What Size Roof Lantern Do I Need? A Room-by-Room Sizing Guide (2026)

Written by the Glazing Store team — online supplier of Korniche roof lanterns, bi-fold doors and rooflights, taking sizing calls from builders and homeowners on 01604 347222 every working day. Last updated: 10 November 2026

TL;DR: Order your Korniche roof lantern to the external size of the finished timber kerb, not the hole in the ceiling and not the internal kerb measurement — that single mistake causes more re-orders than everything else combined. As a rule of thumb, aim for a lantern covering 15–20% of the room's floor area, keeping at least 500mm of flat roof on every side. For a 3 x 3m extension that is typically 1000 x 1500mm (£923.24); for 4 x 3m, 1500 x 2000mm (£1,237.68); for 5 x 3m, 1500 x 2500mm (£1,389.32); for 6 x 4m, 2000 x 3000mm (£1,770.71). All inc VAT.

Contents

1. What size do you actually order — kerb or lantern?
2. How do I measure the kerb correctly?
3. What size roof lantern for a 3x3, 4x3, 5x3 or 6x4 extension?
4. One large lantern or two smaller ones?
5. How much taller does a bigger lantern make the roof?
6. How heavy is the glass, and what can two people lift?
7. Will a bigger lantern mean more bars in the view?
8. What size needs a structural opening or building control?
9. Roof lantern size and price table
10. FAQs

What size do you actually order — kerb or lantern?

Every Korniche roof lantern is ordered to the external dimensions of the kerb (upstand) it sits on — the base frame drops over the outside face of the kerb like a lid on a box. Build a kerb with an external footprint of 1500 x 2500mm and you order the 1500 x 2500mm lantern, even though the daylight opening through the ceiling ends up noticeably smaller once kerb timber, insulation and plasterboard reveal are taken off. Three measurements get confused:

Measurement	What it means	Do you order to it?
Structural opening	The hole framed in the flat-roof joists	No — usually 100–150mm bigger than the finished kerb each way
External kerb size	Outside face to outside face of the finished upstand	Yes — this is your order size
Internal kerb size / visible glass	What you see looking up from inside	No — smaller by the kerb thickness and frame width on all four sides

From behind the counter: the call we take most often starts "I measured 1400 x 2400 inside the kerb, what do I order?" With a kerb built from 100 x 50mm timber the outside is 1500 x 2500mm, and that is the lantern. Go smaller and it falls through the hole; go bigger and it sits proud with no bearing. Measure the outside of the kerb, at the top, once it is covered — that is the only number we need.

How do I measure the kerb correctly?

1. Measure the top of the kerb, not the bottom — kerbs get built slightly splayed, and the lantern lands on the top face.
2. Measure both ends and both sides. More than about 3mm difference and you will see a tapering shadow gap along one rafter.
3. Check the diagonals. Equal diagonals means square; it takes 30 seconds and saves a re-order.
4. Measure over the finished covering. EPDM, GRP or felt dressed over the kerb adds a few millimetres per side.
5. Check the kerb is level and order to the outside face exactly. The ridge follows the kerb — 8mm out over 3m reads as a leaning ridge — and you should never deduct 5mm as a "tolerance".

Trade tip: write the four measurements and the two diagonals on the kerb in marker before you phone the order through. When we read the size back, you are reading the timber, not a note on your phone.

What size roof lantern for a 3x3, 4x3, 5x3 or 6x4 extension?

Two rules do most of the work. Aim for a lantern of roughly 15–20% of the room's floor area — under about 12% it reads as "a window in a ceiling", over about 25% you lose the flat roof you need for falls and outlets. And leave at least 500mm of flat roof on every side of the kerb for the roofer to dress the membrane into.

Extension (internal)	Floor area	Lantern we'd suggest	Glazed area	% of floor	Price inc VAT
3 x 3m	9m²	1000 x 1500mm	1.5m²	17%	£923.24
4 x 3m	12m²	1500 x 2000mm	3.0m²	25%*	£1,237.68
5 x 3m	15m²	1500 x 2500mm	3.75m²	25%*	£1,389.32
5 x 3m (calmer)	15m²	1250 x 2500mm	3.12m²	21%	£1,286.36
6 x 4m	24m²	2000 x 3000mm	6.0m²	25%	£1,770.71
6 x 4m (two lanterns)	24m²	2 x 1500 x 2000mm	6.0m²	25%	£2,475.36
7 x 4m	28m²	2000 x 4000mm	8.0m²	29%	£2,296.46

*A 3m-deep extension only has ~2m of width left over once a 1500mm kerb sits in it, so 1500mm-wide lanterns are the practical maximum on a 3m depth. Prices are Glazing Store's live figures for the Korniche Aluminium Roof Lantern in clear 1.2 W/m²K glazing, correct as of 21 August 2026.

