The most common form of natural roof lighting used in commercial and industrial buildings comprises single skin, glass-reinforced plastic (GRP) sheets matching the profile of the metal roofing, running from ridge to eaves in continuous lengths of up to 12 m, sometimes longer. For other applications such as housing, polycarbonate sheeting is more commonly used. Polycarbonate is available in stock sheet lengths of up to 7.2 metres.
Profiled PVC roof lights are now rare as they suffer from embrittlement when exposed to UV.
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.
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.
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.
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.
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.