Mesh vs Conventional Rod Air Termination: How the Choice Gets Made

Ask two people how a building gets its lightning protection and you will often hear the same mental image: a single rod on the roof, catching the strike. It is a tidy picture, and for some structures it is close to right. For most of the facilities we work on, it is not the whole story, and the gap between that image and what the standard actually requires is where protection quietly fails.

Air termination, the part of a lightning protection system that intercepts a strike before it reaches the structure, is rarely a choice between one rod or none. It is a question of which method, or combination of methods, gives complete coverage for a particular building at a particular protection level. Here is how that decision actually gets made.

The three methods the standard recognises

IEC 62305-3 (and AS/NZS 1768:2021, which aligns with it) recognises three air termination methods, and they are designed to be used together rather than as rival options:

• Conventional rods (Franklin rods). Vertical air terminals that protect the volume beneath them. Effective for discrete features such as plant, antennas, and small roof areas.
• Mesh conductors. A network of conductors laid across a roof at a defined spacing, protecting large flat or gently contoured surfaces.
• Catenary (suspended) wires. Horizontal conductors strung above a structure, used where rods or mesh are impractical.

One method that does not appear on that list is the so-called enhanced air terminal. IEC 62305 does not recognise these air terminals, and the underlying physics remains contested in peer review. Our designs use conventional rods, mesh, and catenary systems only. If a competitor is proposing an air terminal that claims an enlarged "attractive radius," that is a flag worth questioning.

The lightning protection level drives everything

Before the method is chosen, the lightning protection level (LPL) has to be set, and that comes from the risk assessment, not from preference. The LPL maps to a class of lightning protection system (I to IV), and that class fixes the geometry every method must satisfy. Higher protection means a denser capture network, a smaller rolling sphere, a tighter mesh, and closer down conductors.

Values per IEC 62305-3. The point of the table is not the individual numbers, it is the relationship: the protection level is the input, and the air termination geometry is the output. You do not pick a mesh size and hope it is adequate. You establish the required class, then design the termination to meet it.

The rolling sphere method: the common test

Whichever method is used, the rolling sphere method is how coverage is verified. Imagine a sphere of the radius set by the protection level rolled across the structure from every direction. Anywhere the sphere touches the building before it touches a protection conductor is an exposed point that needs an air termination. Anywhere the sphere cannot reach is protected.

This is why a single rod is rarely enough. On a large or complex roof, a sphere can settle into the gaps between features and contact the surface in places a lone rod never shields. The rolling sphere makes those gaps visible, and it is the basis of the 3D RSM modelling we use to confirm coverage on geometrically awkward facilities before anything is installed.

So when mesh, and when rods?

The honest answer is that most real facilities use both, and the split follows the building:

Mesh suits large, accessible surfaces

For a broad flat roof, a mesh network at the spacing its class requires (5 m × 5 m at LPL I, widening to 20 m × 20 m at LPL IV) gives even, predictable coverage and distributes the current across many paths, which reduces the thermal and electromagnetic stress on any single conductor. Mesh integrates well with the roof structure and with bonding to structural steel.

Rods suit discrete features and elevated points

Anything that projects above the roof plane (HVAC plant, stacks, antennas, lift overruns) tends to attract the strike and needs its own protection. Conventional rods placed by rolling sphere analysis protect these features directly, often working alongside the mesh that covers the surface around them.

Catenary suits the awkward cases

Where neither rods nor mesh sit comfortably, for example over an open yard or a sensitive structure that cannot carry mounted hardware, catenary wires provide an overhead capture path. They are the least common of the three but solve specific geometry problems.

What this means for your facility

The right air termination design is not a product choice, it is the outcome of a sequence: establish the protection level from a defensible risk assessment, fix the class and its geometry, then select and position the methods that achieve complete rolling sphere coverage for your specific building. A design that starts from "we will put some rods up" rather than from the protection level is working backwards, and it is the kind of design that looks fine on a drawing and leaves gaps in three dimensions.

Our approach is to model coverage before committing to hardware, so the termination design is verified rather than assumed. For the deeper standards detail, Skytree Scientific has published a guide to IEC 62305-3 design, inspection, and maintenance that covers the air termination methods in full.