Sizing a roof lantern for a kitchen extension

The size of a roof lantern should be worked out from the room it lights, not from the space available on the roof. A common mistake is to pick the biggest lantern the flat roof will physically carry and centre it. Sometimes that is right. Often it drops far more light into the middle of the room than the corners ever see, and the money would have been better spent on a lantern sized to the daylight the space actually needs.

The starting figure is the glazed area as a share of the floor area beneath it. For a single-storey kitchen extension with ordinary ceiling height, a glazed roof area of somewhere between 10 and 20 per cent of the floor area gives a bright, evenly lit room without tipping into problems. Below about 10 per cent the far reaches of the room stay dull, particularly on a deep extension where the rear wall is a long way from any glazing. Above roughly 20 per cent you are into territory where heat, glare and winter heat loss all start to work against you, and where the room can feel more like a conservatory than part of the house.

Take a typical rear extension of 4 metres by 5 metres, so 20 square metres of floor. Ten per cent is 2 square metres of glass, which a 1.5 by 1.5 metre lantern comfortably provides. Twenty per cent is 4 square metres, roughly a 2 by 2 metre unit. Most kitchens of that size land happily between those two, and where in the band you sit depends heavily on which way the roof faces, which is dealt with further down.

These are working figures for a single-storey extension with a flat or shallow-pitched roof and a ceiling around 2.4 metres. They assume a lantern roughly centred over the room. Deeper rooms, darker finishes and any wall of full-height doors all shift the sums, but the pattern holds.

The glazed figure is the aperture, the daylight opening, not the outer frame dimension. A lantern quoted as 1.5 by 1.5 metres externally has a slightly smaller glazed area once the frame is taken out, so if you are working to the middle of the band, round up rather than down. On a big open-plan kitchen and dining space, one very large lantern is rarely the best answer. Two smaller units, or a lantern paired with a run of rooflights over the dining end, spread the light more evenly and give you more control over which parts of the roof face the sun.

One more adjustment. Bi-fold or sliding doors across the back wall already pour daylight into the rear of the room at eye level. Where a room has a full glazed wall to the garden, you can sit at the lower end of the band and let the lantern handle the middle and front of the room, the part the doors cannot reach.

The floor tells you the size you want. The structure tells you the size you can have, and where it can go. A roof lantern does not sit straight on a flat roof. It sits on a timber upstand, a raised kerb built up off the roof deck, which lifts the glass clear of standing water and gives the flashing something to turn up against. The upstand needs a clear margin of flat roof around it on every side, so the true limit is not the size of the roof but the size of the roof minus that perimeter.

Below the roof, the opening in the ceiling has to be framed. On a new extension this is designed in from the start, with the roof joists trimmed around the aperture and doubled or tripled where they run alongside it to carry the load the interrupted joists would have taken. The wider the lantern across the direction the joists span, the more load lands on those trimming members, and past a certain width the answer is a steel beam rather than timber. None of that stops you having a large lantern. It just means the size is a structural decision made with the builder and, where a beam is involved, a structural engineer, not something to settle by eye on site.

There is also the pitch of the glass itself. A roof lantern is a raised, pitched structure, typically rising at somewhere between 15 and 25 degrees to a central ridge or a hip. That pitch is what lets rain and debris run off, and it is why a lantern reads as a distinct feature rather than a flat panel. The taller the lantern, the more presence it has in the room and from the garden, but a very tall lantern on a small extension can look top-heavy, so the height is chosen against the footprint, not maximised.

Roof lanterns are sold in a grid of standard sizes, and there is real money in staying on that grid. A standard-size aluminium lantern is made to a set pattern and priced accordingly. A bespoke size, made to your exact aperture, carries a premium and a longer lead time. The trick is to design the aperture around a standard size wherever the room allows, and only pay for bespoke where the space genuinely calls for it.

Rectangular lanterns above about 2.5 metres on the long side are usually built with an intermediate rafter bar so the individual glass units stay a sensible, liftable size. That bar is barely noticeable from below and it keeps the glass panes within the weights a crew can handle safely on a roof. Once a room needs more glazed area than a single 2 by 3 metre unit gives, two lanterns almost always beat one enormous one, both for the light distribution and for the handling on the day.

If your aperture falls a few centimetres off a standard size, it is often cheaper to adjust the framing of the opening by that amount than to order a bespoke lantern to match an arbitrary hole. Decide the lantern size first, then build the aperture to suit it. This is one of the things worth settling before the roof structure is fixed, and it is a normal part of a roof lantern installation rather than an afterthought.

Size is not only about square metres. A lantern that is correctly sized for daylight can still look wrong if its shape fights the room. Two proportions do most of the work here.

The first is the aspect ratio. A roughly square room takes a square lantern well. A long thin room, such as a side-return kitchen, looks and lights far better with a rectangular lantern whose long axis runs the same way as the room. Dropping a square lantern into a long room leaves the two ends underlit and the middle over-bright, and it reads as an afterthought from the garden.

The second is the margin around the lantern. As a rough guide, aim to keep the flat roof margin between the edge of the lantern and the parapet or wall at a fairly even width all the way round, and try not to let the lantern occupy the entire roof. A lantern that runs almost wall to wall can look crammed in, and it leaves no room for the upstand and flashing to be built properly. A little breathing space around the unit is not wasted roof, it is what makes the finished thing look deliberate.

Ceiling height matters too. In a room with a standard 2.4 metre ceiling, a large lantern brings the sky right down into the space and can feel generous. Under a very high vaulted or raised ceiling, the same lantern can feel small and distant, and this is a case where going up a size, or choosing a taller-pitched lantern, earns its keep.

Here is where the size question and the aspect of the roof meet, and where the sums change sharply depending on which way your extension faces. A roof lantern faces straight up, so at the height of summer it collects sunlight at a far more direct angle than any wall window. Scale that up and the effect is not subtle. Every extra square metre of glass you add to a south-facing lantern is another square metre of collector pointed almost square-on to the midsummer sun.

Across south Essex this is not a minor consideration, because Southend-on-Sea faces south over the Thames Estuary and the rear extensions here so often look onto a south-facing garden with an open horizon. A 1930s semi with a knocked-through kitchen and a large south-facing lantern is, on a July afternoon, a solar collector with a dining table under it. The bigger the lantern, the more heat and the more glare arrive, and they arrive at ceiling height where the hot air already wants to sit.

This does not mean sizing a south-facing lantern small. It means that on a south or west-facing roof, the point where a larger lantern stops being a bright, pleasant room and starts being an uncomfortable one comes sooner, so you sit lower in the glazing-to-floor band, nearer 10 to 15 per cent than 20. And whatever the size, the glass specification carries the load. On a south-facing lantern the number that decides whether the room is liveable in summer is the G-value, the share of the sun’s energy the glass lets through, not the size alone. A large lantern in solar-control glazing at a G-value around 0.3 can be more comfortable than a smaller one in ordinary clear glass at 0.6. On a north-facing roof, where there is no direct beam to manage, you can size right up to the top of the band and fit clear glass for all the daylight you can get. The orientation decides both the size and the glass, together.

A lantern brings heat in at the highest point of the room, and warm air rises, so the top of the room is exactly where you want a way for that heat to leave. Sizing the lantern generously without any means of ventilation is how a room ends up warm and stuffy on still summer days. A lantern with an opening vent, or a separate opening rooflight nearby, lets the stratified hot air at ceiling level escape far faster than any window at head height can manage.

On a large lantern, or on a high ceiling where nobody will reach a manual handle twice a day, this is the point where an electric opening unit earns its place, often with a rain sensor that closes it automatically. The bigger the glazed area, the more the ventilation matters, so the size of the lantern and the way it opens are worth deciding at the same time rather than treating the vent as an extra.

The practical size limit for a room, then, is not usually the roof or even the structure. It is the point where the glazed area brings in more light and heat than the room can comfortably handle across the year. That ceiling is lower on a south-facing roof and higher on a north-facing one, and it moves with the glass you choose and the ventilation you fit. This is why the size, the aspect, the glass and the opening are all one decision.

Size interacts with the rules in a few ways worth knowing before you settle on a dimension. A new or enlarged rooflight in a dwelling is notifiable under Building Regulations. Part L covers the thermal performance, and it sets a limit on the U-value of the glazing regardless of how large the lantern is, so a bigger lantern does not escape the thermal standard, it simply means more square metres held to it. Part K covers safety glazing, and any glass overhead needs a laminated inner pane, which is standard on a properly specified lantern of any size.

On the planning side, a roof lantern on a house is generally permitted development where it projects no more than 150mm above the existing roof plane and sits below the ridge, but a raised, pitched lantern can breach that 150mm limit, so a larger or taller lantern is more likely to need a check with the local authority than a low-profile flush rooflight. Flats and maisonettes, listed buildings, conservation areas and any property under an Article 4 Direction sit outside the permitted-development route altogether. In this part of Essex the Leigh Cliff and Leigh conservation areas carry an Article 4 Direction that removes permitted-development rights for changes to roofing and windows, so a planning application is needed there where it would not be a few streets away. Treat all of this as general guidance and confirm your own address with the local authority before ordering. New extension rooflights are notifiable whatever their size, and that notification to Southend-on-Sea City Council, or the relevant authority for your address, is made on your behalf as part of the installation.

You can turn all of this into a short brief that gets you a lantern sized properly the first time. Have these to hand before anyone quotes.

1. The room’s internal dimensions and ceiling height. Floor area is the starting point for the glazed area, and ceiling height affects how large the lantern needs to feel right.
2. Which way the roof plane faces. South and west push you towards the lower end of the glazing band and towards solar-control glass. North lets you size up and keep clear glass. This is the first thing to establish, not the last.
3. What else lights the room. A full wall of doors to the garden changes how much the lantern has to do, and usually lets you size it a little smaller.
4. Whether the opening is being framed now or already exists. On a new extension the aperture can be built to suit a standard lantern size and save you the bespoke premium. On an existing roof the opening may fix the size for you.
5. How the lantern will ventilate. Decide the opening vent or a separate opening rooflight alongside the size, especially on a larger lantern or a high ceiling.
6. The glass specification, in numbers. Ask for the U-value and, on any south or west-facing lantern, the G-value, written down for the size you are ordering. A large lantern with the wrong glass is a bigger mistake than a small one.

Sized from the room, checked against the structure, matched to the way the roof faces and paired with the right glass and ventilation, a roof lantern lights a kitchen extension the way it should: evenly, generously and without a hot spot under it in July. If you want the glazed area and glass worked out for your own room and aspect before you commit, ask us for a specification with the size and the G-value written down for each roof plane.