What size roof lantern do I need?

The lantern earns its keep by the light it puts on the floor and the walls, so the floor area of the room is the number to start from. Measure the room below, take a fifth of that area as a first target for the glass, and see what shape of lantern gets you there. A kitchen of 18 square metres wants somewhere around 3.5 square metres of glazing to feel properly daylit rather than merely topped up.

Shape matters as much as area. A long galley room takes light better from a lantern that runs along its length, while a squarer room reads best under a lantern closer to square. If the extension has bifold doors down one side already pulling in light, you can lean the lantern towards the darker half of the room rather than centring it out of habit. The aim is even light across the whole floor, not a bright pool under the glass and gloom in the corners.

There is a ceiling to this. Push much past a quarter of the floor area in glass and you stop gaining useful daylight and start gaining heat, glare and winter heat loss instead. A fifth is a comfortable target for most rooms. A sixth is plenty if the room already has good glazing at eye level.

A lantern does not sit on the roof surface. It sits on a raised kerb, or upstand, built up around a hole trimmed into the flat roof deck, and that opening is the real limit on size. The rafters or joists you cut through have to be trimmed and doubled around the gap so the load they were carrying is carried around the new opening instead, and the wider you go, the more structure has to be rerouted.

On a standard timber flat roof over an extension you can usually trim a generous opening without difficulty, but the moment a lantern spans wide enough that the trimming timbers alone cannot carry the load, a structural beam comes into it and the cost and the build sequence change. This is worth knowing before you fall in love with a size. It is also why the honest answer to sizing is often settled on site, once the roof build-up is open and the spans are visible.

The upstand adds height too. A kerb of around 150 to 300 millimetres lifts the glass clear of standing water and gives the flashing something to turn up against, and the lantern frame sits on top of that. If your roof projects only 150 millimetres above the surrounding plane to stay within permitted development, that kerb and frame height is part of the sum. Our roof lantern installation work always starts from the opening and the upstand, because a beautifully specified lantern over a badly built kerb is a leak waiting to happen.

Most manufacturers sell roof lanterns in a grid of stock external sizes, and buying to that grid is cheaper and quicker than a bespoke unit. Common footprints run from around 1 by 1 metre up to about 2 by 3 metres and beyond, in steps of roughly 250 or 500 millimetres. If your opening can be trimmed to land on one of those sizes, that is usually the sensible route.

Bespoke is worth it in a few clear cases:

• An awkward span. If the gap between the walls or the usable roof area falls between two stock sizes, a made-to-measure lantern uses the full width instead of leaving you with a smaller stock unit and a wider frame of blank roof around it.
• A very large room. Past about 3 metres in length, a single-ridge lantern can look heavy, and a run of two lanterns or a purpose-made design keeps the proportions right.
• A specific look. Slimmer sightlines, a particular pitch to the glass, or a contemporary flat-topped profile rather than a traditional ridge are all easier to get from a custom skylight design than off the shelf.

Whichever route you take, quote sizes carefully. A lantern is usually described by its external frame dimensions, but the opening you need to trim in the roof is smaller than that, and the glazed area smaller again once you subtract the frame and the ridge bar. Ask the supplier for all three figures so the daylight you are actually buying is clear.

Size and glass specification are the same decision, because the heat a lantern lets in scales directly with its glazed area. Double the glass and you double the solar gain for the same weather, and that is where orientation stops being a detail and becomes the whole question.

Southend sits facing south across the Thames Estuary, so a great many rear extensions here have their flat roof pointing at the summer sun for most of the day, with an open horizon and reflected light off the water on top. A modest lantern in clear double glazing on that roof might be comfortable. Scale it up over a kitchen island and the same glass turns the room into a greenhouse by mid-afternoon. The larger you go, the more the glass has to work, which means asking for a lower G-value: solar-control double glazing sits near 0.3 against roughly 0.6 for clear, and it blocks a good share of that heat before it reaches the room.

So the sizing conversation and the glazing conversation belong together. If you want a large lantern on a south-facing extension, budget for solar-control glass as part of the size rather than as an afterthought once the room turns out to overheat. This is the heart of our energy-efficient skylight installation approach, and it is why we ask which way the roof faces before we talk about dimensions. North-facing rooms can carry a bigger area of clear glass happily and take the free winter warmth. South-facing ones need the size and the glass settled as one.

Work through it in order. Measure the room and take a fifth of the floor area as your daylight target. Sketch a lantern shape that spreads that glass evenly across the floor rather than pooling it in one spot. Check that shape against the roof structure, allowing for the trimming and the kerb, and see whether it lands on a stock size or wants a bespoke unit. Then, and only then, fix the glass specification to the orientation, going for a lower G-value the larger and the more southerly the lantern is.

If the room already runs warm, or you are unsure the structure will carry the size you want, that is the point to have someone look at the roof rather than guess from a catalogue. Tell us the room dimensions, which way the roof faces and roughly what you are picturing, and we will come back with a size that suits the structure, a glass specification to match the orientation, and a price. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the hole you want to put in it.