The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
Daylight glare levels, diffusion, and location are more important than light intensity, and these conditions should be determined to ensure the plastic sheeting is suitable for the intended use or purpose of the building.
Normal roof light areas range from 5% of floor area for warehouses to 10 – 15% for industrial buildings, and 20% for sports halls and for factories where intricate work is done.
For urban areas where air pollution reduces the level of natural light, or where roof lights are double skin or are specified tinted, the proportion of roof lights should be increased.
Typical light transmission for new double-skin roof lights is around 70%, and diffusing agents can be added during manufacture to minimise glare and solar gain. Roof lights will perform better, maintain maximum light transmission, and last longer if they are kept clean by washing down with water and mild detergent, at 1 – 2-year intervals. Although films and gel-coats will vary in their resistance to surface deterioration, any dirt or lichen build-up will shorten the life of roof lights.
The light and heat transmission of plastic roof lights can be reduced by adding tints to polycarbonate sheeting or pigments to GRP sheeting. Such tints can lower the light transmission to as low as 25% compared to 90% for clear sheets, and heat transmission can be reduced by over 50%.