Lightning protection
Give the current a controlled path to earth, so it does not find its own way through the building.
Last updated 2026-09-05
What is Lightning protection?
Lightning protection does not stop a building being struck. It accepts that a strike may happen and gives the current somewhere deliberate to go. The system intercepts the strike at a known point on the roof, conducts it down the outside of the structure on a defined path, and disperses it into the ground - all in preference to the current finding its own route through the structure, the services or anything standing next to them. Three elements do that work: the air termination network on the roof, the down conductors on the way down, and the earth termination network in the ground. A fourth, less visible element - bonding - ties the protection system to the other metalwork of the building so that the two are held at a similar potential during a strike.
Whether a building needs protection at all, and what form it takes, is not a site decision and not a supplier's decision. The designer carries out a risk assessment that weighs the exposure of the building, what it is made of, what it contains, how many people use it and what the consequences of a strike would be. That assessment produces a required level of protection, and the geometry of the system - where air terminations go, how many down conductors there are and how they are spaced around the building - follows from it. The same assessment covers protection of the electrical and electronic systems inside, because a strike to or near a building puts surges onto incoming power and data services as well as putting current down the outside.
On most modern buildings the protection system is not a separate cage bolted to the outside. Structural steel frames and the reinforcement in concrete frames are highly conductive and are commonly used as part of the down conductor path, with bonds made to the frame and continuity established through the structure. That approach is cheaper and more effective than an external network, but it has to be planned with the structural engineer and the frame contractor and executed while the frame is being erected - the connections cannot be made once the concrete has been poured or the cladding hung. The system is then inspected and tested at completion and re-inspected periodically through the life of the building, because a protection system that has corroded, been disconnected during a re-roof or been cut through by another trade is worse than none, since everybody assumes it is still working.
How does Lightning protection work, step by step?
- 1
Step 1: Assess the risk and decide what is needed
The designer carries out a risk assessment for the building. It considers the exposure of the site and the height and shape of the structure, the materials it is built from, what it is used for and what it contains, the number of people present and the consequences of a strike - loss of life, loss of service, loss of heritage or loss of value. The output is a decision on whether protection is needed and, if so, the level of protection required. That level is what drives the whole design: the density of the air termination network, the number and spacing of down conductors, and the requirements for surge protection of the systems inside. No part of that is a rule of thumb that a contractor should apply on site.
- 2
Step 2: Design the system with the structure, not after it
The specialist designer sets out the three elements together and coordinates them with the building design. Air termination positions are agreed with the roof designer so that they do not conflict with the waterproofing, the plant, the fall arrest system or the solar array. Down conductor routes are agreed with the facade designer so there is a path from roof to ground that is not interrupted by insulation, rainscreen or glazing. Earth termination positions are agreed with the ground works and the drainage layout. Where the structural frame is to be used as part of the conductor path, that is agreed with the structural engineer at design stage, because it changes what the frame contractor has to do during erection.
- 3
Step 3: Install the air termination network
The air termination is what the strike is intended to hit: a network of conductors, rods or mesh across the roof, following the ridges, edges and corners where a strike is most likely to arrive, with separate terminations for plant, masts, chimneys and anything else standing proud. It is fixed so that the roof covering is not perforated and the fixings do not become the weak point of the whole system - clips, weights and bonded pads rather than screws through a membrane. Metallic roof elements, handrails, plant frames and cable trays on the roof are bonded into the network so that they cannot become an alternative and uncontrolled path.
- 4
Step 4: Route and connect the down conductors
Down conductors take the current from the roof to the ground. They are distributed around the perimeter so that the current divides between several paths rather than concentrating in one, and they run as directly as the building allows - sharp changes of direction and loops are avoided because a lightning current does not follow a tight bend willingly. Where the steel frame or the reinforcement is used as the conductor path, the connections are made progressively as the frame is erected and are recorded as they are made. Test joints are provided so that each down conductor can be disconnected for testing later without dismantling the system.
- 5
Step 5: Install the earth termination network
The earth termination disperses the current into the ground. It commonly takes the form of driven rods, buried tapes, a ring conductor around the building or a connection to the reinforcement of the foundations, and which of those is used depends on the ground conditions and on what the designer has specified. The network is installed with the ground works, because most of it is below finished level and none of it can be added conveniently afterwards. Inspection pits are provided at test points so that the earthing can be reached for testing through the life of the building without excavation.
