6.3.3 Air-termination systems for buildings with metal roofs

Modern industrial and functional buildings are often equipped with metal roofs and façades. These usually consist of metal sheets or plates with a thickness between 0.7 mm and 1.2 mm, as shown in Figure 6.3.3.1. A direct lightning strike on such a metal roof can cause melting at particular points and vaporisation of the material, resulting in a hole in the roof. The size of the hole depends largely on the energy of the lightning strike and the material properties of the roof. However, the actual damage often lies in the consequential damage. Water can penetrate through the resulting hole, leading to moisture damage to the roof insulation and the ceiling below. This means that the building is no longer guaranteed to be rainproof, and the damage often remains undetected for days or weeks.
A specific example of damage, which was analysed by Siemens' Lightning Information Service (BLIDS) at the time of the incident, illustrates this problem (Figure 6.3.3.2). A lightning current of approximately 20 kA struck an unearthed sheet metal cover and caused a hole (detail A). In addition, there were flashovers in the cornice area to natural metal components in the wall, which were also damaged (detail B).