From the series of articles "Complete Information on the Lightning Arrester Protection Zones".
Read Part 6 "What Can the Lightning Protection Grid Be Used For?"
It is clear that the lightning protection grid does not provide the protection for the earthed metal elements of the roof. There is no special evidence for this. To provide the protection reliability more than 0.5, the lightning arrester must be higher than the protected object. And it must be much higher. It cannot be provided by waterproof materials on the roof or a thin concrete layer over steel bars in the reinforced concrete plate. If the roof really needs protection against the direct lightning strikes, we must use rod or wire lightning arresters, preferably multiple. They will provide a reliable protection with relatively low heights above the roof.