A common rule of thumb is rooflight glass equal to about 10–20% of the floor area of the room it lights, adjusted for orientation, ceiling depth and how bright you want the room. The rooflight must also fit the structural opening between joists, and in an extension the total glazing allowance applies.
Is there a rule of thumb?
For general living space, many designers start at 10–20% of the floor area in rooflight glass. North-facing roofs and rooms with deep plaster reveals need more; south-facing kitchens may need less, or solar-control glass, to avoid overheating. It is a starting point, not a regulation.
How do joists affect the size?
Flat roof joists usually run at 400 mm or 600 mm centres. A rooflight wider than the gap means trimming joists and adding trimmers, which is routine in new build but adds cost in an existing roof. Running a long rooflight along the joists is often easier than across them.
What are standard rooflight sizes?
Manufacturers list stock sizes from about 600 × 600 mm up to 1500 × 3000 mm as single units, with internal (clear opening) and external dimensions both quoted. Always order from the upstand's internal or external dimension as the maker specifies — not from the ceiling hole.
One big rooflight or two smaller ones?
Two or three smaller units spread light more evenly along a long room and can be cheaper to lift and fit. One large unit gives a dramatic single 'window to the sky'. Mixing a fixed unit with an opening one often gives the best of both.
How deep should the reveal be?
Deep reveals cut the light and make a rooflight feel smaller. Splaying the reveal outward toward the room spreads light further. Plan the plaster box with the rooflight, not afterwards.
This guide gives general information, not legal or structural advice. Rules change and every property is different — check with your council or building control for your address. We review our guides regularly; the date above shows the last review.
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