In New Zealand, two groups of plastic roof lighting materials are commonly used with profiled metal cladding:
Thermo-setting: GRP translucent glass-reinforced polyester.
Thermo-plastic: acrylic, uPVC and polycarbonate.
Thermo-plastics softens and collapses under heat, and GRP can distort at 80⁰ C but will yellow more rapidly above 60⁰ C. Distortion temperatures varies between different materials and thicknesses.
As the performance of plastic sheeting is related to both the thickness and profile, designers should be aware that a nominal weight per square metre does not relate to different profiles.
The weight per square metre or mass per unit area has a tolerance of roughly 10%. However, it is a confusing measure because the test provisions of AS/NZS 4257 require the measure to be calculated from the area of the profile, including the overlap. Because the mass of plastic sheeting varies between profiles, the thickness will vary.
This is the opposite measure to metal, where one thickness will have a varying mass per square metre for each profile.
When different profiles have the same mass per square metre, they have different thicknesses. It is not, therefore, possible to compare the performance of plastic sheeting by the weight, and it is recommended that all plastic roof lighting sheets be described by thickness.