Keep the lantern's proportions close to the room's: a long, thin side return reads far better with a 1000 x 3000mm (£1,307.41) than with a square 1500 x 1500mm dropped in the middle.

One large lantern or two smaller ones?

For the same glazed area, one lantern is almost always cheaper; two make sense when the room has two distinct zones or the roof is broken up by a steel or chimney.

Option	Glazed area	Price inc VAT	Notes
1 x 2000 x 3000mm	6.0m²	£1,770.71	One kerb, one set of flashings, one delivery
2 x 1500 x 2000mm	6.0m²	£2,475.36	£704.65 more, two kerbs to build, two potential leak paths

You pay twice for base frames, ridges, hip ends and cills, so the bigger the lantern, the cheaper the daylight: £739 per m² at 1000 x 1000mm, £370 per m² at 1500 x 2500mm, £265 per m² at 3000 x 5000mm.

From behind the counter: we recommend two lanterns roughly one time in six, nearly always an L-shaped kitchen-diner where one central lantern would sit over the junction and light neither zone. If your room is a single rectangle, buy one lantern and spend the difference on the 1.0 W/m²K glass.

How much taller does a bigger lantern make the roof?

Ridge height rises with the shorter span, not the longer one. At a typical 20° pitch the ridge sits roughly half the width × 0.36 above the kerb top, plus your kerb height (commonly 150mm):

Lantern width (short span)	Approx. ridge above kerb at 20°	Plus a 150mm kerb
1000mm	~180mm	~330mm
1500mm	~270mm	~420mm
2000mm	~365mm	~515mm
2500mm	~455mm	~605mm
3000mm	~545mm	~695mm

Those are indicative geometry, not manufacturer drawings — confirm the ridge height on your order acknowledgement before finalising planning drawings. It matters twice over: permitted development limits an extension's total height including what sits on the roof (see the Planning Portal's guidance on extensions), and under a first-floor window a wide ridge can end up awkwardly close to the cill.

How heavy is the glass, and what can two people lift?

Glass weighs about 2.5kg per m² per millimetre of thickness, so a roof lantern sealed unit lands around 28–32kg per m² with the toughened outer, laminated inner and spacer. On that basis:

Lantern size	Glazed area	Estimated glass weight	Handling
1000 x 1500mm	1.5m²	~45kg	Two people, straightforward
1500 x 2500mm	3.75m²	~110kg	Two people per pane; the panes split the load
2000 x 3000mm	6.0m²	~180kg	Two to three people, suction cups worth hiring
3000 x 5000mm	15.0m²	~440kg	Plan lifting properly — this is a crane or mechanical-lift job

Lanterns arrive as a flat-packed frame plus individual glass units, so nobody lifts the total in one go — but each pane on a 3m-wide lantern is a serious two-person lift onto a roof. The HSE's manual handling guidance is worth five minutes before delivery day; we cover the numbers in how heavy is a roof lantern and the method in our beginner's guide to installing a Korniche roof lantern.

Trade tip: self-installing with a short span of 2500mm or more? Price in a glazing robot or genie lift on day one — it is the commonest reason a DIY lantern stalls halfway through a Saturday.

Will a bigger lantern mean more bars in the view?

Yes. Rafters divide the lantern into glazed bays, and a longer lantern needs more of them, so a 2000 x 4000mm carries roughly twice the bars of a 2000 x 2000mm. What you control is where they fall:

• Run direction. Rafters cross the short span, so bars read as lines across the lantern's width. Run the length along a dining table and the bars sit perpendicular to your main sightline.
• Internal frame colour. White internals against a white plastered reveal all but vanish; matt black frames the sky deliberately. Both cost the same — see the colour combinations on the product page.

If bars are the deciding factor, a flat rooflight gives one uninterrupted pane — see skylights vs roof lanterns and our rooflights page.

What size needs a structural opening or building control?

Every lantern needs a properly formed opening: joists trimmed with doubled-up trimmers, kerb built off that. Whether you need a structural calculation depends on the span you interrupt, not the lantern's price.

Situation	What is usually required
Lantern up to ~1500mm wide, opening trimmed within the joist run	Doubled trimmers, no separate steel in most domestic flat roofs
Lantern 2000mm+ wide, or the opening cuts more than two joists	Structural engineer's calculation; sometimes a steel trimmer
New extension	Covered by the extension's own Building Regulations application
Retrofitting a lantern into an existing flat roof	A building notice or full plans application to your local authority, with thermal performance to Approved Document L

New and replacement rooflights must meet the U-value standards in Approved Document L; structural work falls under Approved Document A. Both glazing grades we sell (1.2 and 1.0 W/m²K) are specified with that in mind, and building control will want the figure in writing — it is on the product page and your order acknowledgement.

