How big can a roof lantern be?
A standard self-supporting aluminium lantern will run to roughly three metres by six metres before it needs help. Beyond that, lanterns are still available, but they stop being a catalogue item and become an engineered structure with a steel or a reinforced ridge inside them, and the price and the programme change accordingly.
That is the ceiling on the aluminium. In practice most extensions hit a different limit first: the roof cannot lose that much structure, or the unit cannot physically be got into the garden. This page works through each limit in the order it usually bites.
The largest sizes actually available
Domestic ranges from the main lantern manufacturers are typically listed up to about 2.5 by 5 metres as a stock size, with bespoke sizes made to the millimetre above that. The widest span is the constraint rather than the length, because width sets how far each rafter bar has to reach from the eaves beam to the ridge without sagging.
Lengths are easier. A lantern six or seven metres long is not unusual over a wide rear extension, because the ridge can be reinforced along its run and additional rafter bars simply carry more of the load down to the base frame at closer spacings.
Rafter bars, and how far aluminium spans unaided
Each rafter bar is a thermally broken extrusion carrying glass weight, wind uplift and, in a bad February, snow. Deeper sections span further. A shallow 50mm bar might be rated to around 1.8 metres of slope length, while a deep 100mm or 130mm structural bar will reach 3 metres or more.
Manufacturers publish span tables for this and they are not suggestions. When a bar is asked to reach past its rating the result is deflection, the glass gasket loses its even compression along the bar, and the seal is the first thing to complain. Where the span runs out, the answer is a deeper bar or a steel flitch inside the ridge, not a hopeful order.
The single pane, and where glass runs out
Individual panes have their own limits, set by the glass processor rather than the lantern maker. A toughened and laminated double glazed unit for overhead use is normally comfortable up to about 2.5 by 1.5 metres. Larger panes are made, but the toughening bed, the laminating autoclave and the delivery frame all set boundaries.
The practical consequence is that a wide lantern gains bars rather than bigger glass. Doubling the width of a lantern does not double the pane size, it adds another bay. That is worth knowing before you set your heart on the fewest possible glazing lines.
The opening, and what the roof structure allows
A lantern needs a hole under it, and the hole is often the real limit. Cutting joists in a flat roof transfers load to trimmers, and trimmers span between the remaining joists. Below about 1.5 metres this is routine timber work. Beyond it, and particularly where the opening runs across the joist span rather than with it, a steel becomes the sensible destination.
On the interwar semis and post-war stock across Chelmsford and Braintree, the flat-roofed rear addition is often built with modest joists at 400 centres, which limits how much can be removed without a calculation. Where a steel is involved, structural openings and steel for a roof lantern sets out what the engineer produces and what it does to the programme.
Getting a large lantern onto the roof
Lanterns arrive as a kit, which helps, but the glass does not. A 2.4 by 1.4 metre toughened and laminated unit weighs in the region of 120 kilograms. Two fitters can manage a modest pane on a low roof from a scaffold tower. Larger panes want a glass lifter, four people, or on a tight site a crane or a spider lift.
Access is therefore a genuine size constraint on a terraced house with a side alley, and it is one of the first things a survey looks at. A Victorian terrace in central Chelmsford with no rear vehicle access will sometimes cap the practical pane size well below what the catalogue allows.
Wind and snow at large spans
Loads scale with area, and they scale unhelpfully. A lantern in an exposed position near the Blackwater estuary or on the reclaimed flats at Canvey sees higher wind pressures than one tucked between two houses in a Colchester terrace, and uplift on a large shallow lantern is significant.
Snow is the other case. A shallow pitch holds snow rather than shedding it, so a wide, low lantern carries more imposed load per bar than a steeper one of the same footprint. That is why very large lanterns are often specified a little steeper than the aesthetics would suggest.
Where a lantern stops being a stock item
There is a fairly clear step in every supplier’s range. Below it you are buying from a size list with a short lead time. Above it, the lantern is drawn, the bars are cut to order, the glass is made as a special and the lead time roughly doubles.
The step is not only about price. A bespoke lantern is manufactured to the kerb dimensions taken on site, which means the kerb has to be built and measured before the lantern is ordered, not the other way round. That sequencing matters on a fixed programme.
Size bands, and what changes at each step
| Footprint | Structure below | Lantern build | Handling |
|---|---|---|---|
| Up to 1.5m x 2m | Timber trimming | Stock size, shallow bars | Two fitters, tower |
| 1.5m x 2.5m to 2m x 3m | Timber trimming or light steel | Stock or near-stock | Two to three fitters |
| 2m x 3m to 2.5m x 5m | Steel usually needed | Deep bars, made to size | Glass lifter likely |
| Above 2.5m x 5m | Engineered opening | Reinforced ridge, drawn scheme | Crane or spider lift |
Deciding a maximum for your extension
The useful order is to work down, not up. Establish what the roof can lose without a steel, then check what will fit through the access, then look at what daylight the room below actually needs. Very often the third answer is smaller than the first two, and sizing a lantern for the room below covers where that number lands.
Leave a border of flat roof around the lantern as well. A kerb needs something to be dressed into, and a lantern pushed hard against a parapet or a party wall leaves the covering nowhere sensible to terminate. Three to four hundred millimetres of roof on each side is a reasonable working minimum, and it is one of the details a survey settles before a size is confirmed on a lantern installation.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements