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When it comes to the design and performance of glass rooflights, it’s usually aesthetics and thermal performance that are top of the list of specification considerations. However, the performance of the rooflight for safety, as an overall part of the roof, is another important factor.
It is important to note that there is no legal requirement for rooflights to be non-fragile; it is only relevant if the customer has specified that requirement. However, it would generally be considered good practice to make roofs non-fragile where possible.
A risk assessment should be undertaken to control the risks around any roof work, both during construction and for subsequent maintenance activities, with relevant measures being put in place dependent on the roof type. All overhead glass does have to be a safety glass, either toughened, laminated or wired glass. Heat strengthened glass is not a safety glass. This will be discussed later, in relation to which glass type is most appropriate for various locations.
Glazed roofs can be classified in one of the following four types:
Once the roof class has been defined the correct specification of rooflight for a particular project can be identified.
The main standard that applies to rooflight specification is BS5516-2. This gives recommendations for design, properties and maintenance of sloping glass and plastics glazing sheet materials in overhead situations in the envelope and interiors of buildings. It includes those situations where the sloping glazing extends down to floor level.
The Advisory Committee for Roof Safety red book, ACR[M]001:2014 defines the tests for non-fragility.
For glass rooflights the ACR document references CWCT test guidelines, which have been adopted specifically for glass roof lights. It is important to note that not all roof lights have to be CWCT tested.
Within the CWCT guidelines, of particular note are TN66, TN67 and TN92.
TN66 – This Technical Note provides guidance on safety issues relating to access to glass roofs and maintenance of glass roofs which are not accessible by the public but where people carrying out maintenance to the roof, or to other equipment mounted on the roof, may walk, fall or drop objects onto the glazed part of the roof.
TN67 – This Technical Note describes the testing and assessment of glass roofs to establish compliance with the classification system set out in TN66 and described above.
TN92 – This Technical Note defines ‘deemed to satisfy’ criteria and a simplified test method for glass for use in class 2 roofs.
Ultimately, the type of rooflight and its application will define the requirements. Will the rooflight be used for access, ventilation, light or space?
The choice of glass used for the rooflight will depend upon the roof classification required for a particular project (as defined by the roof class, above).
At this stage, the following factors are important to consider:
The intended use of the rooflight is an important aspect to consider. In the case of walk on glass, which is outside the scope of TN66 & 67 and has no single specification to cover all potential uses, the glass must be designed to accommodate floor loads in accordance with EN1991-1-1. The glass make up can differ considerably between standard domestic loads in a private home compared to crowd loads in public buildings such as shops, restaurants and museums. Therefore it is critical that the specifier understands completely the intended use of the glass so that proper precautions can be made when calculating the glazed unit.
The obvious issue with regards to location is access; can people gain access to the glass? Consideration should be given to where the glass is situated where it could be indirectly affected by other activities carried out on the building. If maintenance tasks are going to be carried out above the glazing such as cleaning windows or maintaining guttering or drainage systems, can tools be dropped on to the glass from above? In addition to this, the location of the glass may also determine the size, shape and weight of each glazed section that can feasibly be specified, so that the glass can be manoeuvred into position and installed to acceptable standards.
The obvious risk is injury to anyone that may accidentally fall or stumble onto a rooflight that is fragile due to the drop below the rooflight. If anyone were lying unconscious on a rooflight having fallen from a height onto it, then there is also a potential risk to anyone that needs to go onto the rooflight to assist an injured person. Finally, there is also the risk to building occupants below the rooflight from both objects falling through the glass as well as fragments of glass falling from the rooflight itself. The tests defined in TN67 take all these considerations into account. A class 1 or class 2 roof will meet these requirements.
A further consideration is what is involved in replacing a damaged rooflight. If access and inconvenience are concerns then a class 1 roof may be more appropriate. If the risk of damage is low and replacement would be relatively easy then a class 2 roof may be adequate. Where a class 3, ie fragile roof, has been chosen then it is most important that there should be clear notices and warnings of a fragile roof structure.
Cost is something everyone in the supply chain should think about. This can range from the extra cost when glass specifications have to be increased, to the long term cost benefits that those increased specifications may bring during the life of the building. Thermal performance is a good example of this. Also consider the potential cost of having to replace glass if it does get damaged; larger expanses of glass are often specified for aesthetics, but would splitting the glazed section into smaller parts be more beneficial if one of those parts gets broken?
The CWCT TN67 test reduces the chance of any potential hazards arising from either people or objects impacting with the glass by implementing two types of drop test which ensure the specification of safe, practical roof lights:
– The ‘Hard Body’ test – this involves the impact of a steel ball upon the surface of glass. This is to reflect an object being dropped onto the rooflight or a person stumbling onto the rooflight with carrying a toolbox or other, heavy/hard object.
– The ‘Soft Body’ test – this involves dropping a 45 kilo sandbag onto the surface of the glass, to reflect a person falling on top of the roof light. Any panes of glass that have not broken at this stage must then be broken before two sandbags totalling 180 kilos are placed on the glass to replicate someone who is injured and another who must access the roof light in order to help them. The 180 kilo load has to continue to be supported by the broken glass for up to 30 minutes.
The above procedure is then repeated on two further samples so that a total of three units have been tested to check for consistency of results. In all samples the nothing must pass through the glass and any fragments of glass that fall to the ground below the tested panel will be measured and counted to assess the risk to anyone below from falling glass pieces.
When testing for non-fragility, the sample roof light should be the same size, configuration and support method as it will be on the actual project. If previous test data is available for glazing of the same size or larger panes and as long as the support system is the same then the previous test data can be referred to.
In some instances, using the guidance contained in TN92 and designing within these parameters can remove the requirement for bespoke testing. Despite this, the standard doesn’t cover all eventualities, and so additional testing may still be required.
Testing needs to cover the actual size and precise glass specification to be used on a bespoke rooflight. For these exacting requirements three samples must be supplied for testing. This can result in an inflated project cost as the manufacture of the 3 test units would have to be included.
In summary, a correctly designed rooflight should comply with all relevant standards appropriate to the end use and location and follow the appropriate testing procedures. A thorough specification will provide the end user with a product that enables the use of space and light, with the added peace of mind that the quality and safety of their chosen rooflight is as good as it looks.
For more information on correct rooflight specification contact the technical team or book a CPD.
Why do need this? - Additional Information
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