Roof lightning protection mesh: cell size, fixings, roof-mounted equipment

Cell size by protection class

Under DSTU EN 62305-3:2021, cl. 5.2.2, Table 2, the largest mesh cell is: class I — 5×5 m, class II — 10×10 m, class III — 15×15 m, class IV — 20×20 m. These are limits, so cells larger than these are not allowed.

The class of the system comes from a risk assessment (DSTU EN 62305-2). How it relates to the rolling sphere radius and the protection angle is covered in the article on protection zone calculation.

Of the three acceptable placement methods for air terminals (angle, sphere, mesh), the mesh suits flat surfaces best (cl. 5.2.2; Annex E, E.5.2.2). The rolling sphere method is acceptable in all cases.

How the mesh is laid on flat and pitched roofs

Conductors run along the roof edges, along the overhangs and along the ridge if the slope is steeper than 1/10 (Annex E, E.5.2.2). The rest of the area is divided by conductors into cells no larger than the table values. If the roof surface exceeds the cell size of Table 2, extra conductors are added (E.5.2.4.2).

Two more conditions apply. Lightning current must have at least two separate metal paths to earth, and conductors follow the shortest straight routes. The more down-conductors there are, the smaller the separation distance inside the building (E.5.2.2, note to condition c).

On a flat roof, perimeter conductors are placed as close as practical to the outer edges. If water can collect on the roof, air terminals are fixed above the highest possible water level (cl. 5.2.4, note).

On a pitched roof the conductor follows the ridge. On a small building with a ridge that lies entirely within the zone protected by the ridge conductor, at least two down-conductors run along the hip edges at opposite corners (E.5.2.4.2). Mesh layouts on flat and pitched roofs are in Fig. E.21a–b, example dimensions on a tiled pitched roof in Fig. E.22.

If a tiled roof should not be spoiled by outside conductors, a concealed conductor under the tiles can be used on buildings up to 20 m. Short vertical rods (0.3 m, spaced under 10 m) or plates spaced under 5 m are added to it (E.5.2.4.2, Fig. E.23). A conductor on top of the tiles is easier to inspect, and a direct strike is less likely to damage the tiles themselves.

Pipes, ventilation and climate equipment on the roof

A metal roof structure can be left without rods if it stays within all three limits at once: up to 0.3 m above the roof, up to 1.0 m² in total area and up to 2.0 m in length of an element (E.5.2.4.2.4). If even one limit is exceeded, the structure is protected by a rod, and a concealed metal element without a separation distance is bonded to the air termination system with at least one conductor (E.5.2.4.2.4).

A non-conductive structure that does not fall within the protected zone of the rods needs no extra protection if it projects no more than 0.5 m above the surface formed by the air termination system (E.5.2.4.2.4).

Two cases are singled out. If wires or metal pipes run from the structure into the building, they can carry a large part of the current inside, so such structures are protected by air terminals. And structures with electrical or information-processing equipment must lie within the protected zone of an air terminal regardless of size (E.5.2.4.2.5).

A structure is protected by a rod tall enough to keep it entirely inside the protected zone (by the sphere or the angle from Table 2). The separation distance between the rod and the structure must meet cl. 6.3. A rod for a chimney by the angle method is shown in Fig. E.22b.

Fixing conductors: fixing centres

Conductors on roofs and walls are fixed with spacers and clamps, conductive or non-conductive (E.5.2.4.2). The fixings must withstand wind, weather and work on the roof (same clause).

Table E.1 gives the recommended distances between fixing points.

DSTU EN 62305-3:2021, Annex E, Table E.1 (informative).

Combustible roofs: distance to conductors

On a combustible roof, conductors are not laid flush. At least 0.1 m is kept between conductors and flammable material (E.5.1.1; cl. 5.2.4), and at least 0.15 m for reed roofs without steel supporting beams. Other options: a larger conductor cross-section, or a thermal insulating layer between the conductor and the covering (E.5.2.4.1).

Metal roofing: can it be pierced

If the covering must not be pierced, the mesh is lifted above the roof on supports: air terminals are placed so that the sphere of Table 2 touches no part of the roof, including standing seams (E.5.2.4.2, Fig. E.26). Supports may be non-conductive or conductive. With conductive supports the connection to the sheet must withstand a share of the lightning current.

A lightning strike can puncture the waterproofing on metal and other roofs, and water then leaks in far from the strike point. This is one argument for an air termination system (E.5.2.4.2).

Whether the sheet itself can count as an air terminal is decided by cl. 5.2.5: reliable electrical joints between sheets, thickness per Table 3, no insulating covering, and no dangerous heating of the inner surface at the strike point where there is a risk of overheating or ignition. The designer carries out this check.

Mesh or rods: what the standard says

The standard gives no criterion for choosing between rods, wires and mesh: all three are treated as equivalent (E.5.2.1). The conditions of the site decide, and the designer chooses the method (E.5.2.2). Below is what the standard says on this.

• a complex roof shape: the rolling sphere method suits structures of complex shape, while the mesh is especially suited to flat surfaces (cl. 5.2.2; E.5.2.2);
• a combustible roof: an isolated system should be considered if the thermal and explosive effects at the strike point are dangerous to the building or its contents (cl. 5.1.2). Separate masts stand at the separation distance from the building, Fig. E.13;
• roof structures carrying electrical or information-processing equipment must lie within the protected zone of an air terminal (E.5.2.4.2.5).

The POLYGONAL company makes free-standing lightning rods on multi-sided bent poles and masts of 5–40 m. Roof air terminals are made to the customer’s drawings or technical specification. The height and protection radius of a rod can be estimated with the protection zone calculator, and how the whole system is built is described in the article on lightning protection of masts and towers. A steel pole is sometimes used as both air terminal and down-conductor: see the article on the steel mast.

What to send for lightning rod selection

Send the building height, the type of the object and the wind region. Any data you do not have can be left out; we will clarify. This article does not replace the lightning protection design of a building: the class of the system, the mesh layout and the connections are set by the designer and must be carried out by qualified specialists. Selecting a pole and mast for a lightning rod can start from the catalogue of 6–40 m lightning rods.