The short answer
The right size is set by the room below, the roof around it and the structure beneath, not by a catalogue number. Most lanterns are made to measure, so we take the dimensions at the free survey and size the lantern against the room, the light and the load the roof can carry. The fixed written quote states the exact size.
Start with the room, not the lantern
People asking what size roof lantern they need often start from a figure they have seen online. A better start is the room. How deep is it? Which way does it face? Where do people sit, cook and walk? A lantern that lights the dining table and the end of the worktop is doing a useful job. One placed arbitrarily in the middle of the roof may light a stretch of floor nobody uses. The first step at survey is to stand in the room and see where the light is wanted. It is the same approach we take across our roof lantern installation work.
Roof lantern sizes in practice
Roof lantern sizes are not a fixed list like the standard sizes of a roof window. A lantern is generally made for the opening, so its length, width and height are specified for your roof. Some makers offer standard module sizes at the lower end of the range, and we mention those where they suit. For most extensions, the sensible approach is to choose the footprint that suits the room and let the maker build to it.
The proportion rule we use
A lantern should look as though it belongs to the roof. A common guide is to leave a clear margin of flat roof around the lantern on all sides, so the roof reads as a frame for it. Too small, and it looks lost on a big roof. Too large, and it crowds the edges, leaves too little room for the kerb and the roof junctions, and can bring in more heat and glare than the room can handle. We usually sketch two or three sizes on the roof plan and compare them with the room below before settling on one.
What limits the size
- The roof structure: joists or beams must carry the weight and the wind load of the lantern and its kerb.
- The room below: the opening needs a clean line, with no clash with beams, walls or fixed fittings.
- The upstand height and falls: the surrounding roof needs room for the waterproofing to turn up the kerb.
- The planning position: height and projection can matter, and the size of the unit affects both. The lantern planning answer goes into it.
- Delivery and lifting: a very large lantern may arrive in sections and need lifting equipment.
Sizing for light and heat
A larger lantern gives more daylight, but it also gives more heat gain in summer and more heat loss in winter. In a south-facing room, a more modest lantern with a solar control coating often feels better than a large one with plain glass. A north-facing room can take a larger lantern with a standard specification. The glazing choices change what a given size does, which is why size and glass are decided together. For existing dwellings, a lantern must meet 2.2 W/m²K under Part L.
Height and pitch
Footprint is only one dimension. A lantern also has a height, set by its pitch, and a taller lantern adds more volume to the room but stands further above the roof. A steeper pitch sheds water and leaves more readily, and it brings in more side light. A low pitch looks sleeker and sits closer to the roof, which can matter for planning and for the neighbours. We show you the profile as a drawing so you can judge the look before you order.
Ridge direction and the line of the room
The ridge usually runs along the longest dimension, though not always. We align it with the lines of the room below, such as the run of the kitchen units or the line of the doors, so the lantern looks planned rather than dropped on. Where the extension is square, the choice is about the view and the sun path.
Sizing a lantern to the opening you already have
If you are replacing an existing lantern, the choice is simpler. The old footprint is measured, the kerb is checked, and the new lantern is made to suit it. Slimmer modern frames can give more glass within the same outer size. If you want a bigger lantern than the old one, the roof structure has to change, and the job becomes a new opening. Our kerb check shows what we look at first.
How we confirm the size
Nothing is ordered on a guess. At the free survey we measure the roof opening, the room below and the structure, and note the access for delivery. The lantern is then specified by its outer size, its height and its pitch, and we agree the dimensions in writing before ordering. The fixed written quote states the size, the frame, the glass and the finish inside, with the reveal splayed where the roof depth allows, then plastered and made good. If the size changes later, we re-quote in writing. The work is fitted to current Building Regulations, and the Building Control notification, where it applies, is arranged by us.
