From the series of articles "Lightning protection of residential and public buildings - answers to frequently asked questions in the design".
The answer to this question is simple. In modern residential and office buildings, the first things to suffer from lightning are the roof and equipment that is installed on it. The designed lightning rods are provided to protect them. Are lightning-proof roofs possible? They certainly are. Lightning cannot do anything with the steel sheet 4 mm thick. For copper this value should be increased to 5 mm, for aluminum - 7 mm. Such ponderous roof can be imagined somewhere on the royal palace or mansion of a pop star. Typical houses get along with metal tile or metal sheets with metal thickness of less than 1 mm. According to the requirements of the standard IS-153-34.21.122-2003 such roof can be left unprotected, provided that it is grounded and there is no combustible structural elements such as trusses and sheathing underneath it. For Russia, this requirement is practically not feasible, because the domestic builders mainly use wood for roofs. In such circumstances, the roof has to be protected.
And what to do with dielectric roof? Today, for environmental reasons we have stopped using slate, so we can only talk about modern synthetic roofing materials. They are not an obstacle for lightning. The potential of the channel head, even at the most moderate strength of lightning certainly exceeds several million volts. This is sufficient for an electrical breakdown of almost any roof. Lightning channel will penetrate through it and complete its mission on some grounded structural element, for example, on a metal farm, pipe or electrical network wire (for lightning, the capacity of 380 or 220 V is not distinguishable from zero!). The consequences of such events may be severe enough.
Lightning won't destroy roof of reinforced concrete slabs. At the point of contact with the concrete, the electrical breakdown will occur and the channel will penetrate to the metal fitting of the plate, over which the lightning current wil be spreaded, heading into the ground. I believe that there will appear a shell on the concrete , which doesn't pose danger to the mechanical strength. On the roof of reinforced concrete slabs it is necessary to provide hydro and thermal insulation coating. It may be combustible. This coating seems to be in need of protection from contact with the lightning channel. In any case, according to the requirements of the regulation AD 34.21.122-87 a lightning-protection mesh is put on the covering. Probably, any lightning protection specialist will find it difficult to explain how this mesh with the cell size of 12 x 12 meters, elevating over the concrete slab to a few millimeters, will intercept the lightning discharge. Many experiments have shown that for an effective operation of a lightning rod, it should rise above the protected object to a definite and quite noticeable distance. For example, according to the requirements of the standard IS-153-34.21.122-2003 it should be not less than 15% of the building height, even at relatively low reliability of protection 0.9. We are therefore talking about excesses in meters, but not in millimeters. However, the mesh is still used on the concrete slabs, and ignition of bituminous coatings, the most common in our country, is quite a rare thing. I cannot give a sensible explanation. Maybe that a coating layer of a few millimeters on a concrete substrate cannot be ignited by lightning due to the intense abstract of heat? It is most probably so. An experimental verification in the laboratory is required. So far, nobody held it.
E. M. Bazelyan, DEA, professor
Energy Institute named after G.M. Krzyzanowski, Moscow
- Series of articles about lightning protection for beginners
- Series of webinars about grounding and lightning protection with Professor E. M. Bazelyan
- Elements of external lightning protection
- Consultations on the selection, design and installation of grounding and lightning protection systems