From behind the counter: the awkward conversation is always a retrofit into a 1990s flat roof. Owners assume the lantern is the job; it rarely is. Trimming the opening, forming the kerb, correcting falls and re-covering usually costs more than the lantern.

Roof lantern size and price table

Our stock sizes with glazed area and cost per square metre, so you can see where the value sits. Order size = external kerb size. All prices include VAT and are correct as of 21 August 2026. Delivery is free on UK orders over £1,000.

Order size (external kerb, mm)	Glazed area	Price inc VAT	£ per m²	Suits
850 x 850	0.72m²	£662.28	£920	Utility, en-suite, hallway
1000 x 1000	1.00m²	£738.92	£739	Cloakroom, small landing
1000 x 1500	1.50m²	£923.24	£615	3 x 3m extension
1000 x 3000	3.00m²	£1,307.41	£436	Long galley side return
1250 x 2000	2.50m²	£1,142.83	£457	4 x 3m extension
1250 x 2500	3.12m²	£1,286.36	£412	5 x 3m extension
1500 x 2000	3.00m²	£1,237.68	£413	4 x 3m kitchen-diner
1500 x 2500	3.75m²	£1,389.32	£370	The UK's most-ordered size
1500 x 3000	4.50m²	£1,542.62	£343	Full-width 5–6m extension
2000 x 2000	4.00m²	£1,354.90	£339	Square open-plan zone
2000 x 3000	6.00m²	£1,770.71	£295	6 x 4m kitchen-diner
2000 x 4000	8.00m²	£2,296.46	£287	7 x 4m open plan
2500 x 3000	7.50m²	£2,006.58	£268	Orangery-style room
3000 x 3000	9.00m²	£2,912.82	£324	Big square roof
3000 x 5000	15.00m²	£3,968.89	£265	Full orangery roof

Daylight gets cheaper the bigger you go. Stepping from 1500 x 2000mm to 1500 x 2500mm adds 0.75m² of sky for £151.64 — about £202 per extra square metre, under half the average rate you are paying overall. If you are torn between two sizes and the kerb can take it, go up.

For what drives the final figure — glazing grade, frame colour, installation — see our companion article on Korniche roof lantern prices, or browse the full Korniche roof lantern range. To read your sizes back to someone, call 01604 347222.

FAQs

Do I order a roof lantern to the kerb size or the opening size?

To the external size of the finished kerb (upstand): the base frame sits over the outside face, so the kerb's outside dimensions are your order size. The structural opening is usually larger and the visible glass smaller — neither is the order size.

What size roof lantern do I need for a 3 x 3m extension?

A 1000 x 1500mm lantern, £923.24 inc VAT, suits a 9m² room: about 17% of the floor area in glass with room left for the roof falls. If the structure allows, 1250 x 1500mm is £84.66 more at £1,007.90.

What size roof lantern for a 6 x 4m extension?

A 2000 x 3000mm lantern, £1,770.71 inc VAT: 6m² of glazing, around 25% of a 24m² floor, with 500mm of flat roof left on all sides. The same area as two 1500 x 2000mm lanterns costs £2,475.36, so only split it if the room has two zones.

Is one big roof lantern cheaper than two small ones?

Almost always. One 2000 x 3000mm lantern is £1,770.71; two 1500 x 2000mm lanterns covering the same 6m² are £2,475.36 — £704.65 more, plus a second kerb and flashings. Cost per m² falls from £739 at 1000 x 1000mm to £265 at 3000 x 5000mm.

How high does a roof lantern sit above the roof?

Ridge height follows the shorter span. At a typical 20° pitch expect roughly 180mm above the kerb at 1000mm wide, 270mm at 1500mm, 365mm at 2000mm and 545mm at 3000mm, plus your kerb height (commonly 150mm). Confirm the figure on your order acknowledgement before submitting planning drawings.

How heavy is the glass in a large roof lantern?

Glass weighs about 2.5kg per m² per millimetre of thickness, so a roof lantern sealed unit runs 28–32kg per m². A 1500 x 2500mm lantern carries roughly 110kg of glass in total, a 2000 x 3000mm around 180kg — delivered as separate panes, but each is a two-person lift.

Does a bigger roof lantern need building regulations approval?

The lantern is part of the roof, so it is covered by your extension's Building Regulations application or, for a retrofit, by a building notice. Openings wider than about 2000mm, or cutting more than two joists, usually need a structural engineer's calculation under Approved Document A.

What is the largest Korniche roof lantern you sell?

3000 x 5000mm at £3,968.89 inc VAT in clear 1.2 W/m²K glazing — 15m² of glass, an orangery-scale roof that needs planned mechanical lifting. Full stock size list on our Korniche roof lantern collection page.