Roof Lantern Size Guide

Chapter 1

What the 15 to 20 per cent rule measures

The familiar rule says a roof lantern should cover 15 to 20 per cent of the area below it. It is a fair place to start, but it hides a question that changes the answer by a third on a small room: 15 to 20 per cent of what?

Suppliers quote the rule in two ways. Some apply it to the roof, measured outside the walls and including any overhang or parapet. Others apply it to the floor of the room the lantern lights. On a large extension the gap between the two is modest. On a small one it is not. Take a kitchen that measures 4.0 by 3.0 metres inside. Its floor is 12 square metres, so the rule gives a lantern of 1.8 to 2.4 square metres. Add cavity walls and a small overhang and the roof above it might measure 4.6 by 3.6 metres, or about 16.5 square metres, and the same rule now suggests up to 3.3 square metres of glass. That is a lantern almost 40 per cent larger, for the same room.

We size against the internal floor area of the space the lantern is meant to light, because that is where the light has to arrive. The roof area matters for other reasons (margins, falls, structure and height, all covered below), but it does not tell you how bright the room will be.

The second catch is the word "room". In an open-plan extension that joins the old kitchen, the lantern is not lighting the whole footprint. Wall windows and glazed doors already light the garden end, and the old house lights its own end through its own windows. The zone that needs roof light is usually the deep middle of the plan, and that zone is the area to measure. A lantern skylight sized for the full open-plan floor tends to overshoot, and the extra glass lands where the light was already good.

Treat the rule as a bracket, then. The lower figure suits a room that already has generous wall glazing or faces south. The upper figure suits a deep room, a north-facing one, or one with little else to light it. Going beyond 20 per cent is occasionally right, for a deep terrace room with no wall glazing at all, but it is the point at which heat and glare start to grow faster than useful light.

Chapter 2

Ceiling height and the depth of the light well

Two rooms with the same floor area can need different lanterns, because the light has to travel down a well and then across a taller or shorter room before it reaches anything you look at.

A lantern does not sit on the ceiling. It sits on an upstand at least 150mm above the finished roof surface, and below that is the roof build-up itself: joists, insulation deep enough to meet current Part L, the deck and the plasterboard. From the ceiling line to the base of the lantern frame is commonly somewhere around 400 to 600mm. That short vertical tunnel is the light well, and its effect depends on the lantern's width more than its area.

Think of the well as blinkers. A lantern 2.0 metres wide over a 500mm well barely notices it: the opening is four times as wide as the well is deep, and the room below sees most of the sky through it. A lantern 1.0 metre wide over the same well is only twice as wide as its well is deep, so the walls of the well cut off more of the sky, and the light arrives as a narrower, more directional pool. That is why small lanterns can disappoint even when they satisfy the percentage, and why a slim lantern in a narrow room often benefits from a splayed well, where the plasterboard reveals are angled outwards to widen the spread. The guide to how a roof lantern is finished inside explains the finishing options.

Ceiling height works in a similar way. In a single-storey extension with a ceiling of about 2.4 metres, the pool of light from the lantern reaches the worktop and table while it is still fairly concentrated. Raise the ceiling to 2.8 or 3.0 metres, as newer homes and some architect-designed extensions do, and the light spreads further before it lands, so each square metre of worktop receives less of it. There is also more wall to light. For a ceiling above about 2.7 metres we lean towards the upper end of the bracket, or towards a wider lantern rather than a longer one, because width does more to open up a tall well.

Surfaces make the last adjustment. Light that lands on a pale oak floor and white walls bounces back up and around the room. The same light landing on dark slate and a navy kitchen is mostly absorbed. A room finished in dark colours needs a little more glass to feel equally bright, or a lantern positioned so its pool falls on the lighter surfaces.

Chapter 3

Adjusting for orientation, wall glazing and glass

Once you have a bracket from the floor area and a nudge from the ceiling, three more things push the size up or down: which way the room faces, how much glazing it already has, and what glass the lantern will carry.

Orientation matters more in Cambridge than in a hilly town. The land is flat and the skyline low, so a lantern sees almost the whole sky and, on a clear day, the whole arc of the sun. A south-facing extension will receive direct sun through the lantern for much of a summer day, and a lantern at the top of the bracket there becomes a heat source first and a light source second. A north-facing extension gets steady, soft sky light and very little direct sun, so it can carry more glass without overheating. Our guide to roof lantern orientation and the sun shows how the ridge direction changes the pool of sunlight through the day, and the answer on whether a roof lantern works on a north-facing extension covers the softer case.

