COP v3.0:natural-light; materials

11.2 Materials 

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.

11.2.1 Glass Reinforced Plastic (GRP) 

Glass Reinforced Plastic ( GRP ) combines polyester resin and chopped glass fibre. AS/NZS 4256.3 requires sheets to contain a minimum of 22% glass fibre by mass and to be marked with their classification and weight.
GRP is suitable for in-service temperatures of -10° to 70°C and some GRP sheets are available in a fire resistant grade.
GRP should have a minimum thickness of 1.1 mm, but it is available up to 3 mm thick.

11.2.1A GRP Weight in g/m²

The exterior surface of GRP is covered with either a polyester film or a layer of gel-coat cast as the sheet surface. Sheets can have a film or a coating on one or both sides or have a film on one side and a coating on the other. The performance of GRP is related to both light transmission and durability of the various films, and coatings can provide different performance for each category in different environments.
N.B. Performance in both of these areas is not necessarily directly related.
The roofing contractor must ensure that the correct weathering surface of plastic sheeting is placed uppermost as the durability and warranty depend on placing the sheet the correct side up.

11.2.2 Polycarbonate 


Polycarbonate is a tough, clear thermoplastic polymer with a higher deformation temperature than PVC.
Polycarbonate is manufactured with a co-extruded UV resistant top layer, which will resist weathering, but its durability depends on the thickness of the top layer.
Profiled polycarbonate sheeting has a limited spanning capacity and requires greater provision for expansion than GRP. It is available in thicknesses from 0.8 mm to 1.5 mm but is only available in a limited range of profiles.
Flat multi-wall extruded sheets with one or more air gaps have a thinner wall thickness but derive their rigidity from the shape of the profile. They are limited in spanning capability, but come in wide sheets, fixed into proprietary extrusions, and are suitable as continuous barrel vaults and as double skin roof lights.
Surface coatings provide different levels of durability, chemical resistance and weather resistance.
Polycarbonate sheets must not be fitted above sprinklers as they can drop out in a fire and interfere with the sprinkler's function.

11.2.3 PVC 

PVC is a compound of polyvinyl chloride manufactured with stabilisers but without plasticisers that complies with AS/NZS 4256.2. Profiled PVC roof lights have a limited spanning capacity and need greater provision for expansion than other plastics or metals.

Profiled PVC sheeting ranges from 0.8 mm to 1.5 mm in thickness but is only available in a limited range of profiles. PVC softens at 80°C and will act as a smoke vent when heated during a fire. It has a service temperature of 60°C and a tensile strength of 52 MPa at 20°C.

PVC may not satisfy the 15-year durability requirements of the NZBC; it is not regarded as suitable for commercial or industrial use and should not be used in habitable buildings.
PVC sheets must not be fitted above sprinklers, as they can drop out during a fire and interfere with the sprinkler's function.