Shining a Light On Part L and Rooflight Specification

Rooflights can help flood a space with daylight. However, specifying the right product to comply with Building Regulations is far from straightforward, as Ross Finnie, Sales Director at SIG Design & Technology (D&T), explains.


Rooflights are an increasingly popular means of allowing daylight into a flat-roofed building. In deep-plan commercial buildings, overhead glazing enables daylight to penetrate into the core to create a better, more dynamic indoor environment. In homes, meanwhile, the addition of a rooflight will brighten up even the most mundane of extensions, making the space appear bigger and brighter.

There are a huge number of rooflight products available for flat-roof applications in both the domestic and commercial sectors. These are suitable for both new-build and replacement applications.

But be aware, specifying a rooflight solution that will comply with Part L of the Building Regulations – the section of the regulations that deals with energy efficiency of buildings – is often far from straightforward.

Understanding the regulations

Approved Document L providesguidance on how to meet the energy requirements set out in Building Regulations. It is published in two parts: Volume 1 deals with dwellings; mixed-use buildings and buildings other than dwellings are dealt with in Volume 2.

This article references the 2021 edition of the Approved Document L (ADL), which came into effect on 15th June 2022 (Note: ADL was amended in 2023, but these amendments do not impact the section of the document that deals with rooflights).

A good starting point in the selection of a rooflight is to define its required thermal performance, or the rate of heat transfer through that element, from inside to outside. This is known as the rooflight’s U-value, measured in W/m2.K. In words, this is the amount of heat transmitted in Watts for every square metre of the surface area of that element per degree of temperature difference between inside and outside.

In short, the lower the U-value, the more slowly heat will pass through it and the more thermally efficient it is.

New buildings

A new building must be built to minimum energy standards and requires an energy assessment to demonstrate compliance. ADL also gives a limiting U-value for the fabric elements of a building’s envelope, which includes rooflights.

In other words, this is the worst thermal performance allowed for that element. This is to provide designers with some flexibility in how they meet the total energy performance requirements by using thermally-efficient fabric elements.

The first point where confusion can occur is in the definition of a rooflight. This is because Part L distinguishes between a roof window and a rooflight. If you think these are different names for the same thing, think again.

ADL gives a limiting U-value figure for rooflights of 2.2W/m2.K, whereas for roof windows, the limiting U-value is 1.6W/m2.K for both new and existing buildings.

ADL references the BRE document BR443 Conventions for U-value calculations, which makes a distinction between the two. It defines a roof window as being installed in the same plane as the surrounding roof, with a minimum pitch of 15° (such as a centre pivot window in a tiled roof). Instead, a rooflight is installed on an upstand and so is not in the same plane as the surrounding roof (such as a flat glass rooflight on a flat roof).

The difference in limiting U-values is because the U-values for roof windows are calculated based on them being vertical, whereas the U-value for a rooflight is calculated based on it being horizontal. If a supplier gives the U-value for an element in the wrong plane, it must be adjusted according to the data in BR443.

Another aspect to be aware of is that the U-value is based on the rooflight and its frame. The U-value can also include the kerb assembly when the kerb is supplied as part of the rooflight assembly. Where an upstand to support a rooflight is constructed on site, the upstand should have a maximum U-value of 0.35W/m2.K.

Specifiers need to be aware that this requirement can result in the surface area of the rooflight being greater than the area of the roof opening, which will impact the building’s heat loss calculations.

In addition to the thermal performance, other considerations include the type of glazing. All overhead glass must be safety glass, which is usually annealed laminated glass or polycarbonate.

Loadings are also important because in addition to being able to withstand wind and snow loads, some rooflights may also need to withstand being walked upon, and so may need to be able to withstand the weight of a person.

Existing buildings

Replacement rooflights and rooflight and kerb assemblies must also have a U-value no greater than 2.2W/m2.K. The exception to this is if only the glazing is being replaced as a remedial. In this instance, the glazing does not have to meet the energy-efficiency requirements, but its thermal performance cannot be any worse than the thermal performance of the glazing it is replacing.

Where a dwelling is being extended, the total area of rooflights, roof windows, windows and doors should not exceed:

1. 25% of the extension’s floor area

2. The total area of any windows and doors that no longer exist or are exposed due to the extension.

By contrast, where a building other than a dwelling is being extended, the area of rooflights typically cannot be greater than 20% of the area of the roof.

Clearly, there is a lot to consider when specifying a rooflight. Where a bespoke rooflight is needed, SIG D&T can provide advice on suitable options for a particular application. SIG D&T is also able to draw on the expertise of the SIG360 team when it comes to advice on specific compliance issues.

Where a more cost-effective modular rooflight is required, selection is easy using the SIG D&T online Rooflight Configurator. Using the simple interface, designers and contractors can choose which type of rooflight suits their needs and then pick the specification required, including size, kerb, tints, opening options, ventilation requirements, accessories and more. They can make an enquiry about that specific product or download the appropriate brochure.

Share this article

Login to post comments

About us

Future Constructor & Architect is a specification platform for architects and building contractors, which focuses on top-end domestic and commercial developments.

As well as timely industry comment and legislation updates, the magazine covers recent projects and reviews the latest sustainable building products on the market. Subscribe here.

Privacy policy

Latest updates


Sign up below to receive monthly construction, architecture and product updates from FC&A via email: