Lightning Protection System Design to BS EN 62305
Lightning protection system design is the engineering stage that decides whether a building needs protection, which class of system it needs and exactly where every air terminal, down conductor, earth electrode and bond goes. Earthing Equipment Supplies Southern Ltd designs systems to BS EN 62305 for architects, consultants, developers, main contractors and building owners across Reading, London and the South of England.
Our design service stands on its own or leads straight into installation. You receive a risk assessment report, scaled CAD drawings, a specification and a method statement, prepared by ATLAS member engineers who go on to test and certify the finished system.
Who Our Lightning Protection Design Service Is For
Architects and Consultants
Early advice on roof layout, concealed routes and plant positions, then drawings and a specification you can include in tender documents.
Developers and Main Contractors
A design that is ready before roofing and services are fixed, with coordination through construction to avoid late changes.
Property and Facilities Managers
Design reviews for existing buildings that have been extended, re-roofed or fitted with new plant, plus upgrade designs for older systems.
Heritage and Planning Consultants
Low-impact designs for listed buildings, churches and sensitive sites, prepared with conservation requirements in mind.
Design of a Lightning Protection System: Our Process
Brief and Site Survey
We review drawings, roof layout, construction, services and any existing system, and survey the site where the building exists.
Risk Assessment
A BS EN 62305-2 assessment establishes whether protection is needed, the class required and whether surge protection is justified.
System Design
Air termination, down conductors, earth termination, bonding and surge protection are designed to the chosen class.
Drawings and Support
CAD drawings, specification and method statement are issued, and we support the design through installation and commissioning.
BS EN 62305-2 Risk Assessment: The Starting Point for Every Design
The risk assessment is where design begins. It weighs the size and height of the structure, its location and exposure to lightning, the services connected to it, who occupies it and the consequences of damage, including loss of life, loss of service, damage to cultural heritage and economic loss. The result tells you whether a system is needed, which class it should be and whether surge protection is required.
The assessment also protects the client. A documented result is the evidence insurers, building control and planning officers ask for, and it stops the system being over- or under-specified. As ATLAS members, we work to the BS EN 62305 series and to the guidance published by the Association of Technical Lightning & Access Specialists (ATLAS).
- Structure size, height and construction
- Location and local lightning activity
- Incoming power, data and metallic services
- Occupancy and public access
- Contents, equipment and process value
- Heritage, environmental and service-continuity impact
Lightning Protection System Design to BS EN 62305-3: Class I to IV
The class sets the design parameters for the air termination and down conductor system. Class I is the tightest and is used for the most demanding sites.
| Class | Rolling sphere radius | Mesh size | Down conductor spacing | Typical use |
|---|---|---|---|---|
| I | 20 m | 5 × 5 m | 10 m | Explosive atmospheres, critical infrastructure |
| II | 30 m | 10 × 10 m | 10 m | Hospitals, schools, heritage buildings |
| III | 45 m | 15 × 15 m | 15 m | Offices, commercial and residential buildings |
| IV | 60 m | 20 × 20 m | 20 m | Lower-risk agricultural and storage buildings |
Lightning Rod System Design: Where Air Terminals Go
Rods, roof tape and masts form the air termination system. Their positions are calculated, not estimated, and are checked against every change in roof height and every item of roof plant.
Rolling Sphere Method
A sphere of the class radius is rolled over the structure in every direction. Any point it can touch needs an air terminal or conductor, which is how rod positions are found on complex roofs.
Protection Angle Method
Used for simpler geometries, where the zone protected by a rod of a given height is defined by an angle that reduces as the height increases.
Mesh Method
A grid of roof conductors at the class mesh size, connected to rods at ridges, corners and edges. Suited to flat and complex roofs and often combined with rods.
Masts and Isolated Systems
Tall mast and isolated designs keep strike current away from the structure itself, for sites such as biogas plants and fuel storage. See our lightning protection for biogas page.
Early streamer emission terminals sit outside BS EN 62305 and are designed to NF C 17-102 where a client or insurer specifies them. Our types of lightning protection system page compares all four system types.
