COP v3.0:natural-light;

11 Natural Light 

Clear Roof Sheeting explores the materials and installation methods available to provide natural lighting solutions for buildings in NZ. Other topics include:

  • Types of roof light.
  • Weathering and Durability.
  • Loadings for roof lights.
  • Thermal movement

11.1 Clause G7 NZBC 

NZBC Clause G7 Natural Light requires habitable spaces to have enough natural light through windows and roof lights to safeguard against illness with enough visual awareness of the outside to prevent feelings of isolation.

1. 'The workplace must have suitable and sufficient lighting'.
2. 'The lighting must, so far as reasonably practical, be by natural light.'

All plastic sheets used with profiled metal cladding in New Zealand should comply with AS/NZS 4256.1 and should be tested in accordance with 18.3 Material Density, Melting Point, Expansion And Modulus.

The most common form of roof lighting used in commercial and industrial buildings comprises of single skin matching profiled translucent sheets, running from ridge to eave. Alternatively, discontinuous profiled sheets placed in a chequerboard pattern, barrel-vaults or domes can be fitted on both sloping and flat roofs. All plastic roof lighting materials are supplied with films or surface coatings,  stabilisers for durability, and long-term resistance to weathering and the discolouration termed 'yellowing'.
Roof lights are available to match metal profiles in a range of thicknesses and fire resistant grades. They are differentiated according to structural and safety requirements and imposed loadings.
Roof Lights should comply with the test provisions of AS/NZS 4257.
Only full-length roof lights running from ridge to eave must be used with unpainted galvanized roof cladding to avoid inert catchment corrosion.

To maintain water-tightness and avoid thermal distortion, roof lights should be installed within the spanning capability of the profile, which depends on the profile shape and thickness. Additional support can be obtained from adjacent metal cladding by the use of mid-span supports (see 14.8.4 Midspan Supports)or alternately the thickness of the roof lights should be increased.

Safety mesh should be provided under all translucent sheeting which is accessible and more than 500 mm wide.

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.


11.3 Types Of Roof Lights 


Roof light installations can be grouped into one of the following types:
  1. Single skin.
  2. 11.3A Double Skin Rooflight
    1. Where the roof cladding is fixed to battens and uses a flat multi-wall polycarbonate sheet.
    2. Where an additional sheet is fixed above, as shown in 11.3B Double Skin Rooflight (No Spacers)  and 11.3C Rooflight with Spacers
  3. 11.3D Continuous Vaults.
  4. 11.3E Individual Domes—Light tubes made from acrylic or polycarbonate are usually manufactured with a flashing suitable to weather most roof cladding profiles. 

Proprietary opening window units or skylights made from acrylic also have powder coated frames and are provided with weathering curb flashings, but they should be compatible with the roof cladding and should be installed as required in 9 External Moisture Penetrations.

Acrylic or polycarbonate tubes, skylights or domed roof lights should not drain onto unpainted galvanised roof cladding or onto galvanised fasteners. See 4.12 Inert Catchment.

Proprietary GRP sheets are available that can be used as double skin roof lights. A liner panel GRP or PVC sheet can be used to minimise condensation, but the air gap should be sealed.











11.4 Light Transmission 


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%.

11.5 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 not to damage the film surface when removing mould growth. See 16.7.1 Lichen And Mould.
Where the roof is constructed using composite panels, factory assembled rooflights are the most appropriate rooflight solution. See 15.5 Insulated Panels
The use of double skin sealed rooflight panels will also reduce the risk of condensation.


11.6 Loadings 

11.6.1 Point Load. Walking Traffic 

All plastic roof lights are classified as brittle roofing and are not suitable for roof traffic unless specifically tested under the point load test provisions of AS 4040.1, As/NZS 4040.4, and AS/NZS 1562.3: 2006.

AS/NZS 1562.3: 2006 requires the provision of safety mesh under all plastic sheeting subject to local statutory or national building code regulations. The HSE Act 1992 classifies accessible roof lighting as hazardous and requires the use of safety mesh under or above translucent sheets over 500 mm in width.

Although normal chopped strand GRP of sheet thickness greater than 1.7 mm can resist the impact load to demonstrate resistance to accidental fall, this strength is not expected to be retained for more than 5 –10 years, and the sheeting is therefore classified as brittle. GRP sheet that is reinforced with a woven glass mat, may remain trafficable for 20 years, but this should be proven by testing.

Safety mesh can damage plastic sheeting by expansion movement and walking traffic and should be isolated at the purlin. See 14.8.6 Purlin Protection.

The mechanical properties of plastic roof lights differ from those of profiled metal cladding in that they are more flexible which allows them to deflect to a greater extent without damage. Foot-traffic and or access for maintenance should be considered at the design stage, so one sheet or a reduced width is provided so a worker may step over and not on the roof light.

Safety mesh must be provided under all translucent sheeting which is accessible and wider than 500 mm.
A temporary walkway must be provided for installation where the plastic sheeting is more than one sheet width, and if access for maintenance is required the walkway must be permanent. See 14.6 Walking On Roofs.

11.6.2 U.D.L. Wind And Snow Loads 


The maximum performance of plastic roof sheeting for spanning capability and deflection under a uniformly distributed load depends on the section properties and type of material of the profile. The section properties depend on the number and depth of corrugations and the thickness of the profile. 3.5.6 Section Properties 
The uplift pull-through load performance depends on the number of fasteners per square metre and the type and size of the washer.

Profiles which have deeper ribs are more rigid and will deflect less, but will not provide any greater resistance to pull-over at the fixings, unless the sheet thickness is increased. Greater spans also require a thicker sheet. Additional fixings will increase resistance to pull-over failure at fixings, but will not limit deflection.

Roof lights located in the peripheral zones of high wind design load should have provision for the higher load in this area by the use of additional fixings, reduced purlin spacings or by increasing the roof light thickness. Deflection of plastic roof lighting due to UDL wind or snow loading should be limited to less than 1/30th of the span or 50 mm.

All plastic roof lighting should be tested to withstand wind loads and extrapolation is not acceptable as a statement of performance.

On buildings higher than 10 m or areas located in the peripheral zones of high wind design loads, near verges, eaves or ridges it is better practice for roof lights to be omitted.

Plastic roof sheeting must match the design load of the adjacent metal roof cladding. This can be achieved by using a mid-span support or by increasing the weight of the plastic sheet.

Excessive deflection due to long spans can open up side laps or cause failure in compression at the fixing points.


11.7 Thermal Expansion 


Although the figures quoted below are the theoretical expansion rates of different plastic roof lighting materials, the actual expansion rate will differ. See 7.3.2 Roof Cladding Expansion Provisions.

As PVC and polycarbonate expand almost six times more than steel, they should only be used in lengths not exceeding 3 m. Translucent plastic sheets, however, do not normally reach the same temperatures as adjoining metal sheets.

11.7A Material Expansion Rates

 mm per 50° C per 1 m length

PVC and polycarbonate sheet in lengths up to 3 m require 6 mm oversize holes to all fixings. The fixings should always be fitted in the centre of the holes, which can be achieved by pilot drilling. It is recommended that a stepped drill bit be used to ensure the correct size hole.

GRP roof lights also require provision for thermal movement, but to a lesser degree. See 7.3.2A Favourable Circumstances for Controlling Expansion.

Special screws which drill their own clearance hole are suitable for polycarbonate and GRP sheeting.