Roof Light Sizing Calculator

This calculator estimates the total roof window glazing area a room needs two ways and takes the larger. First, the floor-area rule of thumb: about 15% of floor area for a pitched roof (12% for a flat roof), following VELUX daylight guidance — for most normal rooms this rule governs. Second, a simplified average-daylight-factor (ADF) estimate that lets roof pitch, your target daylight factor and any external obstruction move the number — it estimates the daylight a roof window delivers per square metre of glass (glazing transmittance ≈0.7, a sky-angle factor that falls as the pitch steepens, and a 0.9 maintenance factor) and back-solves the glass area needed to reach the target. This is a first-order proxy, not the full BRE/BS 8206-2 ADF formula, which also needs the room's total internal surface area and surface reflectances. It then picks the largest standard VELUX-pattern size that fits between your rafters without cutting one, and works out how many of that size are needed to deliver the required glazing area — because a single rooflight that fits a 600 mm rafter gap (roughly a 550 mm CK frame) only provides about 0.3 m², so larger rooms need several, or a bigger window in a trimmed opening. It compares your planned number of roof lights against the required area and tells you if you are short, and when many narrow units would be needed it suggests fewer, larger windows in a structural (trimmed) opening instead. It also flags possible over-glazing against the 25% Approved Document L thermal threshold and Approved Document O overheating, selects the flashing kit from the roof covering, and works out lining-kit depth from insulation depth plus plasterboard and a service gap.