The hidden implications and costs of specifying lighter gauge roof cladding
The gauge of profiled pre-finished steel cladding sheet plays a very significant role in structural performance and load-span capability.
It is often tempting to specify lighter gauge material in order to save money, under the misconception that the ordered gauge is within the tolerance of the nominal gauge used for the load span calculations and will meet the structural requirements.
How much will reducing the gauge of the pre-finished steel affect the structural performance of the roof cladding?
The effect of gauge will vary dependant upon the sheeting profile used and also whether the limiting factor is the deflection of the cladding or localised failure.
Which areas of my roof are most affected by gauge?
There are 3 main considerations regarding higher roof loadings:
- The spanning conditions above and below the in-plane rooflights are often different from the rest of the building and can result in lower load carrying ability.
- Areas of the roof may have access requirements for planned maintenance and inspection, resulting in increased loads from work access. These areas often include the areas above or below the rooflights.
- Snow loading can produce significant additional loads on the building. Lighter gauge roof cladding will be more prone to structural failure.
For a typical 32/1000 roof cladding profile, reducing the pre-finished steel gauge from one that meets the requirements of 0.7 mm in accordance with EN 10143:2006 to 0.64 mm in accordance with EN 10143:2006, will result in a 12% reduction in the load carrying capacity for all span conditions.
This assumes that the failure mode in all cases is deflection limited. It is possible, that by reducing the gauge, the failure mode could change to local failure, which could result in a greater reduction in load carrying capacity.
What are the cost implications of using lighter gauge pre-finished steel?
The cost savings associated with the profiled pre-finished steel external sheet being reduced from one which meets the requirements of 0.7 mm in accordance with EN 10143:2006 to 0.64 mm in accordance with EN 10143:2006 are in the region 5%. For a 2000 m2 building, this equates to Approximately £500.
This has to be balanced by the potential costs associated with sheet damage, which could occur during installation or service life:
- A site inspection is likely to cost the client at least £500 plus expenses.
- If remedial work is identified, additional costs to provide safe access, site labour, and transport of replacement material to site are likely to cost at least £1500 plus replacement materials.
- It should also be noted that even after damaged material has been replaced, the remaining roof cladding will be more prone to further damage.
The normally specified gauges for roof and wall cladding are 0.7 mm and 0.5 mm respectively. These gauges have been derived by structural calculations and a record of in service performance.
Using material that does not meet these requirements may result in small initial savings, but can leave the building owner liable to considerable future costs and an overall poorer performing building.