SkyHatch Maintenance Access Rooflight

Part F of the Building Regulations in England sets out requirements for ventilation. The current version is Part F 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 ventilation requirements, albeit the overall aims across the different countries are similar. Appropriate levels of ventilation protect the health of building occupants by avoiding mould and poor indoor air quality. Achieving the right rate of ventilation is essential for meeting current or future regulations in all parts of the UK. Rooflights have a role to play in ventilation control for dwellings, particularly in terms of providing the option of purge ventilation. What are the requirements of Part F 2021? As detailed in Approved Document F volume 1, Requirement F1 is broken down into two separate aspects. Requirement F1(1) is that there shall be adequate means of ventilation provided for people in the building. Requirement F1(2) is that fixed systems for mechanical ventilation and any associated controls must be commissioned by testing and adjusted as necessary. Ventilation strategies can be delivered by mechanical means, natural means or both. Part F 2021 supports each of those options. Requirement F1(1) is met if the ventilation strategy for a dwelling: Extracts water vapour and indoor air pollutants; Supplies a minimum level of outdoor air; Can provide purge ventilation to rapidly dilute pollutants and disperse vapour; Minimises the entry of external pollutants; and Keeps noise to a minimum, can be maintained and provides protection from cold draughts. How can rooflights help to achieve Part F compliance? High levels of ventilation are good for occupant health, but excessive ventilation increases energy use. When heated warm air is lost – in either a controlled or uncontrolled manner, depending on the quality of the construction – the cold air that replaces it must be heated. It therefore makes sense to avoid extra load on the heating system by making ventilation as efficient and effective as possible. Mechanical ventilation systems providing a predictable, consistent and controlled supply of fresh air are increasingly seen as the preferred solution. Simple extract fans are mechanical, but more sophisticated, whole-dwelling solutions can include heat recovery. That means the warm air extracted from the building is used to heat the incoming cold air, lessening the reliance on the building’s heating system. Natural ventilation is mostly driven by external air pressure and air movement, which can fluctuate and might not be entirely dependable on days when it is most needed. Arguably, natural ventilation should be viewed as a supplement to a mechanical system, to take advantage of the days when it is most effective. Paragraph 1.9 of Approved Document F volume 1 says that whatever solution is adopted for a residential dwelling, the overall strategy should be capable of delivering the following. Extract ventilation in rooms such as kitchens and bathrooms. Whole-dwelling ventilation to provide fresh air and remove pollutants not dealt with by extract ventilation. Purge ventilation to remove occasional high concentrations of pollutants. Extract ventilation is only possible via mechanical means, but rooflights can play a meaningful role in the other two areas of ventilation strategy. Where they really excel is in providing additional options for ventilation control, either in individual rooms or the dwelling as a whole, that may not be possible with façade windows alone. Rooflights are an excellent way of helping to achieve cross ventilation through a property where openings on opposite sides of a building are not available or don’t readily align. Using rooflights to go beyond the minimum standard of Part F Like all areas of building regulations, Part F only sets out a minimum standard. The option to go beyond the requirements is always available and is particularly worth considering when it comes to ventilation. A frequent criticism of regulations in recent years is that requirements in one area don’t complement the requirements in another area. The most common example of this issue is how ventilation rates have failed to keep pace with increased levels of insulation and airtightness aimed at improving energy performance. Without a corresponding improvement in ventilation, indoor air quality has worsened, and overheating risk has increased. Changes to the regulations are only just beginning to overcome this disconnect, with Parts F and O now aligning better with Part L. However, designers and specifiers of prestigious homes and high-end apartments can think holistically and deliver a joined-up approach – possibly by adopting voluntary standards as well – 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. Project teams now routinely look to the promised Future Homes Standard, and factor in predicted future climate conditions, to benchmark performance now. Rooflights have always played an important role in homes, providing a level and quality of daylight that vertical windows alone struggle 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.

When selecting the perfect rooflight, energy efficiency is a critical factor to consider. Rooflight U-values measure thermal performance, indicating how well the product retains heat and helps reduce energy consumption. Rooflight U-values – What you need to know The thermal performance of rooflights, including the U-value they achieve, is an essential part of building fabric specification. Balancing outgoing heat loss with incoming daylight and solar energy all contributes to the wider sustainability goals of a project – including the overall thermal efficiency of the building, and the thermal comfort of its occupants. Designers and specifiers therefore need to understand what quoted U-values mean, and the relevance of other performance measures such as Ug-values. This post concentrates on how rooflights allow solar energy in and limit heat transfer out. However, for projects where daylighting is just as important a consideration, factors such as light transmittance and reflectivity must also be taken into account. Why can rooflights have two different U-values? Anybody involved with building design and specification should be aware of U-values as a measure of thermal transmittance (the movement of heat energy) through building fabric from warm to cold. The composition of roof glazing units – including overall size, relative areas of glazing and frame, and the thermal performance of the materials used – varies, so U-values quoted in a specification should be based on an actual product or a detailed calculation model. However, performance specifications may not be clear about whether a whole-unit or centre pane value is required. Manufacturers themselves may not be clear about the type of U-value they are quoting, creating the potential for confusion. In most circumstances, U-values should be for the whole unit, including glazing and frame. Whole-unit U-values can be improved through the use of a warm edge spacer. Traditionally, spacer bars are aluminium but, like any metal, aluminium has a high thermal conductivity. It acts as a thermal bridge, conducting heat from inside the building at the edge of the glass and bypassing features otherwise designed to improve the efficiency of the glazing. ‘Warm edge’ spacer bars use materials with a lower thermal conductivity to slow the rate of heat loss and create a more even surface temperature across the whole glass pane. Centre pane U-values address the thermal performance of the glass only. They appear lower than whole-unit values because the cold bridging effect of the spacer and edge seal are not accounted for. Unfortunately, this means some manufacturers rely on quoting centre pane U-values when they should offer whole-unit U-values. Specifiers can find themselves misled if a centre pane value is made to appear as though it is a better performing product. Centre pane values do have an application, though. They are useful for comparing one glass against another when being used in the same frame, as well as in conservation projects where traditional frame designs offer no meaningful thermal performance. Why should you consider rooflight g-values alongside U-values? Compared to the total surface area of the building fabric, the U-value heat loss through relatively small areas of roof glazing is more than offset by the contribution of solar gains and the reduced use of artificial lighting. The measure of infrared radiation (solar heat) allowed into a building is the g-value. A g-value can be anything from 0 to 1, where 0 represents no solar heat gain and 1 is the maximum possible solar heat gain. It is calculated by dividing the total solar heat gain by the incident solar radiation (the amount of solar radiation received on the surface during a given time). The lower the g-value, the lower the percentage of solar radiation allowed through the glass. Like U-values, performance figures can be quoted for the glass alone or for a complete glazed unit. Better, lower g-values also result in lower light transmission. How do different gases change the thermal performance of rooflights? Most people understand the benefits of having a glazing unit that is more than just single glazed. Double glazing, triple glazing and even quadruple glazing improve the thermal (and acoustic) performance by introducing sealed layers of gas between the panes. A well-known feature of products is to fill the sealed space between panes with an inert gas like argon, whose thermal conductivity is some 34% lower than still air. Some manufacturers use krypton and xenon, both of which offer further improvements in thermal efficiency, but which are more expensive. Do low emissivity coatings impact on rooflight U-values? The function of low emissivity (low-e) coatings is to impact on the loss of solar radiation back out of the building. A material’s emissivity determines the amount of thermal radiation emitted from its surface. Low-e surfaces emit less thermal radiation, and glazing units benefit from this through the application of a microscopic coating of tin, silver or zinc to certain faces of the glass panes in the unit. In contrast to the short-wave radiation from the sun that heats the building interior, the heat energy transferring back through the building fabric, from warm inside to cold outside, is long wave radiation. Glass with the low-e coating reflects long wave radiation, effectively keeping more heat energy in the building. There are two types of coating: hard and soft. Hard coat is applied while the glass is still molten, whereas soft coat is applied later in the process. Hard coat is more durable, as its name suggests. Soft coat remains delicate, is only applied to the sides of panes facing into a sealed airspace, and has a lower emissivity than hard coat. The difference in emissivity between the two means argon-filled glazing with a hard coat treatment will typically offer a centre pane U-value of 1.4 W/m2K, while a soft coat treatment will see that improved to 1.1 W/m2K. It’s a meaningful distinction, yet some manufacturers will simply claim their glazing to be “low-e”. Making a hard coat treatment sound like a similar benefit to soft coat is another reason for specifiers to be clear about the features of the products they’re selecting.

