Can you have roof windows in a solid conservatory roof?

People use the words loosely, but building control does not. Approved Document L treats a roof window as one that follows the slope and lies flat within it, while a rooflight stands proud of the roof on a kerb or upstand. A pyramid lantern or a flat glass rooflight on a raised frame falls into the second group. A hinged window that sits flush with the tiles falls into the first.

The difference matters in three ways. It sets the thermal value the unit has to meet, it changes how water is shed round the edges, and it changes how much daylight reaches the middle of the room. Flush roof windows suit a plain pitched roof with a good slope. Lanterns suit a flatter hip-ended roof where you want a tall, bright centre. Many solid conservatory roofs use one lantern and one or two roof windows together, which spreads the light across the room and gives you more than one opening for ventilation.

A solid roof is heavier than the glass or polycarbonate it replaces. Cutting an opening in it removes rafters, so the remaining timbers must be trimmed and doubled around the hole, with the load passed on to the frame and the foundations. Not every conservatory frame was designed for this. Planning Portal advice is to ask an engineer or surveyor to inspect the frame when a replacement covering will weigh noticeably more than the old one, and a conservatory with openings cut into it fits that case.

For that reason we look at the frame before we talk about window sizes. Where the structure is sound, the work is straightforward. Where it is not, the answer may be a steel or timber upgrade, or a smaller opening, or a lantern placed on the strongest span. Ask whoever quotes for the job who has checked the structure and whether the findings are in writing. Our page on foundations for a solid roof covers the other half of the load path.

When a solid roof is built, building control looks at its energy performance, and the glazed units set into it are part of that. Table 4.2 of Approved Document L 2021 limits a new roof window to 1.4 W/(m²·K); a unit rated Window Energy Rating Band B also meets the minimum. A kerbed rooflight may reach 2.2 W/(m²·K), and an upstand built on site 0.35. The 2026 edition takes effect on 24 March 2027, and its Table 3.2 repeats those values.

You can read those numbers off the manufacturer's data sheet, so there is no need to take anyone's word for it. Ask for the sheet of the exact unit, not the range, and ask that it goes into the paperwork building control sees. A cheap unit with a poor value can hold the whole roof back. A good unit in a badly insulated upstand loses most of its advantage at the edges, which is why the kerb or the frame detail is checked as carefully as the glass.

Almost every leak around a roof window or lantern starts at the edge, not the glass. On a flush unit that means the flashing kit, the soakers at the sides and the apron at the bottom. Soakers are the small folded lead or metal pieces worked in with each course of tiles to turn water back over the joint. On a kerbed lantern it means the upstand, the cover flashing and the way the roof covering is dressed up to the kerb.

Lead is the usual choice for the flashings, in a weight chosen to suit the position. What matters is that the lead is dressed neatly and fixed so that it can move with the roof, not squeezed tight and sealed with silicone as the only defence. The same care is needed at the top of the unit, where a back gutter carries water around it. If you already have a lantern that weeps, our page on a leaking conservatory roof explains how we trace it.

Think about what the windows will do in July before choosing them. A south-facing room with a big fixed lantern can end up hotter than it was under the old glass, even with solar-control glazing. An opening roof window lets hot air leave at the highest point, which helps ventilation as well as light. Many roofers offer electric or rain-sensing units, and a blind that fits the frame is cheaper to fit now than to hang awkwardly later.

Size matters too. More glass brings more light, but also more heat in summer and more loss in winter, so a few well-placed units usually do better than a roof that is mostly window again. Within our conservatory roof replacement section, if you are weighing options, our guides on a solid conservatory roof and on skylights and roof windows sit alongside this one. After the survey and photographs of the frame, you receive our advice on layout plus a written quote giving a single figure.