How Big Can a Roof Light Be? Sizing a Bespoke Unit for Your Extension

The single most common question we get asked at the factory is some version of "how big can I go?" It is the right question: daylight is the whole point of a roof light, and an under-sized unit in a large kitchen extension is the mistake you notice every day for the next twenty years. Because every unit we make is made to order, the honest answer is "bigger than you probably think" — but there are real limits worth understanding before you cut the roof.

Start with the room, not the roof

A useful starting point for a habitable room is glazing equal to around 15–20% of the floor area. A 20m² kitchen-diner therefore wants roughly 3–4m² of roof glazing — a single 2000×1500mm unit, say, or a pair of 1500×1000mm units placed over the key zones: the table, the worktop, the sofa. North-facing rooms and rooms with deep floor plans sit at the top of that range.

What actually limits the size

• Glass weight. A double-glazed unit weighs roughly 25–30kg per square metre, so a 3000×2000mm roof light is a 150kg-plus lift that needs proper manpower or mechanical handling to get onto the roof safely.
• Roof structure. The joists around the opening carry the load the removed timbers used to carry, plus the glass. Anything much over a metre wide normally means doubled-up trimmers — your builder or structural engineer will confirm.
• Access. The unit has to get from the delivery vehicle, through or around the house, and up. A 3.5m single pane that cannot turn the corner of a terraced side return is the wrong specification, however good it looks on the drawing.

One large unit or several?

Up to around 3m on the longest edge, a single frameless roof light gives the cleanest, most dramatic result. Beyond that, a run of two or three framed units is usually the smarter engineering: each unit stays at a liftable weight, the roof keeps more of its structure, and the rhythm of repeated glazing looks deliberate rather than compromised.

Measure the structural opening, not the ceiling

The dimension we manufacture to is the internal size of the upstand — the timber kerb the unit sits on — not the plasterboard aperture below it. Measure the internal opening at the top of the upstand in millimetres, in two places on each axis, and give us the smaller figure. If the upstand is not built yet, agree the roof light size first and build the upstand to suit; it is far easier that way round.

Not sure what your opening can take? Send the sizes and a photo through our bespoke roof light enquiry page or call 03330 902 592 — we quote exact made-to-measure prices the same day, and every unit is manufactured in our Liverpool factory with a 20-year guarantee.