Overhead glazing - designing out risk
The best way to achieve safe overhead roof glazing
Falling through fragile roofs and rooflights has been identified as one of the main causes of the 79 deaths and thousands of injuries that occurred in the construction industry between April 2001 and March 2002.* These statistics are sufficiently alarming for the Construction Division of the HSE to have singled out fragile roofs for a 2004/5 priority programme. Rooflights, for example, are particularly obvious elements of a roof assembly that should be able to support the instantaneous loads imposed by persons stumbling or falling onto them.
Designers' Responsibilities
The Construction (Design and Management) Regulations (CDM) are now ten years old and a recent HSE "Designer Initiative" found that about a third of the designers questioned
"demonstrated little or no understanding of their responsibilities" and that "designers were often abdicating their responsibilities to reduce risk in relation to work at height by leaving it to the principal contractor" and "contractors were struggling to control risk which could have been eliminated or considerably reduced by good design".
These findings come ahead of proposed changes to CDM regulations that are expected to be launched in October 2006.
The CDM requirement for designers to "eliminate or reduce risk" leaves them with an onerous task, so it is hardly surprising that they look to contactors to contribute with their practical experience of construction methods. What seems to be happening, however, is that contractors are being brought into the design process far too late for their ideas to be incorporated or used effectively.
Another problem for designers to address is the task of researching the market to find those products and systems that will go together to provide the safest solutions and it is clear that manufacturers have a key role to play here, by providing clear, concise and honest information about the performance of their products in terms of safe construction. Designers must be made aware of any minimum-risk options that are available to them if they are to carry out their obligation to consider the health and safety of those who construct, occupy, maintain or use the building during its working life, as well as those who may eventually demolish it!
What type of overhead glazing should be specified?
The roof glazing industry has witnessed several high profile glass failures over the past decade, often as a result of the spontaneous breakage of toughened glass, and sometimes caused by the presence of manufacturing impurities known as nickel sulphide inclusions. When toughened glass breaks, it tends to fall in large clumps of diced glass that, dropping from a height, can cause severe injury. The risks are such that there are serious calls for the UK Building Regulations to be aligned with those of the USA and many European countries, where laminated glass must be specified in overhead situations. The basis for this preference is the belief that, in the event of failure, laminated glass will retain its structural integrity, due to its laminated construction.
This may not, however, be a sufficiently robust argument to justify the use of laminated glass in overhead situations where one of the main UK requirements is for non-fragile products to be specified.
Non-Fragile Roof Assemblies
The only test currently accepted by HSE to classify non-fragile roof assemblies, including roof glazing assemblies, is contained in the Advisory Committee for Roofwork document "Test for Fragility of Roofing Assemblies, (Second Edition) ACR[M]001:2000", also known as the "Red Book". When standard 6.4mm laminated glass was tested recently in accordance with this prescribed test method, it failed spectacularly; offering no resistance whatsoever to the standard 45kg impactor dropping onto the test sample from the regulation height of 1.2m.
Glass is also heavy to lift and handle. Great care is needed to avoid breakage and damage. All this adds to the on-site installation risks - and to the overall costs. There are going to be cases where a designer must have the 'clear sky' effect of glass, but in many situations the only real requirement of roof glazing is to provide natural light. This can be achieved effectively using other, very strong, lightweight materials.
Multiwall polycarbonate glazing
Multiwall polycarbonate, for example, can be factory fabricated into gasket-less aluminium framed panels that are interlocked on site - the Twinfix Multi-Link-Panel. The panels are lightweight, can often be handled easily by two men, are factory manufactured off-site and are very quick to install.
25mm multiwall polycarbonate has a very good life expectancy and is now used extensively on a wide range of project types. Its complicated six-wall structure gives it a Value of 1.5 W/m2K. This is more than good enough for most roof glazing applications, whilst a range of standard finishes allows various shading and light-diffusion effects. Built-in filters ensure that not only the glazing itself, but also the contents of the building, are fully protected from the harmful effects of UV radiation. 25mm multiwall weighs only 3.5 Kg/m2, compared with 6mm glass that weighs 15 Kg/m2. And, the noise sometimes associated with rain falling on thinner polycarbonate glazing sheet is significantly reduced by the use of the 25mm product.
But the most significant feature of 25mm multiwall polycarbonate is that it can be manufactured in a panel system - Multi-Link-Panel NF (Non Fragile) - that complies with the HSE requirements for non-fragile roof assemblies and conclusively answers the question of how to specify non-fragile roof glazing. When faced with the problems of roof glazing, designers, specifiers and contractors will find that multiwall polycarbonate can, in fact, provide most of the answers.
Ian Waddington
Technical Manager
December 2005
