Understanding Rooflights

The rooflight market has expanded rapidly over recent years with our growing desire to flood our homes with natural light. Whether it is a rooflight for a loft conversion or a sliding skylight, there are many manufacturers out there who provide some great solutions.

Rooflights fall into two categories depending on the roof they sit within: those on a pitched roof, typically fixed or venting units set into the roof slope, and those on a flat roof, usually sitting on a raised upstand or curb above the waterproofing. The two bring different design and detailing considerations, covered throughout this post.

Although the exact detailing will vary depending on the roof construction and rooflight manufacturer, the principles remain consistent: maintaining waterproofing, insulation, airtightness and structural support throughout the junction.

Key Components of Rooflight Detailing

To achieve a well-detailed rooflight, it is important to pay attention to several critical components:

Glazing Systems

The glazing specification plays an important role in determining the overall performance of a rooflight. Modern rooflights are available with a wide range of glazing options that can improve thermal performance, reduce solar gain and enhance acoustic insulation.

Double and triple glazing are the realistic starting points for most residential rooflights. Single glazing typically returns a U-value in the region of 5.0-5.8 W/m²K, which falls well short of the Approved Document Part L backstop values (around 1.6 W/m²K for new dwellings, 1.8 W/m²K for replacements), so it’s rarely specified outside unheated outbuildings. Laminated and toughened glass provide the safety and security benefits needed to help satisfy Approved Document Part K.

For most residential projects, double or triple glazing incorporating low-emissivity coatings will provide a good balance between energy efficiency and daylight. Where overheating is a concern, particularly on south-facing roofs, solar control glass can help reduce the amount of heat entering the building while still allowing high levels of natural light.

The final specification should always be considered alongside the orientation of the roof, the size of the glazed opening and the intended use of the space.

For U-value calculation methodology, the BRE report BR 443 and the National Association of Rooflight Manufacturers‘ (NARM) technical guidance are useful reference points when comparing figures quoted by different manufacturers and worth checking whether a quoted U-value is centre-pane or whole-unit, since the difference can be substantial. 

Framing and Supports

The framing and support structure of rooflights must be carefully designed to withstand wind loads, snow loads, and other environmental stresses. Timber, steel, and aluminium are commonly used materials, with considerations for durability, aesthetics, and compatibility with the building’s roof assembly.

On a flat roof, it’s also worth establishing early on whether the rooflight will ever be walked over for maintenance access, or because the roof doubles as a terrace since this changes the loading that the frame and any upstand need to carry, as well as the glazing specification itself. 

Upstands and Waterproofing

The junction between the rooflight and the roof covering is one of the most critical areas of the detail. Flat roof rooflights are generally installed on raised upstands that lift the glazing above the roof surface and reduce the likelihood of standing water reaching the frame.

The roof waterproofing should be carefully dressed up the upstand in accordance with the roofing manufacturer’s guidance to maintain a continuous weatherproof barrier. The required upstand height and waterproofing arrangement will vary depending on the roof covering and the rooflight system being used, so these elements should always be designed together rather than as separate components.

Ventilation

Opening rooflights can contribute to a room’s purge ventilation strategy under Approved Document F, which requires an openable area of at least 1/20th of the room’s floor area (for opening angles over 30°) to clear moisture and pollutants quickly. The choice between manual and motorised actuators affects long-term usability. Motorised units are easier to operate at height but add a maintenance item.

Regulatory Considerations for Rooflight Detailing

Compliance with building regulations is important in the design and detailing of rooflights in the UK. Key regulatory considerations include:

Building Regulations

The design of rooflights may be influenced by several Approved Documents depending on the type of project and the role of the rooflight within the building.

Structural openings should comply with the requirements of Approved Document A, while Approved Document L sets standards for the thermal performance of the building envelope, including limiting heat loss through glazed elements and reducing thermal bridging around junctions.

Where opening rooflights contribute to the ventilation strategy, the requirements of Approved Document F should also be considered. For new dwellings, Approved Document O may influence the size, position and specification of rooflights to reduce the risk of summertime overheating. In some situations, Approved Document B may also apply where rooflights form part of a smoke ventilation strategy or are incorporated into fire-resisting construction.

Approved Document K is also relevant wherever a rooflight could be fallen through or come into contact with people, particularly on accessible or trafficable roofs. Its requirements for safety glazing and non-fragility apply to dwellings as well as other building types, even though some of its other provisions are restricted to non-dwellings. 

