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Lightning risk
assessment

Do you need protection, and to what class?

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The question it answers

A lightning risk assessment answers two things: does this structure need protection, and if it does, how much. It is the calculation in BS EN IEC 62305-2, and everything downstream depends on it. The protection class it produces sets the mesh size, the down conductor spacing and the surge protection scope. Skip it and you are guessing at all three.

It also produces the answer some clients are actually after, which is a defensible no. If the calculated risk sits below the tolerable risk in the standard, the assessment is the document that says so, and it is what you hand to an insurer or a surveyor who has asked why the building has no lightning protection.

We have an accredited BS EN 62305 assessor in house and use current assessment software, so the calculation and the design that follows it are done by the same team.

What it weighs

  • Risk of loss of human life, including injury from touch and step voltages.
  • Risk of loss of service to the public, where the building carries power, telecoms, water or gas.
  • Risk of loss of cultural heritage, which is what brings churches and listed buildings into scope.
  • Risk of economic loss, covering the structure, its contents and the activity inside it.
  • Physical damage from a direct strike.
  • Failure of internal electrical and electronic systems from surge.

Existing buildings

Most of the assessments we do are on buildings that already have something on the roof, usually designed to BS 6651 before it was withdrawn in 2008. The question then is not whether to protect the building but whether what is there is still valid under the current standard.

Where it is not, we set the shortfall out in order of risk. That lets you improve the system in stages across more than one budget year instead of treating it as a single replacement project.

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What you get

A written assessment recording the inputs we used, the risk components calculated, the comparison against the tolerable risk in the standard, and the resulting protection class. Where protection is required it states the class and the surge protection requirement, which is enough for a designer to work from or for us to take straight into design.

Where protection is not required, it says that in terms an insurer or a building control officer will accept, and it records the assumptions so it can be revisited if the building changes use.

Lightning protection design

Common questions

What is a lightning risk assessment?

It is the calculation set out in BS EN IEC 62305-2 that decides whether a structure needs lightning protection and, if so, to what protection class. It compares the calculated risk to the structure against the tolerable risk in the standard. If the calculated risk is higher, protection measures are required until it is not.

How do you do a risk assessment for a lightning protection system?

You establish how often lightning is likely to affect the structure, then assess the consequences across the loss categories the standard recognises: loss of human life, loss of service to the public, loss of cultural heritage and economic loss. The structure’s size, height, location, construction, contents, occupancy and incoming services all feed in. The output is a required protection level, plus whether surge protection is needed.

What information do you need from me?

Location, drawings or dimensions and height, what the building is made of, what it is used for, how many people are in it, what the roof carries, what services come in, and anything irreplaceable inside. If there is an existing system we will want its last test certificate and log book too.

Is a risk assessment a legal requirement?

There is no law that names BS EN 62305-2 specifically. In practice a lightning risk assessment is how a designer, an insurer or a heritage body demonstrates the decision about protection was made properly, and general duties under health and safety law expect risks to be assessed. If you are asked to justify having no protection, this is the document that does it.

What changed in the 2024 edition?

BS EN IEC 62305-2:2024 introduced a single risk that combines loss of human life with loss due to fire, and replaced the old lightning strike density with ground impact point density as the basis for estimating how many hazardous events to expect. Assessments carried out under the previous edition can produce a different answer, which is worth knowing if you are relying on an older report.

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.

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