Roof lantern size guide: sizing the glass to the roof it sits in

When a quote says a lantern is 1,500 by 3,000 millimetres, ask which of three numbers that describes. The answer changes what you can fit and how bright the room will be.

The first number is the external kerb size. This is the outside face of the upstand that the lantern frame sits on, and it is the figure manufacturers use to name their lantern skylight sizes. The second is the structural opening, the hole cut or framed in the joists. It is smaller than the kerb by the thickness of the upstand on each side, so a timber kerb built from 50 millimetre stock loses around 100 millimetres in each direction. The third is the visible glass, what you actually see from the kitchen floor once the frame, the ridge and the eaves beam have taken their share.

On a small lantern the gap between kerb size and visible glass can feel large, because the frame is roughly the same width whatever the overall size. That is one reason a lantern that looks generous on a drawing can look modest from below. We always write all three figures on the quote, so there is no guessing about which one the price refers to.

• Kerb size: sets the footprint on the roof and the product you order.
• Structural opening: sets the joist work, trimmers and any beam.
• Visible glass: sets the daylight and the look from inside.

A lantern needs a clear margin of roof on every side. That margin carries the membrane up the upstand, gives the water somewhere to go and lets a person reach the glass to clean it.

Start with the perimeter of the extension. On one side there is usually a parapet or a wall abutment; on the others there is a fascia, a gutter or an edge trim. The kerb should stand well clear of each. As a working starting point, we look for around 500 to 600 millimetres of flat deck between the kerb and any parapet or abutment, and a little more on the side where the water is heading. That strip gives room to dress the membrane neatly into both corners without a tight, crowded detail.

The falls matter just as much. A flat roof is laid to a designed fall of about 1:40 so that, allowing for settlement and tolerances, it still drains at 1:80 or better once built. A lantern kerb is a dam across that fall. Water running down the roof meets the high side of the upstand and has to divide around it. If the kerb sits too close to the high edge, or too near the outlet, the water stalls in the corners and sits against the membrane. On a long extension draining to one gutter, it pays to set the lantern slightly towards the high end, so the water has a clear run on the low side.

The upstand height feeds into this too. The kerb needs to rise far enough above the finished roof surface for the membrane to turn up it and be sealed properly, and 150 millimetres above the finished surface is the usual minimum for the waterproof turn-up. A deeper kerb also lifts the frame away from standing water and snow. Our page on roof lantern kerbs and upstands covers the build-up in detail, and how high a roof lantern sits explains what the finished height looks like from the garden.

A lantern should read as part of the ceiling, not as a hole punched in it. That means matching its shape to the room and placing it where the room is actually used.

Look at the flat ceiling that will be left around the opening. If the plasterboard strip on either side is narrower than the lantern itself, the ceiling starts to look like a frame around glass, and the room can feel top-heavy. If the lantern is tiny in a wide room, it looks like an afterthought. A comfortable result leaves a clear band of flat ceiling on every side, wide enough to carry downlights, a speaker or a blind track, and never so thin that it looks like trim.

Shape follows the room. A square room of about 4 by 4 metres suits a square or near-square lantern. A long, narrow extension across the back of a terrace suits a rectangular lantern running with the long side, which keeps the ridge parallel to the rear wall. For very long rooms, two lanterns in line can sit better than one stretched unit, because each keeps a sensible span and the ceiling between them gives a natural break over a dining table. There is more on this in our answer on which lantern shape suits a long, narrow extension.

Position matters as much as size. Think about what sits below:

1. The kitchen island or dining table, which usually wants the glass directly overhead.
2. Wall units and tall larder cupboards, which should not run into the lantern's light well.
3. Bifold or sliding doors, which already bring in low light, so the lantern can sit back from them and light the deep end of the room.
4. Extractor hoods, which need a solid ceiling to fix to and duct through.

Centring the lantern on the room is the default, but centring it on the island is often the better choice for how the space is lived in. We mark the kerb on the ceiling plan against the kitchen layout before anything is ordered.

These are starting points to test against your own roof, not fixed rules. Every figure is an external kerb size, and every one gets checked against the falls, the joists and the ceiling layout on site.

If your room falls between two rows, the lantern does not have to. Made-to-measure lanterns are built to the kerb you actually have, so the size can follow the ceiling rather than a catalogue. Our answer on whether a roof lantern can be made to measure covers when that is worth doing and when a standard size fits well enough.

A lantern does not simply scale up like a photograph. As the footprint grows, the ridge rises, the bars multiply and the frame changes character.

Aluminium lanterns are built at a fixed roof pitch, typically around 20 to 25 degrees. At a fixed pitch, the ridge height is set by the short side of the lantern. A 1,000 millimetre wide lantern stays low and neat; a 2,000 millimetre wide one rises much higher above the kerb, which is worth checking from the garden, from the neighbour's side and from any first-floor window that looks down onto it.