As always, the relevant Building Regulations should be considered as part of the overall building design rather than in isolation.

Fire Safety

Where a rooflight sits within a fire-resisting element, or is relied on for smoke ventilation, it needs to satisfy Approved Document B — covered above. Fire-rated glazing specification and firefighting access should be confirmed with the rooflight manufacturer and fire engineer on a project-by-project basis. 

Falling, Collision and Impact (Approved Document K)

Approved Document K covers protection from falling, collision and impact, and several of its requirements apply directly to rooflight glazing in dwellings. Where a rooflight sits somewhere a person could fall through including flat roofs used as terraces, green roofs maintained on foot, or roof glazing within reach of an accessible area — Part K (Requirement K2) calls for either a protective barrier or non-fragile glazing, the latter typically demonstrated through the ACR[M]001 test for non-fragility of large element roofing assemblies, with products rated Class B or above generally considered suitable for accessible roofs. 

Requirement K4 additionally means any glazing people might come into contact with must either resist impact or break safely, which is where toughened and laminated glass specifications come in. Some of Part K’s other provisions including its requirements for manifestation of glazing and safe cleaning access apply to buildings other than dwellings rather than to houses directly, so it’s worth checking which parts are relevant to your specific project type.

Thermal Performance and Airtightness

Maintaining the continuity of the insulation and airtightness layers is one of the key challenges when detailing rooflights. Without careful design, the rooflight opening can become a weak point within the building envelope, increasing heat loss and creating areas where condensation may occur.

Insulation should continue as close as possible to the rooflight frame, while the airtightness layer should be carefully sealed around the opening to avoid unwanted air leakage. These junctions are becoming increasingly important as buildings are designed to achieve higher levels of energy efficiency, with greater emphasis placed on reducing heat loss through thermal bridges.

Designing to Reduce Overheating

As buildings become better insulated and more airtight, managing overheating has become an increasingly important part of residential design. Rooflights can make a valuable contribution to daylighting and natural ventilation, but they can also introduce significant solar gains if they are not carefully considered.

The amount of heat entering through a rooflight will depend on a number of factors, including its orientation, size, glazing specification and whether any external shading is provided. South and west-facing rooflights are generally exposed to the highest levels of solar radiation and are therefore more likely to contribute to overheating during the summer months.

Opening rooflights can form part of a natural ventilation strategy by allowing warm air to escape from high-level spaces. This stack effect can be particularly effective within vaulted ceilings and open-plan rooms. However, ventilation alone may not be sufficient where large areas of roof glazing are proposed.

For new dwellings in England, Approved Document O requires designers to consider the risk of overheating. Rooflights may form part of the glazing assessment and should be considered alongside window sizes, orientation, shading and the overall ventilation strategy. Depending on the design approach being used, compliance may involve limiting solar gains through glazing specification, reducing the area of glazing, providing external shading or demonstrating adequate ventilation.

Considering these issues early in the design process will often lead to better daylighting, improved occupant comfort and a more balanced design.

It’s worth noting that Approved Document O applies to new residential buildings only. It doesn’t currently extend to extensions, conservatories or loft conversions carried out to an existing dwelling. That said, the same overheating principles are just as relevant to a kitchen extension with a large flat rooflight, and many designers apply the Part O methodology voluntarily as good practice even where it isn’t a formal requirement. 

For extensions specifically, the more immediate regulatory control on glazing area comes via Approved Document L, which sets a notional limit of 25% of the extension’s floor area for combined window and rooflight glazing. Beyond this, compliance has to be demonstrated through a SAP calculation showing an equivalent or lower carbon impact. 

Planning Permission & Building Control

Many rooflights on existing houses benefit from permitted development rights and don’t need a planning application, provided they don’t project more than 150mm above the plane of the existing roof slope and aren’t installed on a roof slope or wall fronting a highway. 

However, these rights can be restricted or removed in conservation areas, on listed buildings, or where an Article 4 direction applies, so it’s always worth checking with the local planning authority before assuming they apply.

Loft Conversion Details

If you’re designing a rooflight for a loft conversion specifically, our Loft Conversion Details book covers the key junctions in depth including rooflight installation within a pitched roof with annotated 2D and 3D details and downloadable files. The LC42 detail shown in this post is taken from the book and is also available to download in the Detail Library.

You can download a free sample to see how it’s put together before you decide.