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Hero

Lightning protection
design

Drawings and specification to BS EN 62305.

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How we design

Design starts with the risk assessment, not with the roof. BS EN 62305-2 tells us the protection class the building needs, and the class then sets the mesh size, the spacing of the down conductors and the rolling sphere radius the air termination has to satisfy. Working the other way round produces systems that either cost too much or do not comply.

From there it is a geometry problem. We take the roof plan and elevations, apply the protection method appropriate to the shape of the building, and place conductors so the whole structure sits inside the protected volume. On a simple rectangular roof that is quick. On a spire, a mansard, or a building with plant and masts on the roof, it is not.

We use current design software to model this, and we have an accredited BS EN 62305 assessor in house, so the assessment and the design are done by the same people.

Design outputs

  • Protection class, taken from the BS EN 62305-2 assessment.
  • Air termination layout, using mesh, rods or protection angle as the roof geometry requires.
  • Down conductor positions and spacing for the class.
  • Earth termination design sized against measured soil resistivity.
  • Separation distances to metallic services and internal systems.
  • Surge protection scope under BS EN 62305-4.
  • Materials, sizes and fixing details.

Using the frame

On new buildings the structure itself is usually the best down conductor available. We fit connections to the pile caps and establish continuity within the frame using continuous strip or starter bars, so the current path is built into the building rather than bolted onto it afterwards.

This has to be designed and installed in step with the groundworks and the frame. It is the single biggest reason to bring a lightning protection designer in at concept rather than at fit out.

Talk to us at concept stage

Working with designers

Most of our design work arrives through an architect or a building services engineer rather than the building owner. We are used to that. We will attend design team meetings, mark up your drawings rather than issuing a separate set that has to be reconciled, and flag the points where our system and someone else’s package are going to collide.

The usual collisions are roof mounted plant added after the air termination was set out, metallic services routed inside the separation distance of a down conductor, and cladding details that leave nowhere to fix a conductor. All three are cheap to solve on paper.

Common questions

What does a lightning protection design include?

A drawn layout of the air termination network, down conductor positions and earth termination system, the protection class it is designed to, the materials and sizes, the separation distances that have to be maintained, and the surge protection requirement carried over from the risk assessment. It is issued as drawings and a specification an installer can price and build from.

When should the design be done?

As early as the frame design. If a reinforced concrete or steel frame is going to be used as the down conductor system, the connections at the pile caps and the continuity through the frame have to be designed before the concrete is poured. After that the only option is a surface mounted system, which costs more and is visible.

Which protection class will my building need?

That comes out of the BS EN 62305-2 risk assessment rather than being chosen up front. Class I is the highest level of protection and Class IV the lowest. A building holding something irreplaceable, or where a strike would put lives at risk, will land at a higher class than a low occupancy shed of the same height.

Can you design a system for a listed building?

Yes, and it is a large part of what we do. The design problem on a listed or heritage structure is routing conductors so the system works without altering the appearance or damaging the fabric. That usually means following existing lines in the stonework, using finishes that weather in, and agreeing the route before consent is sought.

Need a quote?

Send us the address, a roof plan if you have one, and what you already know about the building. We will tell you what is needed and what it costs. Call 01892 667258 or use the form.

Talk to Braileys
01892 667258