Can You Add a Rooflight to an Existing Flat Roof?

A retrofit flat rooflight is not just glass dropped on a hole. Four things happen in order, and each depends on the one before.

First, the opening is marked from inside, over the spot you want lit, and checked against the joists above. Flat roof joists run in one direction. If the opening lines up with the gaps between them, only one or two joists need cutting. If it falls across several, the trimming is larger.

Second, the cut joists are supported by new timbers either side of the opening, so the roof carries the same load as before. This is the structural part, and it is why cutting a new opening counts as building work. The council or a registered building control approver is notified, and we deal with that where it applies.

Third, a kerb is built around the opening and insulated. If you want the detail, the kerb guide at kerbs and upstands goes through it.

Fourth, the covering is lifted back from the opening, turned up the kerb and sealed, and the rooflight is fixed in place. Last, from inside, the ceiling is opened and the light well is lined.

The covering matters most. A flat roof in GRP, single-ply membrane or felt can all take a rooflight, but each joins to the kerb in its own way. Where the covering is in good condition, the new upstand can be bonded into it. Where it is worn, brittle or has been patched, opening it up is a chance to find out how much life it has left. If the roof needs re-covering soon anyway, doing it with the rooflight saves opening the roof twice.

Access is the second factor. A single-storey roof reached from the garden is quick to work on. A flat roof high above a narrow side return takes more setting up. Inside the room, we need to know whether there is a loft void above the ceiling or whether the ceiling is part of the roof build-up, because that changes how the light well is formed.

The third factor is the fall of the roof, which affects how water moves around the new kerb. The slope question has its own answer, whether a flat rooflight needs a fall.

Permitted development usually covers house rooflights under Class C, with conditions you can read in our planning guide. Before ordering, ask the council if the property is a flat, a listed home or inside a conservation area.

On energy performance, a flat roof unit is a rooflight, so the Part L figure of 2.2 W/m²K applies, and we choose glass that beats it.

No daylight rule exists for English houses, so how many rooflights you add, and how large, is up to you. That freedom is what makes the position worth thinking about. The guidance on choosing the position helps with that decision.

What the survey looks at

The free survey checks the room and the spot, the joist direction, the covering, the access and the height of any upstand needed. The quote you receive is fixed and written. For a simple single unit the roof work takes a day or two, and a written 10-year workmanship guarantee covers it.

Cutting a ceiling is the part people worry about. We protect the floor and furniture below, open the ceiling opening only when the roof above is ready to be closed at the end of the day, and clear up as we go. If the weather turns, the opening is weathered over so the room stays dry.

You will hear the work and see a temporary change to the ceiling for a day or two. After that, the light well is lined, the edges are finished neatly and the daylight lands where you planned it. You can then redecorate the room to suit.

If the rooflight is going into an extension you are still building, the guide to single-storey extension rooflights is worth a look as well. The easiest time to fit a rooflight is while the roof is open, and the guide shows what to settle at that stage.

If you have a flat roof and a dark room and are not sure a rooflight will work, book the free survey. We will look from above and below and tell you plainly what is involved. Other unit types are on the flat roof rooflights hub.