In this article, Ian Smith, Senior Manager at aluminium façade specialists Wicona, looks at some of the challenges and key considerations for specifying roof glazing.
There have been huge technical advances in roof glazing in recent years, giving specifiers the opportunity to increase natural light whilst still retaining a high performance, weather tight roof. This growth in demand has been partly driven by the move away from fully air conditioned buildings with centralised building management systems, towards naturally ventilated and naturally lit environments that give more control to occupants.
Dramatic features can be created that will enhance the building design and the internal environment, from large unsupported glazed roof spans to complex shapes such as domes, facets, atria, and pyramids.
In addition to the considerations of thermal and solar control that also apply to vertical façades, sloped glazing is overhead, so it has to be safe. And if the glass forms part of a roof area that needs to be accessible, for example for cleaning, it may need to be treated and specified as a glass floor.
The Principles of Sloped Glazing
Sloped glazing applies the same principles of vertical façades to roofs. It uses pressure equalisation and insulated thermal breaks, and the drainage is via a cascade system where each transom and rafter drains to an adjacent member which in turn drains to the outside.
Components are often standardised with the vertical curtain wall so fully integrated glazed structures can be created and different profile options help specifiers meet project-specific requirements for height and spans.
What to be Aware of at the Specification Stage
- Involve the systems specialist at the earliest stage. Façade manufacturers have tremendous expertise which specifiers can draw on to produce the optimum glazing solution, and should be involved at an early stage in the design of the atrium or glazed roof. This can avoid potential problems of budget overruns, overheating in summer, unnecessary heat loss in winter and water tightness failures. Use the systems company’s expertise to specify a building structure that will best accommodate the required loadings and the product’s allowable tolerances.
- A slope of less than 10° is generally not recommended. A sloped roof needs a sufficient incline to ensure effective water run off, so very flat roofs should be avoided. A slope of less than 10° could fall outside design limits.
- Ensure you specify a system that prevents water and dirt collecting on the outside. If the system uses external pressure plates or horizontal caps, water and dirt can become lodged and a dirt line could further affect drainage, as well as be very unsightly. A good roof glazing system will be engineered to drain water from the top side of the transom cap. Longer transoms could also deflect in the middle, causing water collection, so be aware of the implication of the length of transoms on drainage. Alternatively silicone joints for glass-to-glass glazing will ensure that water runs straight off the glass.
- Consider how the roof glazing will be cleaned and maintained. Make provision for access to clean the glass and to meet any health and safety requirements. Will fall arrest safety systems need to be attached for cleaning, for example? Do you need to take account of additional loads if a cradle is to be used for cleaning? The systems supplier should provide a maintenance guide and may recommend a cleaning regime twice a year. If the guidelines are not followed, paint guarantees could be affected.
- Make sure any opening lights or smoke vents are integrated with the glazing system. The vents should also have been independently tested with the glazing system and drain to the outside to prevent water ingress. And ensure there are adequate smoke vents to comply with legislative requirements.
- Profiles should be sized to meet the appropriate loadings. These could include wind, snow, glass and cleaning loads as well as the dead load of the building. Size the rafters (vertical profiles or mullions) and the transoms (horizontal profiles) to take account of the anticipated loads.
- When designing a pyramid structure, divide each side up into an uneven number of glazed panels. This should make construction easier and avoid multiple rafters converging at the apex.
- For overhead glazing, regulations generally require the use of safety glass for the inner pane. The internal pane should then be laminated so in the event of breakage, the glass will not shatter and fall, causing injury. The external pane, particularly if it is solar control glass, should be toughened to prevent cracking as a result of thermal shock. The thickness of the glass specified will also affect the loading the system has to accommodate.
- Do you need to reduce solar gain? Controlling solar gain can have important implications for heating and cooling the building. Brise soleil can be incorporated into the roof glazing system either internally or externally to prevent over heating if the roof incline is south facing. Solar control glass could also be specified and products are available that offer a high level of light transmission as well as reflecting out solar gain.
- How many levels of drainage are required? If the roof has a complex configuration, for example incorporating hips or valleys, several levels of drainage may be required rather than two that would be adequate for a more simple roof design. Does the system you have selected have this option?
- Consider the implications of designing larger spans. A more economical solution could be to use structural steelwork to achieve particularly large spans. The size of the aluminium sections will also depend on the size and weight of the glass panels. Large panes of glass may need to be craned into position, making replacement glazing very costly. A general principle is to use panes that can be man handled without the need for a crane, provided there is easy and safe access to the roof.
- There should be no compromise on the thermal performance of the roof glazing compared to the vertical façade. Early systems did not perform well around the eaves of a building – make sure the system you select is well engineered to effectively integrate the vertical and sloped elements or the connections to the building structure, particularly if the roof is sloped onto different planes.
- Would a dry glazed system be more effective for your project? A dry glazed system such as the Wicona Wictec 50 roof glazing system, does not rely on sealants for water tightness. This means that the installation phase would not be comprised or delayed by poor weather conditions. Applying silicone sealant in wet weather could result in water ingress later on.
- Follow CWCT guidelines for specification. CWCT standards are now the benchmark for glazing design and fabrication. The standards are particularly stringent on allowances for deflections so best practice would ensure compliance with CWCT recommendations.
Many of the key considerations for specifying roof glazing are no different to designing vertical façades. Specifiers need to take account of the importance of complete glass and frame U values, independent verification of thermal performance, air and water tightness and safety to ISO standards, as well as solar control and ventilation for occupier comfort. But there are also special requirements for sloped glazed roofs – loadings, cleaning and maintenance, effective drainage, smoke vents and spans.
Systems manufacturers have a wealth of knowledge and technical expertise in all these areas, and if involved at the early stages, can help realise an ambitious design with the optimum building solution that will delight the occupants and significantly enhance the overall scheme.