CIBSE Guide A provides data which allows us to determine the appropriate design and rooflight area to provide recommended light levels for a given application.Typically, for general retail and manufacturing areas, 500 Lux is the recommended level.
The graph above shows how rooflight 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. This is a typical figure for a light diffusing material like Opal polycarbonate or GRP. 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.
You can see that this will be achieved for 2200 hours with 12% rooflights*. 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 rooflights with varying degrees of light transmission and for different daily time windows.
*12% is the recommnded optimal rooflight area recommended by NARM to support Part L compliance in this type of application.