Glazing bars follow the length. Each glass unit has a practical maximum width, so a longer lantern needs more hip and common bars to carry more panes. A lantern with too few bars carries heavy, wide units; one with too many can look busy from below and throw a striped shadow on the worktop. The bar count also affects the look of the ceiling from the kitchen, and it has a bearing on how the lantern sounds in rain, as the chapter below explains. Our guide to roof lantern glazing bars goes through the counts for common sizes.

On larger or longer lanterns, the frame may need tie bars across the eaves to stop the hips spreading under load. They are a normal part of the engineering on wide spans, but they do cross the opening, so it is better to know about them at the sizing stage than to discover them on delivery day.

The biggest lantern the room could take is not always the biggest one the roof or the regulations will allow. Two checks decide the ceiling on size: the joists and Part L.

Cutting a large opening in a flat roof removes joists. The load those joists carried has to go somewhere, usually into doubled trimmers on each side and trimming joists at each end. For a modest lantern in a roof with joists running the right way, that can be straightforward timber work. For a wide opening, or one where the joists run the long way, the trimmers may not be enough and a steel or engineered timber beam may be needed around the opening. Double-glazed units are heavy, and a large lantern adds real dead load, plus snow load in winter. Our answer on whether a roof lantern needs a steel beam around the opening explains when that point arrives.

Then there is energy. For a new extension, Approved Document L sets a limit on the total area of windows, roof windows and doors, including rooflights and lanterns, of 25 per cent of the extension's floor area, plus the area of any windows or doors that the extension covers up or removes. A large lantern on top of wide glazed doors can push an extension over that limit. When it does, the design has to show compensating performance elsewhere, usually through a calculation. The glass specification also has to meet the Part L limiting value for rooflights. Our answer on whether roof lanterns are energy efficient enough for Part L goes into the numbers that apply to the glass.

Overhead glazing brings safety into it as well. Any glass over a place where people stand should be safety glazing, and a laminated inner pane is the sensible choice for a lantern, because it holds together if it breaks. That choice also helps with sound, which brings us to the last check. Where the work is notifiable, we handle the Building Control notification.

Every extra square metre of lantern is a square metre of the roof where rain lands on glass instead of on an insulated deck. Sizing is the first quiet decision, well before the glazing is chosen.

An insulated flat roof is a heavy, layered build: membrane, board, insulation, deck, ceiling. Rain landing on it is soaked up by all that mass and barely heard. Glass is thinner, stiffer and closer to the room. A large lantern over a kitchen gives a downpour a wide, hard surface to strike, and the sloping panes and aluminium bars carry that sound straight down into the space where you are cooking and talking.

That does not mean choosing a small lantern. It means choosing the glazing build to match the size. On a larger lantern, the useful steps are these: a laminated acoustic inner pane rather than plain toughened glass on both sides, a thicker outer pane, and a sensible cavity between them. Each unit is also seated firmly in its gaskets, so it does not buzz against the frame. On the roof, a solid, well-insulated upstand stops the kerb acting like a drum around the opening. We write the chosen build into the quiet spec that comes with every quote, with the reasons for it.

If rain noise is your main worry, read our answer on whether a roof lantern is louder in heavy rain than a flat rooflight. It compares the two shapes directly and explains which glass choices change what you hear.

These are the points to settle before any lantern is ordered. Each one turns on your own roof, but the reasoning is the same everywhere.

Can I fit the largest lantern my roof will physically hold?

You can fit a large one, but the largest is rarely the best. Leaving a proper margin of flat roof keeps the waterproofing detail clean, keeps the falls working and leaves a comfortable band of flat ceiling inside. Past a certain width you also bring in a beam, a taller ridge and a Part L calculation.

Is one big lantern better than two smaller ones?

For a square room, one well-placed lantern usually reads best. For a long room, two lanterns in line can light the space more evenly, keep each span within easy structural limits and put the glass over the two places you use most, such as the island and the table. The trade-off is a second kerb to detail on the roof.

Do I measure from inside or outside?

Measure the room inside to plan the look, then let the surveyor measure the roof. The lantern is ordered to its external kerb size, which depends on the joist layout, the upstand thickness and the roof build, none of which you can see from the kitchen.

Will a bigger lantern make the room too hot?

It can on a south-facing roof in July. The size, the glass coating and any blinds all play a part, and they are best chosen together rather than one at a time.

When you have a room size and a rough idea of where the light should fall, get a quote. We will measure the roof, mark the kerb against your ceiling plan and set out the lantern size, the glass and the upstand in writing. You can also read more about the full service on our roof lantern installation page.