This graph shows how roof light areas will affect illumination levels. In this example for a single storey building located in London, between the hours of 6am and 6pm daily, with roof lights providing 50% light transmission. You can see that as the roof light area along the bottom of the graph increases, the length of time a given illumination level is achieved, is extended.

So if you are designing for 500 lux (recommended light level for retail and general manufacturing), looking at the yellow line, you can see that with 10% rooflights, 500 lux would be achieved for approximately 2000 hours or 46% of the working year.

If rooflight area is increased to 15%, 500 lux is then available for just over 2500 hours, or 58% of the working year. So a 15% roof light area provides 500lux for 26% longer than a 10% roof light area.

This provides information to help decide the optimum rooflight area – note that there is never a specific solution – judgement is required on a project by project basis, but this example shows how data can be interpreted to inform a decision. Data is available from NARM for different building locations, for roof lights with varying degrees of light transmission and for different daily time windows.

You can also get a deeper insight into rooflighting design by booking our RIBA approved CPD seminar: ‘An introduction to daylighting with rooflights’