Rooflights and roof windows are both popular choices for bringing natural daylight into a property, and rooflights can also provide a variety of other benefits, such as natural ventilation and access to roof terraces. When selecting a roof window or rooflight for pitched roofs, there are several specification considerations to think about – and one factor that could play an important part in your decision is the pitch of the roof. Roof pitch affects drainage, determines how the units should be installed and even what type of products you can use. Below, we explain the different requirements for rooflights and roof windows; this should help you to decide which product is most appropriate for your project. Installing a rooflight on a flat roof When installing a rooflight on a flat roof you should ensure that the product is not fitted completely flat itself. Glass inherently has a degree of flex when installed flat (as opposed to vertically as you would find in traditional windows and doors). This is known as ‘deflection’ and means that without pitching the rooflight up slightly at one end, rainwater will accumulate and begin to ‘pond’ on the glass. When this evaporates it can leave unsightly marks and stains behind. Manufacturers therefore recommend a ‘minimum pitch’ to install their product so that rainwater and debris runs off the glass more effectively, keeping the rooflight cleaner for longer. The recommended pitch for rooflight installation on a flat roof For many rooflight applications on flat roofs, there should be a minimum pitch or fall of three degrees. This is enough to ensure that water will drain off the surface of the glass and avoid ponding. The higher the pitch, the more effective it is at draining off the water, so although three degrees is often the recommended minimum pitch, installing the unit slightly higher – for example at five degrees will prove more efficient. To achieve the required pitch, an upstand or kerb is usually constructed around the aperture in the roof, which will accommodate the slope into it for the rooflight to be structurally fixed to. If you are unsure, the rooflight manufacturer should provide drawings to indicate minimum kerb heights, which will allow your builder to calculate how high the top of the slope needs to be to achieve the desired pitch. Steeper pitches for rooflight installation on flat roof Rooflights can be successfully installed at steeper pitches, but for flat roof applications the height at the top end of the upstand will begin to be so great it will compromise aesthetics and potentially contravene planning permissions, which in some cases will limit the height of any structure that can be built above roof level. Depending on the rooflight design, installing on more harshly pitched upstands may result in fouling the framework, so you should always check with manufacturer drawings and recommendations prior to commencing works on site. Other considerations for roof pitch and specification Here are a few more things to think about in relation to roof pitch and specifying rooflights for pitched roofs. ● Deflection – There is a level of flex that will occur in all glazing – and it can be particularly apparent in overhead installations. Wind loads, snow loads and the weight of the glass itself can contribute to the amount of deflection, and this can hinder drainage. ● Orientation – Pay attention to the orientation of the rooflight so that you can maximise. For example, if a unit has a 5m width and a 1m span, it’s better to build the pitch into the span dimension so the water has less distance to travel. ● Capping – Because rooflights have minimal pitch, it’s a good idea to check products closely to ensure there is no external capping around the edges. This is because capping can trap moisture and dirt, which can result in unsightly messes and damage to the unit. The correct pitch for installing a roof window on a flat roof Unlike rooflights mounted onto an upstand, roof windows are installed in-plane, meaning they follow the pitch of the roof and are mounted flush with the surface. Answering the question: “What’s the minimum roof pitch for a roof window?” is also much easier to answer. Because the units follow the existing roof pitch they do not require any additional height at one end to allow for drainage. According to EN 14351-1:2006, roof windows should be installed on roofs with a pitch of at least 15 degrees. Roof windows should be CE marked against this standard. Rooflights, however, cannot be CE-marked because they are usually installed ‘out of plane’ on an upstand and are not considered roof windows. Please note that specific requirements may vary depending on the manufacturer. Always check that the specification meets your roof design.

Glass can become slippery when wet and common sense should be applied when specifying this material for walk-on applications such as walk-on rooflights. This is of particular importance when the glass is being installed where the public can access it. In a private dwelling it is less likely that the glass will be used if it is raining, but the same cannot be said for commercial and public applications. Applying an anti-slip glass surface finish to glass that is designed for walk-on applications should always be considered; the same finish can also provide some obscurity to the glass if required. A screen printed frit that includes particles within the ink to create a rough texture can be applied to the glass in a variety of patterns, which will significantly increase the slip resistance of the glass. Alternatively the surface of the glass can be sandblasted which will result in more diffused light and improved obscurity. Slip resistance is measured using mean Pendulum Test Values (PTV); the higher the figure the better the slip resistance. A PTV of 0-24 has a high slip risk, 25-35 has a moderate slip risk and 36+ has a low slip risk. The test is carried out in wet and dry conditions and the lowest figure is obtained when wet. Generally sandblasted glass achieves a PTV of 50 and fritted glass achieves a PTV of 60, providing better slip resistance than the sandblasted. However both are well above the threshold of 36 to be categorised as having a low slip potential.

We all have a predilection to well lit, inviting spaces, so increasing the use of glazing products in the built environment is almost always an efficient way of improving an internal space. But what is the difference between these glazing products: is a skylight window different to a rooflight, or a rooflight different to a roof window? There are plenty of benefits to bringing natural light into a building – from boosting occupants’ wellbeing, through to saving energy. Roof windows, rooflights and skylights are all popular options for letting the sunshine in and improving the look and feel of an internal area, but what’s the difference between these products – and how do you know which one is the right choice for your project? Is it A Roof Window, Rooflight or Skylight? It’s fair to say that the terminology used to describe glazed units in roofs can be a bit tricky to understand – so don’t worry if you’re not sure what the difference is between roof windows, rooflights and skylights. It’s also fairly common to see terminology being used inconsistently, which increases confusion. Let’s take a look at each product type to help clear things up. What are roof windows? These are probably the easiest to define because they are covered under BS EN 14351-1:2010. The standard stipulates that roof windows must be installed in the same orientation and ‘in plane’ with the surrounding roof, typically at a minimum 15° pitch. Once installed, they should be weatherproofed using a skirt or flashing. Roof windows must be UKCA marked before they can be sold and manufacturers are expected to provide a declaration of performance (DOP) to advise specifiers how each unit performs under test conditions. Typically, this will cover things such as tests to simulate prolonged and heavy rainfall, how the roof window withstands increased air pressure, glass deflection and monitoring any air leakage to ensure the product does not create drafts or allow in damp. Roof windows are usually only available in standardised shapes and sizes and are typically smaller than rooflights. However, new products have recently become available using improved glass specifications that allow much larger sizes to be manufactured. Rooflights.com stocks the sleek Glazing Vision Pitchglaze Roof Window which is a fixed roof window that can be installed in roof pitches between 15° and 60°, it offers completely frameless internal views. What Are Rooflights? ‘Rooflight’ is a generic term that can sometimes mean different things. Typically, the term refers to a glazed unit installed on a flat roof, or where installed on a pitched roof it is likely to be fitted ‘out of plane’ with the level of the tiling. Rooflights are commonly installed using an upstand or kerb system to support the actual product and act as a surface for any weathering to be fixed to; on flat roof systems the upstand will provide enough height to ensure that the rooflight remains watertight. The rooflight quite often has to be installed at a certain height to maintain any guarantees supplied by suppliers of waterproofing systems. Rooflights come in a huge range of designs and styles, and as well as providing natural light, they’re also often used for ventilation or access. Options include frameless fixed rooflights, which offer a minimal appearance and sky-only views, such as the Glazing Vision Fixed Flushglaze Rooflight. Other rooflight options include hinged rooflights, sliding rooflights, fire rated and AOV rooflights, and box rooflights, which are often used to provide access to roof terraces. What Are Skylights? Skylight is another generic term, and it’s often used by manufacturers for a variety of different products, ranging from small-scale domestic units installed on traditional pitched roofs to larger bespoke glazed units designed to be installed on flat roofs or terraces. Skylight is a generic term that can sometimes confuse consumers, since the products it refers to may differ quite drastically in terms of size, scale, function and application. In some cases, the use of the term skylight is actually completely inaccurate as far as the Building Regulations are concerned, where more appropriate terms should be used that are recognised in British Standards. Specification Considerations: Advantages and Disadvantages In most cases, choosing between a roof window or rooflight (or skylight) is easy: if it’s going on a pitched roof, you’ll want a roof window; and it will be rooflights for flat roofs. However, there are other issues to think about. Off-the-shelf design vs customised Roof windows are very popular and widely used having been adopted by many major national house builders. They are robust, reliable and offer value for money. The advantage of standardised sizes and specifications means that they are usually available to buy ‘off the shelf’. The disadvantage of this is their flexibility – particularly when it comes to scale. If you’re looking for larger sections of glazing on your roof then roof windows presently only offer a limited amount of scope. Rooflights (or skylights) tend to be offered in a much wider range of shapes, sizes, specification and function. However, they tend to be more expensive than roof windows due to their bespoke nature, and since they are usually built to order, there will be a lead time of several weeks. External appearance Rooflights and roof windows are both available in frameless designs, which means you only see glass when looking up at the unit from indoors. However, these products look quite different from the outside. Since roof windows are designed to sit flush with the plane of the roof, they offer aesthetically pleasing clean lines, which rooflights sometimes struggle to match. That said, rooflight manufacturers are beginning to respond to client demands for the flexibility and scale of a rooflight, but with a low external profile that can be installed flush with roof tiling lines in the same way that a roof window can.

Glass, when installed overhead in rooflights and skylights must be ‘safety glass’, which is often referred to as Toughened or Tempered Glass. Toughened glass is 4-5 times stronger than standard annealed glass of the same thickness; it’s a key safety feature in the specification of domestic rooflights and skylights as it is designed to crumble into granular type fragments when broken, rather than shatter into jagged shards like annealed glass. This significantly helps to reduce the risk of injury. How is Toughened Glass Made? Toughened glass is produced by passing annealed glass through a furnace, heating it to above 600°C before being rapidly cooled. A balance of high compressive stresses at the surface and tensile stresses in the centre of the glass increase its strength. When the glass breaks it is the release of these stresses that causes it to break into small pieces – usually accompanied by a large bang as the energy is released. Toughened (tempered) glass is a preferred option for rooflights but there can be disadvantages. Firstly, glass needs to be cut to the required size and shape before it’s treated. Secondly, toughened glass has been known to spontaneously fracture for no apparent reason with the most likely culprit being Nickel Sulphide (NiS) inclusions within the glass. NiS is a chemical contaminant that can manifest itself during the toughening process; as it gradually changes state over time it can cause the glass to fracture. This can happen at any time, from a few weeks to many years or not at all. One of the most effective ways of combating this is to subject the glass to 'Heat Soak Testing'. What is Heat Soak Testing? Heat soak testing is carried out during the manufacture of the glass and will filter out around 95% of problem units. This involves heating tempered glass up to 290 degrees Celsius and holding it at that temperature over a controlled period of time. This process accelerates any NiS inclusions reverting back to their Beta state, which could result in glass failure. Whilst more expensive, this method of testing identifies any issues with toughened glass before it’s used in manufacture. This is of particular importance when specifying larger structural rooflights or those used for walk on applications, where heat soak testing is a requirement of Building Regulation. What is Laminated Glass? Annealed laminated glass can also be specified for some rooflight applications, to conform to BS5516 Part 2 – pane sizes above 3m2 and between 5mts and 13mts from floor level should be laminated. Laminated glass is produced by combining two or more sheets of float glass with one or more interlayers. Glass integrity is maintained from a ‘laminated’ interlayer, commonly PolyVinylButyl (PVB), which is processed with heat and pressure under factory conditions. Should any damage occur, the interlayer holds any fragments together preventing them from falling, reducing injury risk even further. Combinations Toughened Laminated or HST Toughened Laminated are also common glass specifications, particularly when the glass has to perform a structural function, as in a glass floor or structural glass fins.