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Snow loading – structural implications for cladding systems

1 February 2011 No Comment

We have seen a noticeable increase in the pattern of snow falling across the UK during the last two winters with snowfalls typical of those across northern Europe being experienced in the UK.

Deep snow, accumulating on the roofs of buildings during these periods, caused many buildings to collapse because the resulting imposed load on the cladding, sub structure and main structure had not been adequately allowed for during the design phase.

Should designers and specifiers make additional allowances for imposed snow loads?

Recent events suggest that allowances should be made for additional loads over and above those shown in the standards. Snow loading on structures, up to the end of March 2010, was dealt with in British Standard BS 6399, Part 3, since that date European Standard EN 1991-1-3 has applied.

The standard and the National Annex associated with BS EN 1991-1-3 allows for a uniformly imposed snow load on buildings, depending upon geographical location, of between 0.6 – 0.8kN/m2 as a minimum. In addition the standards call for local design improvements at valleys, parapets and eaves.

In some exposed locations a base case imposed load of greater than 1.5kN/m2 should be accounted for to meet our changing environment and the additional snow loading. These increased snow loads will either require the specification of stronger structural elements or the adjustment of spanning capability of the structural components.

How can I assess the imposed load on a roof that is covered in snow?

The density of water is 1000kg/m3 and the density of snow is measured as a percentage of this figure. Typically new snow has a density of between 7% and 12% i.e. 70 – 120 kg/m3. However, wind driven snow can reach a density of between 30–40% i.e. 300 – 400 kg/m3.

Old snow, which has gone through a number of freeze/thaw cycles can reach densities as high as 600kg/m3. So for example 10cm snow depth at 100 kg/m3 will apply an additional load of 10kg/m2 on the roof. Denser snow will increase the imposed load.

How can I ensure a cladding system can accommodate greater imposed snow loads?

Tata Steel supply chain partners all provide specific load – span tables for each of their steel cladding profiles. These tables have been produced by the Steel Construction Institute using the latest European design standards.

In addition each of the profiles from these companies have been awarded SCI Assessed as a verification of their structural performance. This gives a designer confidence in the structural performance of the cladding including performance under snow loading conditions.

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