What is the effect of a lightning strike on a metal roof laid on top of a slate roof?

Whether a lightning protection installation is needed and what level (protection) it should be, depend on many factors. It depends, for example, on the size of your roof, at what height it is, the ceramic activity of the area where you live, whether there are tall trees in the area … In principle, a risk analysis should be carried out according to the IEC 62305 standard.

That risk analysis is not necessarily going to say that you have an unacceptable risk because you have a metal roof (without lightning protection installation). It could therefore be quite ok without a lightning protection installation. It is true that installing a lightning protection installation will in any case be better than no installation.

With a metal roof, you can use the roof itself as a lightning conductor. For this, the metal must be sufficiently thick (sufficiently large section to conduct the lightning current) and continuity must be guaranteed (all the metal parts must be mechanically and electrically connected to each other). However, outgoing conductors are still needed, and every outgoing conductor needs a ground. In principle, all metal parts that are located within one meter of the lightning protection installation must also be attached to the installation (this is admittedly very crude). It is common practice to connect the grounding of the lightning protection installation to the grounding of the low voltage installation. Usually this happens on the main ground terminal, giving rise to a star structure (with the main ground terminal as the star point). It is even better to have a common and highly meshed ground all over the building, but that is often considered too much trouble.

Lightning diversion systems are a highly specialized matter. Everything you want to know can in principle be found in the standard IEC 62305. All the paper together is probably almost 10 cm high, chances are you might be a little discouraged as a non-professional. There is also software available from several manufacturers. A department at the University of Mons has also developed software, if I’m not mistaken.

A new edition of the standard will also be released within a few months.