Flat rooflight vs roof lantern, and what each does to the room below
The flat rooflight vs roof lantern question comes up whenever a back kitchen, a single-storey extension or a flat-roofed offshot needs light from above. Both units are rooflights, or skylights in the homeowner's word, and both stand on a kerb in a flat or almost flat roof. Their shapes are what separate them.
A flat rooflight is a sealed glass unit, set in a low frame, that follows the line of the roof. From inside it reads as a single pane of sky in the ceiling. A roof lantern is a small glazed roof: sloping panes rise from the kerb to a ridge or a point, held by glazing bars, so the room gains a feature as well as daylight.
Everything else in this guide follows from that difference in shape: the way the light lands, the height above the roof, the glass area you pay to heat, the cleaning, and the look from the kitchen table.
The room comes first
Glazed doors across the garden end of an extension light the floor beside them. The stretch beside the original house is boxed in by the old rear wall and gets almost nothing from the side, so the roof has to supply its daylight. That is the stretch to look at when you compare the two units, and darkest room first shows how that thinking shapes each of our surveys.
How the light spreads across the floor
A flat unit sends its daylight down in a column roughly the size of the opening. Under cloud, the sky overhead is the brightest part of the sky, so a clear pane delivers a strong, even patch of light directly under it. The edges of that pool of light soften if the reveal, the short shaft between ceiling and glass, is splayed outwards at the ceiling.
A lantern receives light through several faces at once. The sloping panes face different directions, so at any hour at least one of them is catching bright sky, and the room below is lit from more than one angle. Shadows under a cook's own head are fainter as a result. The glass is also spread over a longer footprint, which suits a room that is long and narrow.
Where a flat unit does better
Over a single task, such as a hob, a desk or a basin, a flat rooflight puts the light exactly where it is wanted. In a room that is already bright at one end, a modest flat unit at the shaded end adds natural light without flooding the whole ceiling with glass.
Where a lantern does better
Over a wide family room or an open-plan kitchen and dining space, one lantern lights a long stretch of floor with a softer gradient than a single flat pane manages. Two flat units can match it, with a strip of solid roof between them, but each extra opening adds another kerb to build and weather.
A flat unit lights the floor under its opening. A lantern lights it from several faces, which suits a room that runs long.
Roof height, headroom and planning
A flat rooflight adds only the height of its kerb and frame to the roof line. A lantern adds the kerb, the frame and then the full rise of its slopes, which is why a lantern is the more visible of the two from the garden and from upstairs windows next door. The same height shows inside. A lantern gives extra headroom above the ceiling line, while a flat unit leaves the ceiling height as it was.
That difference has planning consequences. Class C, the national rule on roof alterations that need no planning application, covers a house rooflight that stands 150mm or less clear of the roof slope and does not rise above the roof's highest point. A kerbed flat unit can sit close to that line, depending on the kerb and the make, so the kerb and unit are measured together before an order is placed. A lantern usually rises well past it, so ask the council, or look at whether the extension's own permission already takes it in.
Owners of flats and maisonettes cannot use that right at all, so permission is usually needed, and the council is the place to ask. A listed building needs its own consent, and in some conservation areas an Article 4 direction takes the right away. Every condition is set out in the skylight planning guide, and there is a separate answer on roof lantern planning permission.
Both units need a hole cut in the roof, which changes how it is held up. That counts as notifiable building work, so we notify Building Control.
Heat, glare and glazing
Glass loses more heat than an insulated flat roof, so every rooflight is a trade between daylight and warmth. A lantern usually carries more glass than a flat unit over the same opening, because its slopes are longer than its footprint. That makes the glazing specification matter more on a lantern.
Part L sets the same ceiling for both: a new rooflight, lanterns included, has a U-value no higher than 2.2 W/m²K. Meeting it is the minimum. Triple glazing, warm-edge spacers, gas fills such as argon and low-emissivity coatings all improve the thermal efficiency of either unit, and a lower U-value keeps more warmth in the room.
Summer sun
Overhead glass catches the sun at its highest, so a south-facing extension can warm quickly. A solar control coating holds back the sun's heat. On a flat unit the answer can also be as simple as choosing a smaller opening. On a lantern it is usually the coating, with an opening vent at the ridge if the room has a hob below it.
