With recent government policy shifts towards putting PV on large roofs, it is vital that installers are familiar with the mechanical issues that can arise on typical large commercial roofs, explains Steve Pester, BRE
The design loads of ‘tin shed’ buildings (e.g. steel-clad warehouses) often leave very little margin for extra roof loads to be imposed. The BRE National Solar Centre has seen a number of situations recently in which not all of the relevant factors had been taken into account, leaving roofs vulnerable to damage, or failure.
In general, the services of a structural engineer will be needed to assess the strength of non-domestic roofs, but not all structural engineers are comfortable with calculating anything more than the static loads imposed by solar panels, so it is extremely important that installers understand which other factors must be taken into account.
By way of reminder, there are both static loads (PV system weight and snow loads) and dynamic loads (wind) that must be accounted for.
Most structural engineers will be able to calculate the static loads, but care should be taken to make sure they have included components such as cabling, as well as panels and mountings.
The wind loads are a little more involved – the BRE document DG489 gives some good guidance. However, the installer should make sure that both uplift and downward wind loads have been considered because the combination of the static and wind loads can sometimes exceed the design load of the roof.
After all loads have been taken into account, a safety factor of 1.35 is usually applied to allow for uncertainties and exceptional events, such as storms and maintenance operations.
Once the total loads are known, the number and distribution of array fixings can be calculated from the published strength of each fixing and structural engineer’s advice on the strongest fixing points.