Calculating loadings on cladding

The relevant British Standards (BS) give calculation methods for loading and there are now also several software packages available to assist with these. Note that for agricultural buildings, BS 5502-22 should be followed for calculation of loading and design rules. In designing a metal-clad building, the following loads should be considered.

This is caused by the weight of the roof or wall cladding system and acts vertically downwards on the supporting steelwork. In calculating total load for a roof, dead and imposed loads need to be added together.

This refers in most cases to snow loading, being the largest distributed load that is expected on a roof. BS 6399-3 (1988, with amendments from 1997) details the necessary calculations for imposed roof loading. It is important to note that the latest revision of BS 6399-3 does not consider snow loads to be uniform across the entire roof, but considers the effect of drifting snow and the exceptional loads caused by drifts. It is particularly important to consider the effect of drifting at verge walls – the architect or building designer should consider increasing the purlin spacings and frequency of fixings in this area.

The prediction of snow-loads is notoriously difficult because of the lack of observational data. Care should be taken in calculating imposed loads, in particular for buildings at an altitude over 500m.

This refers to the load generated by pedestrian activities on the roof. BS 6399-3 defines a roof with access as being designed for foot traffic in excess of standard cleaning and repair. Conversely, where the only expected foot traffic is due to maintenance, concentrated imposed loads are considered to be 0.9 kN or the effective snow load, whichever is the greater, on the assumption that spreader boards are used for roof access. For roofs where pedestrian access is expected, the allowable concentrated load should be 1.8 kN or the effective snow load, whichever is the greater.

Generally, the most significant effects of the wind are two-fold: the positive (imposed) force on the walls and the wind suction on the roof. Calculation methods for wind loading of roofs and walls are given in BS 6399-2 (1997 with the latest revisions in 2002). The calculation of wind load takes account of geography, topology and building shape and size, together with the presence of up-wind obstructions. Therefore, it is site dependent and difficult to generalise.

Particular attention should be paid to security of all edge details to prevent localised wind damage.