Existing glazing is the second adjustment. A rear wall of bifold or sliding doors lights the first two or three metres of the room very well. A lantern in that same zone adds little. Where the doors are generous, a lantern at the lower end of the bracket, set back towards the house over the part the doors cannot reach, usually lights the room more evenly than a bigger one centred on the floor.

Glass is the third. Clear double glazing passes more visible light than most solar control glass, and blue-tinted solar control passes noticeably less than the neutral types. If the lantern will carry a tinted or heavily coated glass to keep the heat down, allow a little more area to keep the brightness you expected. The roof lantern glass options guide compares what each type does to light and to heat.

Chapter 4

Shape and proportion: fitting the lantern to the room

Area decides how much light comes in; shape decides where it lands and whether the lantern looks as though it belongs. A lantern of the right area in the wrong proportion lights the wrong strip of floor and reads awkwardly from below.

Run the long side of the lantern the same way as the long side of the room. In a room that is 6 metres across the back of the house and 4 metres deep, a lantern of 1.5 by 3.0 metres laid parallel to the house lights a band across the width of the room. The same lantern turned through 90 degrees pushes light towards the garden doors, where it is least needed, and leaves the corners either side dim.

Keep the lantern's own proportions within a sensible range. Between square and about 1 to 2.5 looks settled on most hipped lanterns. Much longer than that and a hipped lantern starts to look like a ridge of glass with little roof to it; a gable-ended or double-hipped design may suit a very long, thin room better, and our guide to roof lantern styles sets out which shapes suit which houses. A square lantern suits a squarish room with a single focal point, such as a dining table.

Line the lantern up with something inside. The edges of a kitchen island, the width of a table, the line of a run of units or the centre of a doorway from the old house all give the eye a reason for where the lantern sits. When the lantern is centred on nothing, it looks as though it was chosen from a catalogue and the room drawn around it.

Chapter 5

How tall the lantern will stand

A lantern's height grows with its width, not its length, and the height is what neighbours, planners and your own first-floor windows notice.

Most lanterns have hips pitched at roughly 20 to 25 degrees, and the answer on what pitch a roof lantern is explains why. The rise from the top of the upstand to the ridge is half the width multiplied by the tangent of the pitch. In plain numbers, it comes out like this:

The last column adds a 150mm upstand to the rise at 25 degrees. The ridge cap and frame add a little more, and on a roof laid to falls the upstand is taller on the low side, so read these as close guides rather than drawing figures.

Two things follow. First, if height is the constraint, make the lantern longer rather than wider: a 1.5 by 3.0 metre lantern stands no taller than a 1.5 by 1.5 metre one. Second, height is where size meets planning. A lantern added to an existing flat roof can easily project more than the 150mm that permitted development allows for roof alterations, whereas one built as part of a new extension is judged with the extension. The guide to planning rules for roof lanterns goes through both cases. Check the view from any first-floor window above the extension, too: a wide lantern seen from a bedroom directly above is a lot of glass and frame to look down on.

Chapter 6

Margins, joists and reading a lantern size

The lantern also has to fit the roof, which means leaving a border of flat roof around it, respecting the direction of the joists, and knowing which dimension a quoted size refers to.

Leave a margin of flat roof all round the upstand. The roof covering needs room to turn up the upstand, rainwater needs a clear path to the gutter or outlet, and the joists either side of the opening need to land on something. As a working guide, plan on around half a metre between the upstand and any wall or parapet, and more where the roof drains towards that edge. A lantern pushed hard against the walls also looks as though it has been squeezed in.

Joist direction can decide the width. Cutting an opening means cutting joists and carrying their ends on trimmers. A lantern that runs along the joists cuts fewer of them than one of the same area running across them, and in a lightly built roof that can be the difference between doubled joists and a steel beam. The guide to the structural opening for a roof lantern explains what a builder or engineer needs to settle before the lantern is ordered.

Standard lanterns run from about 1000 by 1000mm up to around 4000 by 2500mm, with made-to-measure sizes between and beyond. Those figures are usually the external size of the upstand or kerb the lantern sits on, not the glass you see from inside.

A lantern very much under 1000 by 1000mm is rarely worth the frame. The well starts to dominate, the ridge adds little, and a flat glass rooflight often gives as much light for less height and cost. Our comparison of a roof lantern and a flat rooflight weighs the two on the same extension roof.

Chapter 7

Worked sizes for Cambridge rooms

The numbers make more sense on real room shapes. These four are typical of what gets built behind Cambridge houses, from Victorian terraces to new homes on the city edge.

