Daylighting is essential for healthy living, it is an established fact that ample daylight improves a general feeling of health and well-being in the workforce and improves productivity and safety.

Workplace (Health, Safety and Welfare) Regulation No. 8 and HSG.38 - Lighting at Work state ‘every workplace shall have suitable and sufficient lighting, which shall, so far as is reasonably practical be by natural light’.

The most effective method of providing even, consistent daylight, particularly in large buildings, in through rooflights, which are up to three times more effective than windows at the perimeter of the building.

Diffused lighting should be used to provide even light distribution and avoid glare.

Design Considerations

Design considerations should include:

• Fragility both initial and aged of selected rooflights. (Tested and accredited to ACR[M]001: 2000)
• Light transmission and distribution analysis
• Thermal (U-value) level
• Risk of condensation f-factor including thermal bridging, Psi Value (Y) at perimeter and intermediate spacers
• Durability and functional life of rooflight system (profiled rooflights can be difficult to replace in metal roofs)
• Safe access for maintenance

Health & Safety

All rooflights are designated as fragile materials HSG 33 Health & Safety Roof Work, unless proven otherwise in accordance with ACR[M]001: 2000. A test method is provided in ACR[M]001 for use by competent person(s) to determine the fragility class which is dependant on the rooflight material, number, location and type of fixings. Certificates for fragility class issued by rooflight manufacturers are only valid when the rooflight has been installed in full accordance with the manufacturers recommendations.

New rooflights should be certified by the manufacturer as Class B or better. If a person falls onto the rooflight, either during construction or during maintenance, the materials, regardless of whether they are classified as Class B or C, should be replaced, as the materials may now be, or soon may be, fragile. This information should be contained in the Health and Safety File.

ACR[M]001 states ‘Rooflights of whatever form, which are fitted in or stand above the plane of the roof, should never be walked on, irrespective of their fragility class’. Designers intending to use long lengths of rooflights should therefore consider safe access routes across the roof for persons needing to perform essential maintenance.

Thermal (U-value) Parts L2 and J compliance can be met by the use of triple skin rooflight systems. A major advantage of the installation of factory manufactured triple skin rooflights is their reliable thermal performance which achieves the correct level of heat retention (when calculating whole building Alpha value (a)) and energy conservation and at the same time allows a margin for junction details and site build quality risk.

Condensation Risk
Avoiding condensation risk should be considered in specifying rooflights for use in 0.25W/m²K Parts L2 and J roof systems. BRE IP 17/01 divides buildings into condensation risk categories by the calculation of the f-factor. This indicates that double skin rooflights are a condensation risk and not recommended, as they have a typical U-value of 3.1W/m²K, which significantly increases heat loss.

Trade Off Constraints

Limited trade off within the roof and wall light areas is allowed provided the total heat loss from the building does not exceed the notional values indicated in Parts L2 and J.

Solar Gain (Overheating - Part L2 only)

A provision to avoid solar overheating will limit the permitted area of rooflights to a maximum of 12% of the roof area, unless passive measures such as shading or active thermal storage are included in the design.

Other Rooflight Design Factors

Part L2 includes limits for air leakage, thermal bridging and condensation, these considerations apply to rooflight assemblies within a roof system including end and side lap seals, upstands and junctions on barrel vault and dome type rooflights. Kingspan rooflight construction solutions illustrated fully comply with Parts L2 and J.

Wind Loading

Rooflights are more susceptible to wind damage than metal roofs, it is therefore good design practice to avoid fitting rooflights in the zones of peak wind load around the perimeter of a roof.

Typical Rooflight Layout Options

The layouts indicated are suitable for pitched and curved roofs.

Chequerboard - Most uniform light distribution, but most difficult to build.

Ridge - Reasonable light distribution on small span buildings, but subject to high wind suction loads. Normally barrel vault designs are supplied by specialist manufacturers.

