COP v3.0:natural-light;

11 Natural Light 

Natural Lighting covers the use of translucent roofing material for providing interior illumination. These typically take the form of profiled sheeting, stand-alone units, or flat sheet systems.

Skylights profiled to match the roof sheeting may be made from glass reinforced polyester or polycarbonate. Proprietary stand-alone skylights may have polycarbonate, glass, or acrylic panels. Single or multi-skinned sheet products can be used in conjunction with proprietary support systems to replace glass in conservatory type situations.

Main headings include:

  • Compliance
  • Profiled roof lighting
  • Stand-alone roof lighting
  • Sheet systems

11.1 Clause G7 NZBC 

Objective

NZBC G7 aims to safeguard people for illness or loss of amenity due to isolation from natural light and the outside environment.

Funtional Requirements

Habitable spaces shall provide adequate openings for natural light and visual awareness of the outside environment.

Performance

Natural light shall provide an illuminance of no less than 30 lux at floor level 75% of the standard year.

Openings to give awareness of the outside shall be provided in suitable locations.

11.1.1 Compliance 

The NZ Building Code G7.2 requires adequate natural light and an awareness of the outside environment for housing, old people’s homes, and early childhood centres. In other buildings, natural light is often used to create ambience and reduce the need for artificial lighting.

All plastic sheets used with profiled metal cladding in New Zealand must comply with the structural, durability, and external moisture requirements of the New Zealand Building Code.

All plastic sheeting should comply with the AS/NZS 4256 and AS 1562.3 and should be tested in accordance with the AS/NZS 4257.

11.2 Natural Light Materials 

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.

11.2A Comparison of GRP and Polycarbonate.

GRPPolycarbonate
Available in all profilesAvailable in limited profiles
Available in long runAvailable in standard lengths
Available in different thicknessesAvailable in one thickness
Durability >25 yearsDurability >25 years
Will lose transparency with ageWill maintain initial transparency
Gives diffuse lightGives direct light
Thermal expansion 2 x that of steelThermal expansion 5x that of steel
Available in tinted or opaqueAvailable tinted
Reasonable light transparencyExcellent light transparency

Polycarbonate is more popular for shorter runs and continuous widths, where clarity and stable appearance are considered an aesthetic attribute.

Stand-alone and flat-sheet roof lights may be manufactured from translucent or transparent polycarbonate, acrylic, or glass sheeting.

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

mm=g/m²
1.1
1.2
1.3
1.5
1.9
2.1
2.5
3.0
 1800
2000
2100
2400
3000
3300
4000
4880
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.
PVC

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 Performance – Structure 

GRP Sheeting is measured in weight per square metre. 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 for different profiles of a given weight per square metre. Deeper profiled products are usually supplied as standard at a greater weight per square metre than more material-efficient profiles.

11.3.1 Point Load on Translucent Sheets 

Standard profiled roof lights are classified as brittle roofing and are not suitable for roof traffic. They are normally laid over safety mesh to AS 4859.  (See 13.4.4 Safety Netting)

Trafficable roof lights are normally rendered trafficable by embedding a woven glass mat into the material. They must be tested under the point load test provisions of AS 4040.1, AS/NZS 4040.4, and AS/NZS 1562.3: 2006. Plastic roof lights.

Non-trafficable sheets over one sheet in width should be provided with a walkway, terminate short of ridge and/or eaves, to enable transverse traffic.

11.3.2 Distributed Loads on Translucent Sheeting 

The spanning capacity of profiled roof lights depends on the profile shape, material thickness and composition, the number of fasteners per square metre, and the washer type and size.   

Pull-though of the fastener is the most common failure mode of profiled translucent sheeting, so it should always be installed with a load spreading washer unless otherwise specified by the manufacturer.

Additional support can be obtained from adjacent metal cladding by using mid-span supports, alternatively, the thickness of the roof lights may be increased.

Profiled translucent sheeting must be designed to be capable of withstanding design wind loads, using the manufacturer’s literature.

 

 

11.4 Thermal Resistance 

Roof lights can increase both the heat entering and leaving a building.

Tinted sheeting can be used to decrease heat build-up through radiation. Tinting can be coloured or translucent.

To decrease heat loss and prevent condensation, multi-skinned systems can be employed to provide insulation. In colder areas, UV-stabilised EVA copolymer film is often used under profiled skylights to minimise condensation. Where buildings are heated, or where condensation cannot be tolerated, multi skinned systems are recommended.

 

 

 

 

 

 

 

 

11.5 Light Transmission 

The amount of roof lighting required in a building depends on the building design or use, the external light conditions of the location, and the reflectance of internal surfaces. Regular roof light areas range from 5% of the floor area for warehouses to 10 – 15% for industrial buildings, and 20% for sports halls and factories where intricate work is done.

Proprietary skylights are usually strategically positioned to bring borrowed light into specific areas that are not otherwise naturally lit. Often these are also designed to provide ventilation.

11.6 Installing Translucent Sheets 

To provide for thermal expansion, all profiled roof lighting must be fitted with fasteners installed through an oversize hole and with a load spreading washer. Some fasteners drill their own clearance holes, otherwise pre-drilling is required. Although GRP has approximately twice the expansion as steel, it absorbs less energy from the sun, so the same expansion allowances as used for the adjacent steel sheet can be applied.

Roof netting and safety mesh can damage GRP through expansion movement and walking traffic. GRP can be protected at the purlin by placing a small slip sheet of GRP between the netting and the roof light.

 

 

Polycarbonate sheets expand more than steel, and so are only available in lengths up to 7.2 m. When directly installing polycarbonate sheets, friction tape can be placed on the purlins to reduce thermally induced noise.

Manufacturers recommend side lap fasteners on translucent sheets to maintain the integrity of the side laps.

Turn up stop ends are not easily formed with profiled skylights. To prevent blow over at the upper end of sheets, an angle or odd leg J-shaped channel is normally attached to the upper end of sheets with rivets and sealed with silicon or MS sealant.

 

 

11.7 Stand-Alone Rooflights 

Stand-alone roof lights are available in many configurations with the most common being a glass-faced rectangular or square design, or a dome. Both are available either clear or tinted and may be integrated with a permanent, manually operated, or automated ventilation function.

Rectangular skylights typically consist of a timber or aluminium base which must be flashed into the roof cladding, using one of the methods detailed in 9 External Moisture Penetrations. The designer should select which flashing detail is optimal for the application and detail it accordingly. To enable efficient design and prevent excessive use of over flashings, designers should also consider the placement of skylights in relation to each other and other architectural elements,

Retrofitted skylights, not included in the original design, often result in unsightly, leaky, and excessive over-flashings. Such additions often evade compliance inspections. If skylights are likely to be required, it is strongly recommended that thought be given to their location and design so that they can be appropriately detailed on the consent drawings.

Dome flashings often come with a flashing kit, making them simpler to install, and many have flexible light tubes leading to a ceiling light diffuser, avoiding the need to frame out and line the roof space between the skylight and the ceiling. This allows the upper section to be positioned in an optimal position to facilitate flashing design.

 

 

11.8 Flatsheet Rooflights 

Flat sheets of translucent material can also be used side by side to form a continuous roof light, wall lighting, or conservatory. Materials may be single or multi skinned glass, polycarbonate or acrylic.

These are almost exclusively proprietary systems so design advice should be sought from the distributor/manufacturer.