Rooflights

To comply with this building designers commonly use rooflights as a solution for industrial and commercial buildings. When pre-finished steel cladding is used, these generally consist of matching profiled translucent or transparent sheets. The Metal Cladding and Roofing Manufacturers Association (MCRMA) publication, ‘Recommended good practice for daylighting in metal clad buildings’ provides a valuable source of detailed information.

But what do building designers need to take into account in this area? Rooflight design needs to consider:

• Use of the building.
• Building’s height and shape.
• Type of roof cladding.
• Colours of internal surfaces.
• Effect of any adjacent buildings.
• Electric lighting.
• Heating.
• Thermal performance requirements.
• Rooflight layout.
• Fire performance requirements.
• Safety of maintenance workers.

Rooflights are generally made from Glass Reinforced Polyester (GRP), Poly Vinyl Chloride (PVC) or Polycarbonates. Their thickness varies from between 1 mm and 3 mm, according to material properties, structural requirements, safety requirements and imposed loadings. Surface coatings on rooflights can provide different levels of durability, chemical resistance and weather resistance.

Rooflights are generally constructed as a double skin arrangement on site or, as is increasingly popular, preassembled in a factory. A barrel vault or dome is a special, factory-made double skin rooflight which is fixed to an upstand so that it is above the roof surface. Factory-assembled rooflights can be designed to integrate with most roof cladding system and will minimise heat loss and condensation build-up.

The arrangement of rooflights should aim to give an even distribution of light in the simplest manner possible. There are four general techniques for layingout rooflights.

• Ridge to eaves.
• Midslope.
• Chequerboard.
• Continuous.

The chequerboard approach gives the most even distribution of light, but it has a downside: the wide distribution gives the greatest possible number of interfaces and therefore the greatest potential for problems in construction and service. The simplest arrangement, and the one which minimises the number of interfaces and cut edges, is the ridge-to-eaves option.

Rooflights are less stiff and have less strength than prefinished steel cladding because of their higher flexibility. As a result, the rooflights rather than the pre-finished steel cladding will determine the purlin spacings. Wind suction loads, particularly at roof edges, are critical. Rooflight manufacturers should be consulted to ensure the correct specification in terms of purlin spacing, material type and additional supports.

It should be noted too that rooflights differ from a pre-finished steel roof in terms of fire and thermal performance, and consideration will need to be given to these. The Building Regulations on conservation of fuel and power stipulate that the maximum U-value for rooflights is 2.2 W/m2K and the maximum allowable proportion of rooflights is 20% of roof area, although lower levels can be traded-off with other aspects.

Rooflights are significantly more fragile than steel roof sheeting and should never be walked over. This is of particular importance if a ridge-to-eaves layout is used which will prevent easy access across the roof. Construction Design Management (CDM) regulations refer to rooflights specifically as something which should be avoided from a safety viewpoint. In all cases, the designer should take care to ensure that the position of rooflights is obvious throughout the life of the building, taking into account any expected colourfade. It is common practice to use fasteners with a Poppy Red coloured head around rooflights to ensure their visibility.