Part O of the Building Regulations in England sets out requirements for overheating. It was a new part of the Building Regulations in 2021 and will be intrinsic to the overarching Future Homes Standard due to be introduced this year. Wales and Scotland have their own overheating requirements, albeit the overall aims across the different countries are similar. Limiting the potential for dwellings to overheat is particularly critical as the climate continues to warm. Overheating is not just a comfort issue; it is a health issue and must be addressed as the likelihood of summer heatwaves continues to increase. Rooflights have a role to play in overheating control, especially as the use of mechanical cooling is restricted. What are the requirements of Part O 2021? There is a single requirement to Part O, which states unwanted solar gains should be limited in summer and there should be an adequate means to remove heat from the dwelling. There are two ways to assess overheating risk in dwellings: a simplified approach and dynamic thermal modelling. The simplified approach is generally sufficient for individual homes. It sets maximum limits on glazing areas to prevent excessive solar gains. It also sets minimum limits on ventilation free areas, with the aim of ensuring excess heat is removed via ventilation. Dynamic thermal modelling is carried out in accordance with CIBSE TM59 and is more accurate than the simplified approach. It sets comfort criteria by accounting for various parameters that affect internal temperatures, such as weather data, thermal mass, orientation of the dwelling, occupancy and internal heat gains. Approved Document O makes clear that its purpose is to protect the health and welfare of people within the dwelling. It does not guarantee their comfort. How can rooflights help to achieve Part O compliance? Record-breaking heatwaves in recent years have focused attention on overheating, giving it the same kind of prominence as the effect of cold weather on people’s health. The need to consider our changing climate is leading some planning authorities to ask for overheating to be assessed not just on current weather data, but also weather projections for 2050 and 2080. Dwellings must be able to demonstrate passive cooling strategies that meet the comfort criteria for these predicted future conditions. That is likely to mean increased specification of solar control features on glazing, combined with external shading devices. However, it also expands the role that rooflights can play in helping to achieve comfortable dwellings for the future. Orientation of the building is obviously critical to the likelihood of overheating. East-facing elevations get the morning sun, while south- and west-facing elevations are exposed to the hottest times of day. North-facing elevations, meanwhile, receive no direct sunlight. There is likely to be greater scope for individual dwellings to be designed to maximise the benefits of different orientations, and minimise the drawbacks. That depends on having a relatively unrestricted plot, and little other surrounding development to dictate where and how the dwelling must be positioned. Apartments are often very restricted, sometimes having only a single aspect. Depending on the design of the building overall, larger and more luxurious apartments located at the top of the building might offer multiple aspects. However, the need to accommodate more than one apartment at that level puts limitations on maximising orientation, with each apartment requiring a unique solution based on its different aspects. For both individual dwellings and apartments, specifying rooflights as well as façade glazing increases the options available for controlling solar gains and reducing overheating risk, while still providing high levels of daylight. Since warm air rises, opening rooflights are the most effective way of losing excess heat in conjunction with opening windows to allow fresh replacement air into the building. Positioning rooflights on the opposite side of windows in a dwelling or apartment can help to achieve effective cross ventilation. As well as contributing to fresh air provision, cross ventilation is a passive cooling technique that has long been a feature in hot and humid climates. As we come to terms with living in a warming climate, cross ventilation will be an increasingly important tool for maintaining the health and wellbeing of building occupants. Using rooflights to go beyond the minimum standard of Part O Like all areas of building regulations, Part O only sets out a minimum standard. The option to go beyond the requirements is always available and can be easily justified in this case – we have already seen that Part O doesn’t seek to achieve comfort for the occupants of a dwelling. A frequent criticism of regulations is that requirements in one area don’t necessarily complement the requirements in another area. The typical example is how energy performance requirements have improved, without ventilation requirements improving to the same level. Changes to the regulations are only just beginning to overcome this disconnect, hence the introduction of Part O. Designers and specifiers of prestigious homes and high-end apartments don’t have to wait for the regulations to be completely holistic. They can deliver a joined-up approach now to benefit their clients and add value to their projects. The blank canvas of a new project offers the opportunity to go beyond minimum standards. Project teams now routinely look to the forthcoming Future Homes Standard, and factor in predicted future climate conditions, to benchmark performance now. Rooflights can provide homes with a level and quality of daylight that vertical windows alone struggle to achieve. Perhaps more importantly, they have a significant role to play in helping to mitigate overheating risk, at a time when the regulatory requirement for such mitigation is not fully mature.

Structural floor loadings for walk on glass rooflights should be stated as Uniform Distributed Load (UDL) and Concentrated Load. Although you should expect a manufacturer or supplier to confirm what loadings the glass is designed to withstand, you should not expect them to provide information on the loadings themselves; this remains the responsibility of the specifier. For example, if you are working on a scheme that involves a glazed floor section in a commercial shopping centre, you must provide the loadings that are relevant to this specific application in accordance with British Standards. The supplier will then ensure that the glass provided will meet those demands. In order to establish what these loadings might be you should refer to BS EN 1991-1-1:2002, Eurocode 1: Actions on structures which covers General actions, densities, self-weight, and imposed loads for buildings. Further information regarding UK specific loadings can be found in the UK National Annex to Eurocode 1 (NA to BS EN 1991-1-1-1:2002). The loadings for any glass rooflight designed for deliberate foot traffic will depend on building type and specific use, but typically you might expect to see load capacities of the following magnitude: Domestic applications Uniformly distributed load (UDL) from 1.5kN/m2 Concentrated load from 2.0kN Commercial applications Uniformly distributed load (UDL) 4.0kN/m2 Concentrated load 3.6kN Heavy duty commercial applications Uniformly distributed load (UDL) 5.0kN/m2 Concentrated load 4.5kN Glass specifications for walk on rooflights For walk on specification rooflights the outer pane of glass should always be both toughened and laminated. It should be specifically designed for the pane size and method of support. The outer pane will vary dependant on intended use and will usually comprise of two or more leaves laminated together; occasionally an additional sacrificial layer of annealed glass is used on the surface. The increased thickness of this type of glass results in a heavier product when compared with non-walk on rooflights and consideration should be given to the supporting roof structure and site logistics. Weights may typically range from 95 kg/m2 – 110 kg/m2. You might also be interested to read our post on the difference between non fragile and walk on glass and anti-slip walk on glass. To find our more about specifying rooflights book a CPD which would be delivered by a member of our expert technical team.

The benefits of specifying rooflights and roof windows are clear: they provide high levels of natural light, they can be wonderful sources of ventilation, and they contribute to the thermal comfort of a building. Roof windows and rooflights also provide an aesthetically pleasing addition to the external appearance of a property. But what else should you consider to ensure the specification runs smoothly once it gets to site? Roof windows and rooflights are generally easy to install, but knowing what to expect can help to ensure things go smoothly. We have exclusive insights from Clarkson Builders about how to ensure proper specification and keep work on-site progressing as expected. Here are the things they recommend specifiers should be aware of: Window size Roof windows and rooflights come in an array of shapes and sizes, and choosing the right dimensions for your project is critical. The ‘standard’ for roof windows is 900x600mm, and these are installed between rafters at 600mm centres. Other common sizes are 980x980mm and 900x1200mm – the latter is designed to fit in a double-rafter space. They can also be provided in much larger sizes – for example, the Glazing Vision Pitchglaze Roof Window stocked on rooflights.com can measure up to 3000x1000mm. Structural support The most effective way of supporting a roof window is to use double trimmers for horizontal support and double rafters for vertical support. Double hangers are placed in each corner, and single hangers are located around connected rafters. The double supports comprise two pieces of rafter timber with 600mm centres (either C16 or C24 between 100x50mm or 225x50mm, depending on specification and structural calculations). The rafters should be fixed together with bulldog timber connectors and M12 bolts. When fixing the timber, the head of the bolt should be recessed on the side of the wood to which the rooflight will be fixed. Otherwise, it could protrude into the area beneath the glazing. As with all structural requirements, relevant calculations should be carried out by a qualified structural engineer. Preparing the window aperture Remember that the size of the window aperture may not be the size of the window itself. The actual requirements will depend on the individual roof window, so be sure to consult your desired manufacturer early in the planning stages. When preparing the rafters, consider how elements like the insulation sarking and the ends of the battens will be supported. The roofing underlay should be folded back and fixed appropriately. Other elements will also need to be added to the roof, such as tilting fillets, battens and flashing. Out-of-plane rooflights will also require the construction of an upstand. Site delivery and storage Take time to consider how rooflights and roof windows will get to site – and once there, how will the units be stored to ensure they aren’t damaged. In addition, don’t underestimate the weight of a roof window. A small, 750x750mm laminated rooflight weighs just over 31kg, and the units can reach more than 150kg, depending on the size and type of glazing. Heavier units should also be assessed in terms of Health and Safety. Mechanical assistance should be utilised for moving the roof windows around the site safely. Getting the roof window or rooflight onto the roof In most cases, construction materials are delivered ‘to the kerb’, and this is the case for rooflights and roof windows. Unless you have a particularly obliging driver, you will need to work out how to get the unit onto the roof. As discussed in the previous section, roof windows can be very heavy and lifting them in a way that avoids damage or injury will require careful planning. It could also incur extra costs – you may need to hire lifting equipment for bulky units, or for hard-to-access locations. Instructions and tools Many roof windows come with an installation guide that actually covers multiple products – and those products can look very similar while having slightly different fixings and flashings. If these instructions aren’t followed correctly, it could impact the installation process and how the roof window or rooflight performs. Be sure to check the instructions to ensure that the installation team has the right tools. Depending on the manufacturer, special drill bits may be required. For example, Velux windows use a screw that uses a T20 drill bit that’s commonly used for day-to-day work on building sites. Tiling around roof windows Roof windows are usually found on pitched roofs (unlike roof lights, typically found on flat roofs). When tiling around roof windows, it’s important to ensure that the bottom tile is not too close to the window, as the flashing cannot be installed correctly. The best way to plan the tile gauge is to measure from the bottom of the roof, as this allows for the best finish and helps to ensure the roof window performance isn’t compromised. When using pantiles (these are usually made from concrete but may also be clay in heritage projects), be sure to chamfer the top of the bold roll beneath the window. This should all be explained in the instruction manual, which should outline the distances to install your battens to the sides, top and bottom of the window. With so much to think about, it’s easy to see why it’s so important to look beyond appearance and performance of a building element at the specification stage. If you have any questions or concerns about a product you’re specifying, it’s a good idea to get answers and advice as soon as possible from the manufacturer.

Part L of the Building Regulations in England sets out requirements for the conservation of fuel and power, or energy efficiency as it might be more commonly termed. The current version is Part L 2021, which is expected to be replaced by a new version later this year as part of the overarching Future Homes Standard. Wales and Scotland have their own energy efficiency requirements. High standards of building fabric performance, supported by low U-values and a good standard of airtightness, are essential for meeting current or future regulations in all parts of the UK. Rooflights are part of the building fabric, and so must play their part in delivering energy efficiency as part of overall building performance. What are the requirements of Part L 2021? As Approved Document L volume 1 says, “A new dwelling must be built to a minimum standard of energy performance.” To assess that performance, the dwelling’s specification is compared to that of a theoretical dwelling, called the notional dwelling. The calculation method for making the assessment is the Standard Assessment Procedure, or SAP. The actual dwelling must meet or exceed the targets set by the notional dwelling in the following areas. Primary energy rate Carbon dioxide emission rate Fabric energy efficiency rate The first two are influenced by both the building fabric and the fuel used within the dwelling. The fabric energy efficiency rate is defined entirely by the building fabric. As we are talking about rooflights, which are part of the building fabric, we won’t be discussing heating systems, different fuel types and building services in this blog post. How can rooflights help to achieve Part L compliance? What does meeting the requirements of Part L look like in practice? Generally speaking, dwellings need to aim for well-insulated and reasonably airtight building fabric. Doing so reduces thermal transmittance and keeps the leakage of warm air to a minimum. Glazing of any type must play its role within the building fabric performance, while acknowledging that it doesn’t perform as well as the surrounding construction in terms of U-values. Ideally, the U-value of windows and rooflights should be as low as possible. This is particularly important to minimise thermal bridging, where the rooflight sits within the roof build-up. Failing to address thermal bridging risks undoing all the intended performance goals for the dwelling. Rooflights must be installed within the surrounding roof, without creating a break in the thermal envelope and causing significant thermal bridging. For the first time, Part L 2021 introduced a requirement for photographic evidence to demonstrate that details were being installed in accordance with the intended design. This effort to reduce the difference between what is designed and what is built on site – known as the performance gap – is vital to ensuring that dwellings do not consume more energy and emit more carbon than anticipated. Using rooflights to go beyond the minimum standard of Part L We’ve already acknowledged that Part L only sets out a minimum standard. The option to go beyond the requirements is always available and can be considered preferable. A frequent criticism of national building regulations, especially in recent years, is that the requirements in one area don’t complement the requirements in another area. The most common example of this issue is how levels of insulation and airtightness have increased, without a corresponding improvement in rates to maintain indoor air quality and reduce. Changes to the regulations are only just beginning to overcome this disconnect. Designers and specifiers of prestigious homes and high-end apartments have it within their power to think holistically and deliver a joined-up approach that benefits their clients and adds value to their projects. The blank canvas of a new project offers the opportunity to deliver more than the minimum standards required by building regulations. Project teams now routinely look to the promised and predicted climatic conditions decades down the line, to benchmark performance now. Rooflights have always played an important role in homes. They can provide a level and quality of daylight that façade glazing alone struggles to replicate. And better levels of natural light are good for the health and wellbeing of building occupants and can reduce reliance on artificial lighting – thereby saving energy.

The rise in homeowners extending their properties has increased alongside a growing demand for suitable rooflights for flat roofs therefore, it is important to recognise that installing a rooflight onto a flat roof is not simply a case of installing a section of glass over an opening. This is of course a quick and easy approach that may provide a solution for a while, but in time common issues will develop that can cause unnecessary damage to your roof and building internals. Such issues can be avoided if you specify the right product for the job. The correct or minimum pitch for flat roof fixed rooflights Although rooflights for flat roofs are now much more readily available, product manufacturers often hear similar questions as to how they should be installed, one of the most common being ‘What pitch do I install the unit at?’ There is no single answer to this question due to product ranges being quite diverse. However there is still a methodology behind it, which this post will discuss in relation to the most straightforward fixed rooflights. For planning reasons and aesthetics, customers often ask if the rooflights can be installed onto a flat roof - It is recommended that regardless of overall dimensions, a minimum 3 degree fall is sufficient to ensure that water will drain off the surface of the glass and avoid ponding, a process in which puddles of water on the glass surface evaporate and leave unsightly dirt and staining on the glass. With a minimum 3 degree fall, individual droplets of water will still stay on the glass as they would on vertical glazing, but larger pools of water will not form. In all glazing there is a degree of deflection that is particularly apparent on overhead glazing. A combination of wind load, snow load and the physical self-weight of the glass all contribute to the amount of deflection that is apparent. The glass rooflight specification should be calculated so that the maximum amount of deflection under the worst case loading and on the largest pane sizes does not exceed 25mm. Without the pitch, any deflection in the glass will increase the severity of water ponding so installing the roof light at a minimum 3 degrees is the recommended method for avoiding this problem. Increasing rooflight pitch to 5 degrees For more efficient results and improved drainage, increasing this to 5 degrees, as shown in the following diagram, can be beneficial; the pitch is constructed by building the required slope into the upstand itself. For pitches greater than 5 degrees, the pitch must be built into the roof with the kerb constructed perpendicular to the roof as illustrated below, otherwise the down leg of the framework may foul the external face of the kerb. This could result in a poor fit that means the rooflight is not properly supported. Specification considerations should also be given to the orientation of the rooflight. For example, if you have specified a unit of 5 m x 1 m, it would be wise to build the pitch into the span dimension; the differential between kerb heights on opposing sides is reduced and water has less distance to travel when running off the unit. Specifying Walk on Rooflights The only real exception to the 3 degree rule is where walk-on specification glass is going to be used; obviously the glass needs to be as level as possible for its chosen application. For walk on use, it is recommended to build a 1 degree fall wherever possible to allow some water run off. The risk of ponding is marginally greater but because walk on glass is so much thicker in order to carry the design floor loadings, the deflections relative to pane size will be far less than with standard double or triple glazing. In addition, since the unit is accessible, the glass can be cleaned far more easily so that water ponding is not so much of an issue. Please note that requirements may vary depending on your chosen manufacturer. Always check that the specification meets your flat roof design. It is also critical to consider your supplier when specifying rooflights for flat roof applications. Due to the minimal pitch used it is important to look in detail at the product and ensure that there is no external capping around the edge of the unit, which could trap moisture and dirt, leaving an unsightly mess.

High-quality residential developments are no longer defined only by their location and standard of finish. Stunning architecture, luxurious interiors and high-end amenities are still expected, of course, but there is something else to add into the mix: sustainability. A new home or high-rise development doesn’t have to be labelled as an “eco home” to be part of the sustainability conversation. Whether driven by the client, the architect, or the two working in collaboration, better standards of energy efficiency and comfort, along with healthier interiors, are now normal aspirations. The blank canvas of a new project offers the opportunity to deliver much more than the minimum standards required by building regulations. Project teams now routinely look to the promised Future Homes Standard – not due to be implemented until 2025 – as a benchmark for performance. As a component of the building envelope that must balance energy efficiency with providing natural light and views out, rooflight specification plays a big part in whether that benchmark is achieved. What is the Future Homes Standard? The Future Homes Standard is the name given to proposed changes to the Building Regulations in England, expected to take effect this year. It is not one single standard or document, but the combined result of updates to Part L (conservation of fuel and power), Part F (ventilation) and Part O (overheating), as part of the wider Building Regulations. The current stated intention of the ‘Future Homes Standard’ is for new residential properties to deliver an average reduction in carbon emissions of 75-80% compared to Part L 2013. Thanks to such significantly better performance, homes should not require energy efficiency retrofit measures in future. The eventual decarbonisation of grid electricity will account for the remaining operational emissions, meaning new-build homes will play their part in helping the country to meet its legally binding net zero carbon targets. How can projects be built to the Future Homes Standard today? Until it is consulted on and agreed, we don’t know the precise technical detail of the Future Homes Standard. However, there are two key elements that make it possible to deliver a new residential development today and claim alignment with it. The first is the knowledge of the intended carbon reductions. A development can be assessed against the version of SAP used by the current Part L 2021, with improvements made to the specification such even greater carbon reductions are demonstrated. The second element is Part L 2021 itself. The government introduced Part L 2021 with the specific intention of it acting as a stepping stone to the Future Homes Standard. It lays the groundwork for increased uptake of heat pumps, and the general electrification of properties, alongside much better building fabric standards. These will all be features of the eventual Future Homes Standard. Specifying rooflights to deliver Future Homes Standard levels of performance  Despite uncertainty around broader government policy, businesses are pushing ahead with their plans to help deliver net zero. The construction industry is no different, and projects are routinely looking to achieve performance above and beyond Part L 2021. Part of that is sensible risk management. Planning a project now to current regulations, only to find it then needs to meet the Future Homes Standard in just a few years, would require substantial changes to the specification. Much better to work to that specification now and create healthier, more comfortable and more energy efficient homes. High standards of building fabric performance, supported by low U-values and correct installation, are essential for meeting current or future regulations. Rooflights are part of the building fabric, so choosing high-quality components from a reliable manufacturer is essential. Rooflights have always played an important role in homes. They can provide a level and quality of daylight that facade glazing alone struggles to replicate. Better levels of natural light are good for the health and wellbeing of building occupants and can reduce reliance on artificial lighting – thereby saving energy. With greater focus on sustainability, installed rooflights must deliver the right thermal transmittance (U-value), solar transmittance (g-value) and, where required, ventilation to support the overall energy efficiency and comfort goals. That is true whether choosing from an existing range or having bespoke items created to fit a particular architectural vision Rooflights also must be capable of being installed within the surrounding roof, without creating a break in the thermal envelope and causing significant thermal bridging. Thermal bridging heat losses risk undoing all the intended performance goals for the dwelling as a whole. With overheating now a part of the Building Regulations, rooflights can play an even greater role. Alongside delivering daylit interiors, impressive views and desirable architecture, they can be positioned to limit solar gains. Their potential to provide natural ventilation might also be an answer in situations – such as high-rise developments – where achieving cross-ventilation through a property is not possible.