Earthing and Lightning Protection Design
Earthing and lightning protection design cannot be separated: a strike is only safe if the current reaches the ground. We design the earth termination system and the bonding around it from measured ground data.
Ground Data First
Soil resistivity measured with the Wenner four-point method shows what electrode arrangement the site needs. See soil resistivity testing.
Electrode Arrangement
Vertical rod or ring electrode arrangements selected for the building footprint, ground conditions and the resistance target of below 10Ω.
Integrated Earthing
Lightning, power and communications earthing are tied into a single earth termination system, avoiding dangerous potential differences between them.
Equipotential Bonding
Metallic services, structural steel and roof plant are bonded, or kept at a calculated separation distance, so current cannot arc across a gap.
Surge Protection in the Design
A direct strike is only part of the risk. Surges travelling in on power and data lines can destroy electronics inside a building that has an otherwise sound external system. Where the risk assessment justifies it, we design surge protection to BS EN 62305-4, selecting Type 1, 2 and 3 devices for the incoming supply, distribution boards and sensitive equipment. Installation is covered by our surge arrester installations service.
What You Receive From Our Design Service
Risk Assessment Report
BS EN 62305-2 calculation, inputs, protection class and surge protection recommendation, in a format accepted by insurers and building control.
CAD Drawings
Scaled plans and elevations showing air terminals, conductors, test clamps, earth electrodes and bonding points.
Specification and Materials
Component types, conductor sizes and materials matched to the class, the roof finish and the environment.
Method Statement and Phasing
Installation sequence, access arrangements and a phase plan coordinated with the main contractor.
For a worked example of the process, see our guide to applying structural lightning protection (PDF).
Design in Practice: A Discreet System for a New-Build House in Watlington
A rebuilt, isolated house on a hill needed lightning protection to satisfy its insurer, but the owner did not want the system visible from the ground.
- Risk assessment carried out first to set the protection class
- Soil resistivity test completed to inform the earthing design
- Design drawings and quotation prepared before work began
- Phase plan agreed and coordinated with the main contractor
- 100 linear metres of roof conductor on a metal and clay tile roof
- Installed in 2 weeks, with almost no visibility from ground level
- Tested, inspected and certified on completion
- Surge protection added to the electrical system
Client feedback: “Fast efficient installation, well programmed, good communication.”
Read the full case study View the risk assessment (PDF) View the resistivity test (PDF)
Lightning Protection Design for Listed and Historic Buildings
Historic buildings, churches and spires need protection designed around their fabric. We route conductors discreetly, minimise fixings into historic material and keep visible terminals to the minimum the risk assessment allows. Historic England’s guidance on lightning protection for historic buildings is the reference we design from, and our church lightning protection page covers tower and spire systems in more detail.
How Lightning Protection Design Is Priced
We quote the risk assessment and design package in writing before work begins. The fee depends on:
Size and Complexity
Building footprint, roof geometry, number of buildings and the amount of roof plant to coordinate.
Protection Class and Environment
Class I and hazardous-area designs need more analysis than Class III or IV commercial designs.
Surveys Required
Whether a site survey and soil resistivity test are needed, or drawings alone are enough.
Design Only or Design and Install
Design can be issued as a stand-alone package or carried through to installation by our own contractors.
Why Choose Earthing Equipment Supplies for Lightning Protection Design?
Design, installation, testing and inspection sit under one roof, so what is drawn is what gets built, and what gets built is what gets checked.
Designers Who Also Install and Test
Our designers see the faults that poor design causes when they test existing systems, and they design with that in mind.
- Founder and full members of ATLAS
- Current ATLAS design certification
- Risk assessments to BS EN 62305-2
- Scaled CAD drawings and specifications
- Soil resistivity testing in-house
- Design coordinated with architects and main contractors
- Heritage and discreet system experience
- Post-installation testing and certification by the same team
ATLAS Membership
Founder and full members of the Association of Technical Lightning & Access Specialists, the trade body represented on the BSI committee for BS EN 62305.
