Lightning Protection Systems and the Trades That Touch Them

Why this matters

A lightning protection system is a set of rooftop rods and heavy braided cable that does nothing visible for twenty years and then, for a few microseconds, carries a strike to earth instead of through the attic. It is also the system on the house most likely to be quietly broken by a trade that did not know what it was: a roofing crew that coils the cable and leaves it on the new shingles, a solar installer who unbolts a conductor to fit a rail, a gutter installer who replaces bonded metal with unbonded metal. Nothing fails until the next strike, and then the damage path runs through the work your crew did.

This card is for the trades that touch the system without installing it. Installing and certifying one is specialist work for a lightning protection contractor.

The parts, from the top down

• Strike termination devices, usually called air terminals: short rods along ridges, at roof edges and on chimneys and equipment. They are where the strike attaches.
• Main conductors: heavy stranded copper or aluminum cable connecting the air terminals to each other along the ridge and roof edges.
• Down conductors: the runs that bring the current from roof level to the ground, usually down outside corners of the building.
• Grounding electrodes: ground rods (or other listed electrodes) where each down conductor enters the earth.
• Bonding connections: short conductors and listed fittings tying nearby metal (gutters, vents, a rooftop condenser, gas piping, the electrical service ground) into the system, so a strike cannot jump from the system to that metal through the house. That jump is called a side flash.

Common grounding ties it together: the lightning system's grounding electrodes are bonded to the building's electrical grounding electrode system. NEC 250.106, as adopted by your jurisdiction, requires that bond, and NEC 250.60 says the lightning system's rods may not substitute for the electrical grounding electrodes. Two separate ground systems without a bond is exactly the condition that drives surge current through the house wiring.

What the standards set, and who they bind

NFPA 780, Standard for the Installation of Lightning Protection Systems, is the main U.S. installation standard. UL 96A, Installation Requirements for Lightning Protection Systems, is the parallel standard used for UL's certification of completed installations, and UL 96 covers the components. The Lightning Protection Institute publishes a third installation standard used for its own certification program.

None of these is law for a house by default. NFPA 780 becomes binding when a building code, a contract, an insurer or the owner's specification requires it, and it binds the installer. For most houses the system was the owner's choice. So the obligation a roofer or solar installer carries is not "comply with NFPA 780"; it is the ordinary duty not to damage the customer's property, plus whatever the system's certification requires to stay valid. A system with a UL or LPI certificate may need reinspection after alteration to keep it, which is the owner's interest and the reason to involve the lightning protection contractor rather than improvising.

A few geometry rules from NFPA 780 that a non-installer should recognize, stated for the edition the system was designed to:

• Air terminals along a ridge at intervals of not more than 20 feet, with wider spacing allowed for taller terminals, and a terminal within 2 feet of each ridge end and outside corner.
• At least two down conductors on any structure, with additional ones so the average spacing around the perimeter does not exceed 100 feet.
• No conductor bend sharper than 90 degrees, and no bend radius under 8 inches. Lightning current does not like tight turns; a sharp kink is a point where it can arc off the conductor.

 ROOF PLAN, ridge house, conductors shown
 
   end <2ft          20ft max           <2ft end
   AT------AT----------AT----------AT------AT
   |        main conductor along ridge       |
   |                                         |
   + bond to                     bond to     +
   | gutter                      vent stack  |
   |                                         |
   DC (down conductor,           DC (down conductor,
   |   outside corner)            |  outside corner)
   GR -- bond to service         GR
         grounding electrode
 
 AT = air terminal  DC = down conductor  GR = ground rod

Why your new metal may need to join the system

The governing idea is the side flash. During a strike the system's conductors rise to a very high voltage relative to earth for an instant. Metal close to them that is not bonded sits at a different potential, and if the gap is short enough the current jumps it. NFPA 780 gives a calculation for the bonding distance, which depends on the height of the point and the number of down conductors. Inside that distance the metal gets bonded; outside it, it does not need to be.

This is why ordinary work changes the system's requirements:

• A new metal roof or metal roof panels on a house that had shingles adds a large conductive body directly under the air terminals. It typically needs bonding at intervals, and the terminal mounting changes.
• New metal gutters and downspouts near a down conductor are usually bonded to it. Replacing bonded gutters with new ones and not reconnecting the bond leaves a large metal body unbonded at the roof edge.
• An antenna mast or satellite dish on the roof is a strike attachment point in its own right. It needs either an air terminal protecting it or a bond into the system.
• A rooftop condenser or a new vent stack above the roof plane needs coverage and bonding in the same way.
• Gas piping, especially corrugated stainless steel tubing (CSST), is a known side-flash victim because its thin wall can be perforated by an arc. The National Fuel Gas Code (NFPA 54) and the gas tubing manufacturers' instructions require CSST bonding, and where a lightning protection system is present the fuel gas code points to NFPA 780 for how the gas piping is bonded, in the editions your jurisdiction adopted. A plumber or HVAC tech adding gas piping on a house with a system should flag it to the lightning protection contractor, not decide it.
• A solar array on a protected roof is metal, high, and wired into the house. It needs its own coverage and bonding designed into the system, and the array's equipment grounding is not a substitute.

Never disconnect a conductor and walk away

The rule for every trade that is not the lightning protection contractor: do not cut, unbolt or reroute a conductor without a plan to restore it the same day, and do not leave the house with any part of the system disconnected. A conductor removed and stacked on the lawn has left the house with a set of air terminals that attract a strike and nowhere to send it. That is worse than no system at all.

If the job requires moving part of the system (a reroof is the usual case), the sequence that works is to call the lightning protection contractor before the job is scheduled, agree who removes and reinstalls what, and keep a photo record of every conductor route and connection before anything is touched. Many lightning protection contractors will remove and reinstall around a reroof; some roofers do it under the contractor's direction. Either way, the reinstallation is inspected by someone qualified, and the owner gets a note saying the system was restored.

Weather and the work itself. A crew on a roof with a partly disconnected system during a thunderstorm is standing on the strike point. The FieldHealth lightning card owns crew rules (get off the roof and into a building or a hard-topped vehicle at the first thunder, and stay down until 30 minutes after the last); on these jobs it applies with more force.

Worked example: a reroof on a protected house

A roofing shop is asked to tear off and replace an asphalt shingle roof on a two-story house with a lightning protection system. The ridge is 60 feet long. A walk-around shows five air terminals along the ridge, one on the chimney, a main conductor along the ridge and both gable rakes, and two down conductors on opposite back corners with ground rods. The gutters are aluminum, and at each down conductor a short bonding strap ties the gutter to the down conductor. There is a certificate label at the electrical service.

Check what is there against the plan. Five terminals on a 60-foot ridge gives four intervals of 15 feet if the end terminals sit at the ridge ends, which is inside the 20-foot maximum. Two down conductors on a structure this size meets the two-minimum; the footprint is about 60 by 30 feet, a perimeter of about 180 feet, and two conductors give an average spacing of 90 feet, inside 100 but close enough that an addition or a wider footprint would call for a third. The roofer is not certifying this; the point is to notice if the existing system looks different after the job.

The plan. The roofer calls the lightning protection contractor named on the certificate before signing the contract. The contractor agrees to detach the ridge conductor and air terminal bases on tear-off day and reinstall on the day the ridge cap goes on. In between, the roofer's crew keeps the detached conductors coiled at each corner, still attached to the down conductors, and the contractor is told the dry-in date. The roofer photographs every attachment point, every bond strap and the gutter bonds.

What goes wrong without it. The common version: the crew tears off, removes the terminals because the bases are nailed through the old shingles, finishes the roof, and reinstalls the terminals with new adhesive bases but splices the ridge conductor with a mechanical connector not listed for lightning protection, at a sharp 90-degree kink, and forgets the gutter bond on the east corner. The system looks complete from the street. On the next strike the current arcs at the kink, and the unbonded gutter flashes to the fascia nailing, starting a fire in the soffit.

What changes the plan. If the owner wants a standing-seam metal roof instead, the roofer tells the owner before the contract is signed that the lightning protection contractor must redesign the terminal mounting and the bonding to the roof panels, which is added cost and a schedule dependency. If there is no certificate and no contractor on record, the owner chooses who restores the system; the roofer's job is to state in writing what was there before tear-off, with the photographs, and to not reinstall a system it is not qualified to restore.

How to verify you got this right

• The pre-job photos show every air terminal, every connection, every bond, and the down conductor routes.
• The post-job photos show the same points in the same places, with no new sharp bends and no unlisted splices.
• The owner has a written note naming who restored the system and whether it was reinspected for its certificate.
• Your intake for roofing, gutter, solar and rooftop HVAC jobs asks "is there a lightning protection system on the house?", because the crew should know before arriving, not on the ridge.

References

• NFPA 780, Standard for the Installation of Lightning Protection Systems, edition per the system's design
• UL 96A, Installation Requirements for Lightning Protection Systems; UL 96 for components
• NFPA 70 (NEC) 250.106 and 250.60; NFPA 54 (National Fuel Gas Code) CSST bonding, as adopted
• See related: Lightning and the Outdoor Crew; Whole-House Surge Protection; How Service Work Quietly Removes a House's Hazard Resistance