflight against the room below still applies, it is just working within a smaller box.

Every flat rooflight needs a fall to shed water off the glass, and a dormer deck is no exception, even though it looks flat to the eye. The glazing unit itself is set with a built-in fall, but the kerb it sits on has to carry that fall through, which on a dormer often means a firring piece or a thin tapered upstand rather than the wider tapered insulation used on a big roof. Get this wrong and water sits against the low side of the glass, which is exactly the problem covered in why flat rooflights are not quite flat. A minimum fall of 1:80 is standard practice on the glazing itself, and the deck below should not work against it.

Kerb height matters more on a dormer than it does on a larger roof, because the upstand has to clear the existing covering by enough to stay watertight without looking oversized against a small area of glass. The detail is the same one used for kerbs and upstands on any flat rooflight, scaled to the dormer in front of it.

The trickiest part of the job is rarely the glass, it is the junction between the new upstand and whatever covering already sits on the dormer deck. On felt and GRP roofs the new membrane is dressed up and over the kerb and lapped into the existing surface so there is no cold joint for water to find. On an EPDM roof the same principle applies with a bonded rather than a heat-welded lap. The method is exactly the one used across flat rooflights on EPDM, felt and GRP roofs, and the age and condition of the existing covering usually decides whether it is worth re-covering the whole dormer top at the same time rather than working around it piece by piece.

A dormer already gives a room a vertical window with a view out, so a rooflight in its roof adds a second aperture, this time looking straight up. The two need to be planned together rather than treated as separate jobs. Sit on the bed or at the desk that the dormer serves and the eye moves between the two: out through the cheek window, then up through the glass overhead. If the rooflight is placed without reference to that sightline, the reveal or a kerb edge can sit right where the eye naturally lifts, cutting the sky in half from the one spot in the room people actually use. This is exactly what a sightline sketch is for, showing where the frame and reveal fall from that seat before the roof is opened, so the two views work together instead of competing.

Because the deck is small, the reveal below a dormer rooflight is often shallower than on a large extension, which helps rather than hinders the view, an open sky rather than a letterbox glimpsed down a long shaft.

• The covering already on the dormer roof, and whether it is sound enough to weather a new upstand into, or due for renewal anyway.
• The joist spacing and span across the deck, to decide whether the glass sits between existing timber or needs a trimmed opening.
• The fall already built into the dormer roof, so the kerb and the glazing unit work with it rather than against it.
• Whether the dormer was built under permitted development, and what that means for adding a rooflight to its own roof.

Many dormers were built under permitted development, and adding a rooflight to a roof that already exists does not usually reopen that permission on its own, though this depends on what has already been built at the address and its planning history, so it is always checked before work starts rather than assumed. The question of whether a specific job needs an application is covered in will my flat rooflight need a planning application.

Where the work is notifiable, we handle the Building Control notification and install to current Building Regulations, the same as on any flat rooflight. What that covers for a dormer roof, from structural loading to thermal performance at the upstand, is set out in what Building Control asks of a flat rooflight. Every quote also comes with a 10-year workmanship guarantee, whatever roof the glass is going into.

Does a dormer's roof need strengthening before a rooflight goes in?

Only where the opening is wider than the existing joist spacing. In that case the joists either side are doubled to carry the load the cut section was taking, the same trimming used on any flat roof opening. Where the glass fits between joists already in a useful position, no extra structure is usually needed.

Can a rooflight go on a hipped or catslide dormer roof?

A hipped or catslide dormer top still carries a fall in one direction, and a rooflight can be set into it provided the kerb is built up to keep the glass level, or angled to match the fall, in a way that still sheds water off the glass rather than along it.

Will a rooflight look out of scale on a small dormer roof?

Not if it is sized to the deck rather than to a standard catalogue size. A pane planned around the joist spacing and the reveal below usually reads as part of the roof rather than as an add-on, which is why the survey measures the deck before anything is ordered.

Can the rooflight be added later if the dormer roof is re-covered first?

Yes, and a re-covering is often the easiest point to add one, since the covering is already being stripped back and a new upstand can be built in without disturbing a covering that would otherwise be left untouched.