What is the maximum size for a roof lantern?

Three separate things decide how big a lantern can go, and they bite in a different order depending on the design.

The first is the glass. A double-glazed unit is heavy, roughly 25 to 30 kilograms per square metre for a standard build-up, more once you add toughened and laminated panes. A single pane much beyond about 1.4 by 2.4 metres becomes awkward to make, ruinously heavy to lift onto a roof by hand, and difficult to seat without flexing. So a large lantern is almost never one sheet of glass. It is a frame carrying several panes, and the size question becomes how many panes the frame can hold rather than how big one sheet can be.

The second is the aluminium. The sloped bars, called rafters, run from the low eaves up to the ridge, and like any beam they can only span so far before they deflect under the weight of glass and wind. A slim contemporary rafter looks lovely but has its limits. Past a certain length the manufacturer either deepens the rafter, which thickens the sightline, or drops in an intermediate structural bar or a hip to break the span. That is why a very long lantern gains extra bars across it: they are not decoration, they are what keeps the roof from bowing.

The third is the structure below, which is the one people forget until late. A lantern sits on a builder’s kerb, the raised timber upstand around the opening in the flat roof. The bigger the lantern, the bigger the hole, and the bigger the hole the more of the roof’s original support you have cut away. Beyond a modest opening you need a structural engineer to size a steel or timber frame around it so the roof still carries its own load and anything resting on it.

Most manufacturers publish a grid of stock sizes because they are cheaper to make and quicker to get. Those grids typically run up in 500mm steps and top out around 3000 by 2000mm, occasionally a little more for a two-section design. If a stock size fits your opening, take it: it will cost less and arrive faster than anything cut to order.

Bespoke is where the numbers climb. A made-to-measure aluminium lantern can be built as a run of glazed sections, and the sensible working ceiling for a rectangular lantern is around 4 metres on the long side and about 2.5 metres across, with intermediate rafters carrying the middle. Go larger than that and you are usually into a different product: a linked pair of lantern modules, a full glazed roof, or a flat rooflight run rather than a raised lantern, because the geometry and the weight stop making sense as a single hipped frame.

• Small. Up to about 1.5 by 1 metre. A single-pitch or simple four-pane lantern over a small kitchen or a hallway.
• Standard. Around 2 to 3 metres long by 1.5 to 2 metres wide. The common size over a rear extension, and where the stock ranges live.
• Large bespoke. 3 to 4 metres long, up to about 2.5 metres wide, with added structural bars. Made to order and priced accordingly.

Width tends to be the harder constraint than length. Length can be extended with more bars along the ridge, but width is a straight rafter span from eaves to ridge on each side, so a very wide lantern runs into the deflection limit sooner than a very long one does.

The size you can fit is decided as much by the flat roof as by the lantern catalogue. When you cut an opening for a lantern you remove joists, and their load has to be carried around the gap by a trimmer frame. On a small opening a doubled-up timber does it. On a large one you need a steel or an engineered timber beam sized for the span and the load above, and that is a structural engineer’s calculation, not a guess.

New or enlarged rooflights are notifiable under Building Regulations, and a big lantern touches two parts of them. Part L sets a ceiling on the thermal performance, the U-value, of the glazing you install. Part K covers safety glazing and guarding, which is why the panes in a lantern are toughened or laminated rather than ordinary glass. For a lantern of any real size the glass overhead has to be specified to break safely, and that is a fixed requirement rather than an upgrade. On any install we handle the Building Control notification to the local authority on your behalf, so the paperwork for the opening is not left to you.

There is also the plain question of getting it up there. A four metre lantern frame and its glass are heavy and unwieldy, and a large unit can need mechanical lifting or a bigger team rather than two people and a ladder. That is a cost and an access consideration on a terraced street or a tight side return, and it is worth raising early, because the largest lantern that fits your opening is not always the largest one that fits your site.

Size and heat are the same problem. A lantern is horizontal glazing, so it faces the sky rather than a compass point, and on a clear summer day it collects the high midday sun almost square on. Double the glazed area and you double the heat pouring into the room below. On a south-facing rear extension in Southend, where the estuary gives an open southern horizon and reflected light off the water on top of it, a large clear-glazed lantern turns a kitchen into a greenhouse from late morning onwards.

This is where the specification matters more than the dimensions. The figure to ask about is the G-value, the proportion of the sun’s energy the glass lets through as heat. Clear double glazing sits around 0.6; solar-control glass brings it down towards 0.3, cutting the summer heat by roughly half while the room stays bright. On a big lantern over a south-facing room that is not an optional extra, it is the difference between a room you can use in July and one you cannot. We go into the trade-offs on our energy-efficient skylight installation page, and the short version is that the larger the lantern, the lower the G-value you want.

North-facing is the opposite case. There the glass only ever sees diffuse light, the winter warmth is welcome, and a large clear lantern is a fine idea. Orientation decides the glass; the glass does not care how big the frame is, only how much sun lands on it.

To find the largest lantern that suits your project, work through it in this order. Measure the flat roof and leave a sensible margin of roof deck around the opening rather than cutting to the very edge. Ask a structural engineer, or an installer who will bring one in, what the opening does to the joists and whether you need a beam. Then choose a stock size if one fits, and only pay for bespoke when the opening genuinely needs it. Last, and before you settle the size, decide the glass by which way the roof faces.

If you would rather not do that arithmetic yourself, tell us the rough opening size and the orientation and we will come back with a sensible maximum, a glass specification and a price. Our roof lantern installation page covers the whole job, and you can see typical figures on the skylight installation costs page. Ring 01702 898232 or request a quote, and we will look at what the roof can carry rather than just what fits the hole.