Frames and finishes
An aluminium frame can be made thinner than uPVC and still carry the glass, which leaves more of the opening clear. Good ones include a thermal break, a strip of insulating material that stops the inner face chilling. Aluminium frames are powder coated, often in a RAL colour matched to the doors and windows. Our aluminium or uPVC lantern frames answer compares the two materials in more detail.
Condensation on cold mornings
Any glass in a heated room can mist on its inner face on a cold morning if the room is damp and the glass is cold. A thermally broken frame and good glazing keep the inner pane warmer, and an opening vent clears moisture in a kitchen. Moisture between the panes of a sealed unit is a different matter: it points to a failed edge seal, so the glazed unit is replaced, and we supply and fit the new one.
Weight, kerbs and what happens on the roof
The two units ask different things of the roof. A flat rooflight is the lighter of the pair, and its opening is often sized to sit between two joists, so the trimming is limited to a pair of short timbers. A lantern is heavier, the opening is usually larger and the kerb has to hold a pitched structure that wants to sit dead level. Where the opening crosses joists, each one that is cut is carried by a trimmer, and a wide span may call for a bigger timber or a steel beam. That is structural work, and part of why two units with the same glass area can need very different amounts of building.
The kerb
Both units sit on a kerb, a low box fixed to the deck and insulated on the outside and inside faces. The roof covering, whether felt, EPDM or GRP, is dressed up the kerb and finished under the unit's frame. A flat rooflight needs a kerb that is level and square, because its glass is bonded to a frame with little tolerance. A lantern sits on a larger kerb and needs the same care over level, since a tilted lantern shows it in the line of the ridge. When the old roof is tired, it makes sense to renew the covering around the kerb in the same visit, so the junction is weathered as one job. The page on putting a rooflight into a finished flat roof follows that sequence step by step.
Getting the unit up
A flat unit is easier to carry through a house or over a side wall. A lantern can arrive as a kit of bars and glass panels to be put together on the roof, or as larger sections, and its big panes need careful handling through a house or over a wall. We note the route when we survey, because a narrow passage can rule out one of the two.
Cleaning, upkeep and how each looks from inside
A flat pane catches whatever lands on it. Leaves, pollen and dust sit on a near-level surface until rain moves them, and a dry spell leaves a film. A lantern's slopes shed more of that, though the bars and the ridge give debris a few places to gather. Neither needs much attention, and the routine for a flat unit is laid out in do flat rooflights need much maintenance.
Inside, the two read very differently. A flat rooflight is quiet: a clean rectangle in a plastered ceiling, which suits a kitchen where the units and the floor are meant to be the focus. A lantern is a feature. Its bars frame the sky, and the room takes on some of the character of a conservatory while the rest of the roof stays solid and insulated. People who want that character choose a lantern, and people who want the ceiling left plain choose a flat unit.
One practical point for either: the ceiling opening is plastered and decorated as part of the job, so the reveal looks like part of the room instead of a hole cut in it.
Which one for which back room
The shape of the room and the roof above it guide the choice. This table sets out the usual Leeds extension shapes.
| Room below | Tends to suit | Reason |
|---|---|---|
| Kitchen extension that runs long and narrow | Lantern, set towards the old house | Spreads light down the shaded run |
| Small utility or rear link | Flat rooflight | Low profile, one tidy opening |
| Bathroom under a flat roof | Flat rooflight | Obscured glass option, steam can escape |
| Open-plan kitchen and dining room | Lantern, or two flat units | Light needed over two areas |
A back room with a low ceiling or a roof that is overlooked by bedroom windows points to the flat unit. A deep room with a generous ceiling suits a lantern, and our guide on whether roof lanterns are worth it goes through that case.
Using both
The two are not rivals on every roof. A lantern over the kitchen and a flat rooflight over the utility room next to it is a sound pairing: each unit is matched to the room beneath it. The details of fixed, opening and walk-on versions of the flat type sit on our flat roof rooflights page, and the lantern side is covered in roof lantern installation.
Settling it on the roof
Photographs and drawings only go so far, so we choose on site: the part of the room that needs the light, the joists that carry the roof and the height the unit may reach. After the free survey we write a fixed quote for the room, giving the unit and how big it is, and our workmanship guarantee runs for 10 years. To book, ring 0113 436 0654 and say which back room is the problem.