A side return behind a Romsey terrace

A flat-roofed side return extension on a gault brick terrace off Mill Road might give a kitchen 2.4 metres wide and 5.5 metres long, about 13 square metres. The bracket gives 2.0 to 2.6 square metres, and with a half-metre margin each side the lantern can be no wider than about 1.4 metres. A lantern of 1.0 by 2.5 metres lands at the top of the bracket and suits a room with no wall glazing along its length. At 1.0 metre wide, the well depth matters, so a splayed reveal is worth planning. On a side return close to the boundary, Building Regulations fire rules can also limit how near the party or boundary wall the glazing can go, which is checked with Building Control before the size is fixed.

A kitchen-diner on a 1930s semi

A single-storey rear extension on a 1930s semi in Cherry Hinton might be 6.0 metres wide and 4.0 metres deep, about 24 square metres, with bifold doors across the back. The bracket gives 3.6 to 4.8 square metres. Because the doors light the garden end well, we would start near the lower figure: a lantern of 1.2 by 3.0 metres, 3.6 square metres, run parallel to the house and set back over the table and the join with the old kitchen, where the light is needed. At 1.2 metres wide it stands only about 280mm above its upstand at 25 degrees, which keeps it discreet from the bedroom window above. The roof lanterns for kitchen extensions guide covers the choice of position in more depth.

A garden room in a new Trumpington home

A newer house at Trumpington Meadows with a south-facing garden room of 5.0 by 4.0 metres, a 2.7 metre ceiling and full-height glazing to the garden is the case where the bracket needs the most restraint. The floor is 20 square metres, so 3.0 to 4.0 square metres, and the south aspect and the glazing both argue for the bottom. A 1.5 by 2.0 metre lantern in neutral solar control glass gives the room its overhead light without adding a second greenhouse to one that already has a glass wall.

An L-shaped or wraparound extension

Wraparound extensions, which fill the side return and run across the back of a terrace, are common in the streets around Mill Road and Romsey. Size each arm as its own room. A rear arm of 4.5 by 3.5 metres, nearly 16 square metres, takes a lantern of about 1.2 by 2.4 metres. The side arm, perhaps 2.2 by 4.0 metres, is too narrow for a lantern to sit comfortably and is often better served by a flat rooflight or a slim lantern about 0.8 metres wide. Avoid a single large lantern straddling the inside corner. That corner is usually where a steel post or beam gathers the loads, and a lantern set diagonally across it lights neither arm well and looks odd from both.

Chapter 8

A sizing method you can run yourself

Before a survey, you can get within touching distance of the right size with a tape measure and ten minutes. Work through it in this order, because each step narrows the one after.

1. Measure the internal floor area of the zone the lantern must light, not the whole open-plan footprint, and multiply it by 0.15 and 0.20. That is your bracket in square metres.
2. Move within the bracket. Go lower for a south or west aspect, generous wall glazing, or pale surfaces. Go higher for a north aspect, a deep plan, dark finishes, a ceiling above about 2.7 metres, or tinted glass.
3. Choose a shape whose long side runs with the long side of the room, between square and about 1 to 2.5, lined up with the island, table or units it will light.
4. Check the width against the room minus a half-metre margin each side, and against the height table if windows above or planning limits are close.
5. Note which way the joists run and whether the lantern crosses them, so the structural opening can be discussed early.
6. Round to a sensible manufactured size and confirm whether it refers to the upstand or the clear opening.

What this cannot do is show where the light will land at 8am in February and at 4pm in July. That depends on the orientation of the ridge, the height of the sun through the Cambridge year and the way you use the room, and it is what we work out at a survey and set down in the light plan that comes with every quote. The page on how we plan the light explains that process, and our roof lantern installation page covers the whole service, from upstand to plastered finish.

Chapter 9

Questions people ask

Is one large lantern better than two smaller ones?

Not always. In a long room, or one used in two distinct zones such as a kitchen run and a dining table, two matched lanterns each sized to their zone usually light the room more evenly than one large lantern in the middle, and each stands lower because each is narrower. One lantern suits a room with a single focus and keeps the roof simpler to build and weather.

Can a roof lantern be too big?

Yes. Beyond about 20 per cent of the floor it lights, a lantern tends to add heat and glare faster than useful light, particularly facing south. It also needs a larger structural opening, weighs more, stands taller and puts more glass overhead to shade. If a room seems to need more than that, two lanterns or better placement usually solve it more comfortably.

Should I size the lantern before the extension is built?

Ideally, yes. On a new extension the opening, the trimmers and the upstand can be designed around the lantern from the start, and the lantern is judged for planning as part of the extension. Fixing the size early, alongside the glass and position, is the cheapest point to get the light right. If you are at that stage, book a light survey and we will size it with you before the roof goes on.