Design Considerations when Specifying Rooflights

In the early stages of building design, it is important in a fabric first design strategy to assess key materials and determine what it is that those materials need to deliver.

Often rooflights are overlooked despite offering a wide variety of proven benefits. Correctly specified, Hambleside Danelaw’s Zenon GRP rooflights can not only satisfy regulatory requirements, but also capture the essential essence of natural daylighting. Below are some key attributes of GRP rooflights that must be considered prior to the specification stage.


Although rooflights should never be walked on, it is expected that they achieve a non-fragility rating to reduce risk associated with falling onto the rooflight assembly. All Zenon GRP rooflights are tested in accordance with ACR[M]001 Test for Non-Fragility of Large Element Roofing Assemblies.

Fire Performance

An equally important consideration when specifying GRP rooflights is fire performance. Regulations relating to fire are under constant review, but current information on regulations and Zenon GRP rooflights can be found in our dedicated document.

Thermal performance

U-Value is used to measure the thermal transmission of a building envelope assembly. GRP rooflights often feature a polycarbonate insulation layer to comply with Building Regulations minimum requirement of 1.8W/m²K but it is becoming increasingly common for buildings to target even lower U-Values. This of course will need to be balanced with the rooflight’s light transmission levels as additional layers of polycarbonate sheet result in lower light distribution.

Zenon Insulator is a sustainable way of decreasing the U-Value, without compromising that all-important light transmission. Made from cellulose acetate (a recycled wood pulp), Insulator has a honeycomb structure which runs perpendicular to the plane of the rooflight, resulting in increased light transmission and diffusion.

Critically, from an environmental standpoint, Insulator can be composted at the end of its service life.


G-Value, also known as solar gain, is the directly transmitted solar heat and absorbed solar radiation, which enters the building through a material. Solar gain and light transmission are closely linked – it is impossible to have one without the other. Therefore, when planning a building with high levels of natural daylight it is important to consider and address the solar gain that will inevitably be created.

Light Transmission

It should go without saying, but GRP rooflights should provide the best levels of natural daylight possible. It is shown that incorporating rooflights into the building design can result in up to three times more natural light in the internal space than a window/vertical glazing of the same size.

Natural daylight results in a happier, healthier and more productive work environment; for more information on how natural daylight can benefit the building occupants, take a look at NARM’s Quickguide.

Building Use

The hours in which a building will be in use is another important consideration. This will affect key decisions made about the building design. During daylight hours, GRP rooflights, when installed in the correct way, can provide the levels of daylighting needed to significantly reduce the need for artificial lighting. This may mean that the installation of automated lighting, to respond to varying levels of daylight (winter afternoons for example), a good choice for the building. This will inevitably reduce the demand for lighting energy and associated costs and make the most of the natural daylighting benefits provided by the rooflights.

Embodied Carbon

The last thing on this list should not be mistaken for the least important. Embodied carbon is fast becoming a key consideration when selecting building components. Embodied carbon refers to the Greenhouse Gases (GHGs) expended during the product’s cradle to grave lifecycle. The embodied carbon figure includes the carbon equivalent of GHGs used during manufacturing, distribution, maintenance, and disposal.

Hambleside Danelaw have always considered reducing embodied carbon to be of the utmost importance, which was one of the key drivers behind the development of Zenon Evolution. The manufacturing composition of Zenon Evolution has significantly less embodied carbon than its traditionally manufactured equivalent alternative. Zenon Evolution rooflights can also be used in conjunction with our Insulator honeycomb core to deliver highly efficient - and very low carbon – rooflight configurations.

Learn more about GRP rooflights

Hambleside Danelaw offer a variety of CPD seminars which help increase knowledge levels surrounding GRP rooflights and their specification in industrial, commercial, and agricultural buildings.

To arrange a CPD presentation, please fill in our contact form.

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