From the series of articles "Complete Information on the Protection Zones of the Lightning Arrester".
Read Part 5 "Why Is a Single Lightning Rod Unreliable?
Almost any designer can say that only a lightning grid is more convenient than a single lightning rod. We do not need any calculations for it: 6 x 6 m cells for the second category protection and 12 x 12 m for the third category are used. And you have no problems at all.
Actually, we have one problem, i.e. to read Instructions RD 34.21.122-87 carefully. Its Section 2.11 says: "The protection of buildings and structures of Category II with a non-metal roof against the direct lightning strikes should be made using rod or wire lightning arresters being standalone or installed on the protected object..., with the roof angle of not more than 1 : 8; a lightning protection grid may also be used...". It is clear that the regulatory document provides for the use of the grid for non-metal roofs only, whereas a reinforced concrete plate is not non-metal. It occurs that not only designers but also the Gostekhnadzor's specialists are not always attentive. We have to consider the true grid purpose more carefully.
Read further "7. How to Deal With the Lightning Protection Grid?"