- 6
Step 6: Bond the system to other metalwork
During a strike the protection system rises rapidly in potential relative to everything around it. If a metallic service - a water pipe, a gas pipe, a cable armour, a structural element - runs close to a down conductor and is not bonded to it, the current can jump the gap. That is side flashing, and it is the mechanism by which a strike to the outside of a building causes damage inside it. Bonding ties the protection system, the incoming metallic services and the building's main earthing arrangement together so that they move in potential together. The specialist contractor makes the bonds the designer has specified, and every one of them is recorded, because a missing bond is invisible until the day it matters.
- 7
Step 7: Protect the systems inside
A strike near a building, not just to it, induces surges on power, data and signal cabling. Surge protection devices are installed at the points the designer has determined - typically where services enter the building and again at sensitive equipment - to divert those surges away from the electronics. Coordinating those devices so that they work together rather than against one another is a design task carried out by the designer against the risk assessment. It is a separate exercise from the external protection system, and a building can need one without the other.
- 8
Step 8: Test at completion and inspect periodically
On completion the specialist contractor verifies the installation: continuity through the conductor paths, the integrity of every bond and joint, the condition of the fixings, and the resistance of the earth termination measured at the test points and compared with the designer's requirements. The results and an as-installed drawing showing every element and every test point are handed over. The system is then re-inspected and re-tested at the intervals the designer specifies, and after any work that disturbs it. Re-roofing, facade replacement, solar panel installation and rooftop plant changes all routinely sever lightning protection, and the disturbance is rarely noticed by the trade that caused it.
What are the benefits of Lightning protection?
- Gives the strike current a controlled, known path instead of letting it find one through the structure or the services
- Reduces the risk of fire, structural damage and injury from a strike and from side flashing
- Using the structural frame or reinforcement as the conductor path is cheaper, more robust and less visually intrusive than an external network
- Protects electronic systems through coordinated surge protection, which is often the greater practical risk for a modern building
- Simple, durable materials with a long service life and no moving parts
- The system is fully testable, so its condition can be demonstrated rather than assumed
What are the limitations of Lightning protection?
- Reduces risk; it does not eliminate it, and no system prevents a building being struck
- Only worth having if it is complete - one broken conductor path or one missing bond can undo the design intent
- Must be designed and largely installed alongside the structure, roof and ground works; retrofitting a full system to a finished building is difficult and expensive
- Very easily damaged by later work - re-roofing, cladding replacement, solar installations and rooftop plant all cut conductors routinely
- Depends on a designer's risk assessment; without one there is no rational basis for what has been installed
- Requires a permanent regime of periodic inspection and testing, which building owners frequently allow to lapse
- Earth termination performance depends on ground conditions the designer has to establish, and difficult ground can make the earthing the hardest part of the job
What is Lightning protection best suited for?
What plant does Lightning protection need?
- Air termination components - rods, tapes, mesh conductors and their non-penetrating roof fixings
- Down conductor material, supports, and structural bonding components for connection to steelwork or reinforcement
- Earth termination materials - rods and couplers, buried tape, ring conductors and inspection pits
- Test joints, clamps and bonding connections, with exothermic or compression jointing equipment
- Surge protection devices for power, data and signal services
- Earth resistance and continuity test instruments
- Roof access and fall protection arrangements for installation and for every future inspection
How is Lightning protection quality-checked?
- Designer's risk assessment completed and the required level of protection recorded before design or procurement
- Coordination signed off with the structural, roofing and facade designers where the frame or reinforcement forms part of the conductor path
- Structural bonds made and recorded progressively during frame erection, not sought afterwards
- Roof fixings checked to confirm the waterproofing has not been penetrated and rooftop metalwork has been bonded in
- Every bond and joint recorded against the design, with test joints provided and accessible
- Earth termination installed with the ground works, with inspection pits provided at every test point
- Continuity and earth resistance measured at completion and compared with the designer's requirements
- As-installed drawing showing all elements, bonds and test points issued at handover
- Periodic inspection and testing regime agreed with the client, and re-testing carried out after any roofing, cladding or rooftop plant work