Please note: in some cases, the definitions in this glossary use generic non-technical terms in order to clarify meanings. For full technical descriptions, please consult the rooflight manufacturer.


Automatic Opening Ventilators for smoke ventilation in the event of fire. Opening rooflights adapted for this purpose with smoke-sensor operated opening mechanisms are available.


An inert gas commonly used to fill cavities in double or triple glazed rooflights. Argon provides an additional barrier to heat transfer.


Glass Reinforced Polyester, commonly referred to as ‘fibreglass’. GRP is a lightweight, translucent material that is widely used for profiled rooflights for industrial and agricultural buildings, warehouses and other applications.


The coefficient commonly used to measure the solar energy transmittance of transparent and translucent materials.

In-plane rooflight

A flat or profiled rooflight that is installed in the same plane as the roof itself. Conversely, out-of-plane rooflights protrude above the plane of the roof, as in a lantern light or dome.

Light tube (or light pipe)

These are generic names for rooflights supplied with and fitted above a tube made of reflective material, designed to provide natural light to rooms under roofs where a loft cavity exists.

Low E glass

Low emissivity glass designed to improve thermal insulation. Low E glass has a heat- reflecting transparent metal oxide film applied to one side.


Lumira® (formerly Nanogel) is a granular aerogel material offering unsurpassed thermal insulation, as well as light diffusing and sound reduction properties. Lumira-filled multiwall polycarbonate rooflights provide extremely high insulation values, making them a popular choice for projects where energy efficiency is paramount.

Out-of-plane rooflight

A rooflight which protrudes above the plane of the roof upon which it is installed. Domes, pyramids, barrel vaults and lantern lights are all examples of out-of-plane rooflights.

Sinusoidal profile

‘Wavy’ profiled sheets typically used in agricultural and utility buildings. GRP rooflights are available to match most sinusoidal profiled sheets. Some sinusoidal profiles are also available in polycarbonate.

Trapezoidal profile

Angular profiled sheets typically used in factories, warehouses and commercial premises. A wide range of trapezoidal profiles are available. These give a more modern appearance than sinusoidal profiles. GRP rooflights are available to match most sinusoidal profiled sheets. Some sinusoidal profiles are also available in polycarbonate.


The measure of heat loss in building element such as roofs and floors. It can also be referred to as an ‘overall heat transfer co-efficient’ and measures how well parts of a building transfer heat. This means that the higher the U-value the worse the thermal performance of the building envelope. A low U-value usually indicates high levels of insulation. U-values are a useful way of predicting the composite behaviour of an entire building element rather than relying on the properties of individual materials.


The U-value of a rooflight based on the developed area of the complete rooflight assembly including any frameworks or upstands. It is this value which should be checked against limiting values in The Building Regulations.

Walk-on rooflight

A glass rooflight with laminated glass load bearing outer pane, allowing safe pedestrian access.