£10M Public Liability Insurance
Public liability and professional indemnity cover for design and installation work. Documentation is available on request.
Design Through to Certification
Every design is tested against its target values after installation and certified, with periodic inspection to follow.
Documented Decisions
Risk assessment inputs, class selection and drawing revisions are recorded, so the reasoning behind the design can be audited later.
Lightning Protection System Design — Frequently Asked Questions
What is lightning protection system design?
Lightning protection system design is the engineering stage that decides whether a building needs protection, which class of system it needs and exactly where every air terminal, conductor, earth electrode, bond and surge protector goes. It follows a BS EN 62305-2 risk assessment and results in drawings, a specification and a method statement that an installer can build from and an inspector can later check against.
What does your lightning protection design package include?
A BS EN 62305-2 risk assessment report, the selected protection class, scaled CAD drawings showing air termination, down conductors, earth termination and bonding, a material specification, surge protection recommendations and an installation method statement. Where ground conditions need confirming, a soil resistivity survey feeds the earthing design.
Is lightning protection a legal requirement in the UK?
No single UK law requires lightning protection on every building. The need is established by a risk assessment, and it can also be required by an insurer, a planning or building control condition, or sector rules for sites such as fuel storage, healthcare and education. If you are unsure which applies, the risk assessment is the starting point. This is general guidance, not legal advice.
How is the lightning protection class chosen?
The class (I to IV) comes out of the BS EN 62305-2 risk assessment, which weighs the size and height of the structure, its location and exposure, the services connected to it, who occupies it and what would be lost in a strike. Class I gives the closest conductor spacing and the highest level of protection, Class IV the widest spacing. Explosive atmospheres and critical infrastructure typically need Class I.
How do you decide where lightning rods go?
Air terminals are positioned using the methods in BS EN 62305-3: the rolling sphere method, the protection angle method or a mesh of roof conductors, often in combination. Each class has its own sphere radius and mesh size, and rods are added wherever roof plant, parapets or changes in height leave an area outside the protected zone.
Is earthing and lightning protection design the same thing?
They are designed together but are not identical. The earth termination system is the part of the lightning protection design that disperses strike current into the ground, and it must be integrated and bonded with the building's other earthing. We design both from the same soil resistivity data so that resistance targets, electrode arrangement and bonding all work as one system.
Can you design lightning protection for listed and historic buildings?
Yes. Heritage designs balance protection against appearance and fabric, using concealed conductor routes, careful fixing methods and minimal visible terminals. Historic England publishes guidance on lightning protection for historic buildings, and we work from it alongside conservation officers where one is involved.
Can you review or upgrade an existing lightning protection design?
Yes. We compare the installed system with its original drawings and with current BS EN 62305 requirements, then issue a gap report and a revised design where changes are needed. This is common where a building has been extended, roof plant has been added or the system was installed under the older BS 6651 standard.
How long does lightning protection design take?
After the site survey, design and drawings for a standard building typically take 2 to 5 working days. Large, complex or multi-building sites take longer, and we confirm the programme in writing when we quote. We work with your architect or main contractor so the design is ready before roof and services details are fixed.
How much does lightning protection system design cost?
Design cost depends on the size and complexity of the structure, the number of buildings, the class of protection and whether a soil resistivity survey is needed. We give a written quotation for the risk assessment and design package before work begins. Call 0118 976 0239 for a quote based on your drawings or site.
Does the design need testing after installation?
Yes. The finished system is tested against the design values, including earth resistance and continuity, and then certified, with periodic checks after that. See our lightning protection testing and lightning protection inspection services.
Related Lightning Protection Services
Start Your Lightning Protection Design
Send us your drawings or book a site survey and we will confirm the scope, programme and fee in writing. Call 0118 976 0239 or request a quote online.
This page is general guidance and not legal or insurance advice. The need for lightning protection and the class of system depend on a site-specific BS EN 62305-2 risk assessment, your insurer and any planning or building control conditions. Last reviewed 12 October 2026.
