COP:natural-light; weathering-and-durability

10.5 Weathering And Durability 

Plastic roof lights are resistant to normal weather conditions because they are protected from ultraviolet radiation (UV), temperature changes, and ingress of moisture by surface coatings or laminated films.
Surface protection includes PVF or polyester films, liquid gel-coats on GRP sheets or co-extruded layers on thermoplastic sheets. Their resistance to yellowing, surface crazing and erosion, fibre prominence in GRP and embrittlement all depend on this protective treatment.
The surface protection provided may also determine their resistance to aggressive chemical environments. PVF films give a very good chemical resistance, provided that the film is undamaged during the life of the product. Gel-coats can also be used in chemically aggressive environments that would be unsuitable for metallic coated steel cladding.
Surface protection films and coatings are very weather-resistant but should be kept clean and well maintained, but the translucence of all plastic sheeting will reduce over time, even if the sheets are cleaned regularly.
UV degradation depends on location, orientation to the sun, and on the UV intensity. Some inferior films on GRP sheets can discolour and craze in as little as three years. Higher grade films, however, proved to provide useful light transmission for 15 years.
Certain types of gel-coat and polycarbonate sheet with good protection can provide useful light transmission for up to 20 years. PVC has a useful light transmission life of only 2 – 5 years and it can become brittle due to UV exposure.
Fire-retardant additives may cause fire-resistant sheeting exposed to UV light to discolour more quickly.
All plastic sheeting is subject to mould growth, particularly in areas of high humidity. Take care no to damage the film surface when removing mould growth. See 15.7.1 Lichen And Mould.
Where the roof is constructed using composite panels, factory assembled rooflights are the most appropriate rooflight solution. See 14.5 Insulated Panels
The use of double skin sealed rooflight panels will also reduce the risk of condensation.