Ridge to Eaves - Reasonable light distribution, good buildability, but subject to high wind suction loads at ridge and eaves.
Note: It is recommended that insulated panels should be used from ridge to the first purlin, downslope and from eaves to first purlin, upslope. This layout arrangement is suitable for in plane and barrel vault type rooflights.

Mid Slope - Compromise between chequerboard and ridge to eaves avoiding high wind suction load areas.

Kingspan do not recommend the use of rooflights in banks, i.e. side by side.

Materials

In plane rooflights are made to match the metal roof panel profiles using GRP to BS EN 1013-2 or Polycarbonate to BS EN 1013-4.

Polycarbonate can be vacuum formed to make more complex three dimensional shapes, such as barrel vaults and domes.

The main difference between the materials is their resistance to temperatures.

Heat Loss & Energy Costs

Rooflights can reduce the costs of artificial lighting. However designers should note that rooflights are generally poorer thermal insulators so there will be more heat lost through them than through the cladding itself. So the benefit of providing some natural daylight must be balanced with increases in heating costs.

The costs of heating a building are greater than artificial lighting.

Light Transmission

Light transmission through a double skin rooflight would typically be between 70% and 80%. GRP sheets will generally give diffused light, with little glare. Polycarbonate is clearer, and more likely to increase glare and higher solar gain.

Strength & Thermal Movement

Rooflights are not as strong as the metal roof panels around them. Triple skin rooflights do not act compositely. This limits purlin spacings to approximately 2 metres and more fasteners with larger diameter washers are required to withstand wind uplift forces, particularly at ridge, eaves and verge.

For example, on trapezoidal rooflights, primary fasteners with 32mm diameter washers would normally be fitted in every trough across the profile at each purlin position. Rooflight side laps onto metal faced panels should be fixed with stitching fasteners with head/washer diameter of 12mm, at a maximum spacing of 400mm.

To accommodate differential thermal movement between rooflights and metal panels, care has to be taken to specify correct fasteners, washers and site drilled hole sizes.

Kingspan’s rooflight construction details illustrate assembly, sealing and fastener information.

Weatherproofing

In principle where through fixed metal cladding panels are used, the rooflights simply end lap and side lap in the normal way.

However, rooflight material thicknesses are typically 2 to 3 times thicker than the external facings of cladding panels. This means that laps do not “nest” precisely on top of each other. End laps are therefore more difficult to seal correctly and consequently there is increased risk of water ingress. Therefore rooflights should be specified in the longest lengths available to minimise the number of end laps.

Only barrel vault or dome type rooflights should be used on low pitched roof slopes (between 1.5° and 4°), with no primary through fixings. See construction details for KS500/1000 Kingzip and KS1000 LP/CR.

Fire

The following table shows how various rooflight materials perform when subjected to high temperatures:

GRP is more resistant to high temperatures than the other rooflight materials, but each material has potential benefits in a fire.

The Building Regulations and Standards require that inner linings have designated value for surface spread of flame. i.e.

Lifetime Durability

KS1000 Safespan rooflights have a life expectancy of up to 25 years.

Barrel vault rooflights, KS1000 BV and KS1000 BVZ, have the following life expectancy
- GRP of up to 25 years
- Polycarbonate up to 20 years and 10 years against discolouration
Life expectancy is dependant on the building’s location, external and internal environment and correct installation. Regular cleaning during this period will help to maintain optimum light transmission.

Note that polycarbonate should not be used over dark coloured sheets or with dark seals as this would lead to increased temperatures which may cause softening, distortion, and degradation.

Note: Polycarbonate sheets must be isolated from plastisol coated panels by barrier tape or suitable sealant.

External Facings & Internal Lining Classifications

When rooflights are to be installed within 6 metres of a boundary they must be non melting so GRP has to be used. Elsewhere polycarbonate materials can be fitted.

The melting behaviour of polycarbonate is often seen as an advantage because it allows a fire to vent automatically, releasing heat and smoke from the building, thereby assisting fire fighting.