Learning about lightning protection

• Photo courtesy of TLSmith Consulting Inc., Rockton, Ill.
• Photo courtesy of TLSmith Consulting Inc., Rockton, Ill.
• Photo courtesy of TLSmith Consulting Inc., Rockton, Ill.
• Photo courtesy of TLSmith Consulting Inc., Rockton, Ill.

Editor's note: This article is based on a paper presented at the 12th International Roofing and Waterproofing Conference.

Lightning-protection systems commonly are installed on roofs located in areas prone to frequent lightning strikes. Although roofing contractors typically do not install lightning-protection systems, when such systems are integrated inadequately into a roof system or not maintained, roof-related problems can arise and the lightning-protection systems may be rendered ineffective.

Manufacturers of lightning-protection system components, roofing material manufacturers and roof system designers typically provide vague or inadequate details for securing lightning-protection systems to roofs and protecting roofs from damage by lightning-protection systems. The following information should help clear up some confusion and provide ways to avoid potential problems.

Franklin rod system

The most common lightning-protection system is the traditional Franklin rod system, which is based on the lightning rod Benjamin Franklin invented in 1752. This type of system does not prevent lightning from striking a building; rather, it controls a lightning strike and prevents damage to nonconducting parts of a building by providing a low-resistance path for the discharge of lightning energy.

The Franklin rod system consists of placing air terminals (commonly referred to as lightning rods) around a roof's perimeter (and also in the field of the roof on large roof areas) and along ridges of steep-slope roofs. The air terminals are connected with either copper or aluminum conductors, or cables. Metal bodies, such as mechanical equipment and roof hatches, often need to be "bonded" to a lightning-protection system by secondary conductors connected to the primary conductors that interconnect the air terminals. There also are instances where a metal body needs to be protected by air terminals and primary conductors that form part of the lightning-protection system.

Conductors are attached to a roof and/or parapet with various types of connectors, or clips, spaced at a maximum of 3 feet (915 mm) on center. The roof conductors are connected to through-roof or through-wall connectors, which are specially made bolted assemblies. Down conductors are connected to the interior side of through-roof or through-wall connectors and then connected to ground rods.

Until a few years ago, air terminals had sharp, pointed tips. However, because research has shown blunt tips are better receptors of lightning strikes than conventional sharp-tipped terminals, blunt-tipped terminals are becoming more common. In addition to their improved receptor properties, blunt-tipped terminals offer a safety advantage. If someone on a roof were to accidentally fall or step on a terminal, impalement is less likely with a blunt-tipped terminal. Additional impalement protection may be provided by blunt-tipped terminals that have spring-mounted bases. Spring-mounted terminals may be advantageous on roofs that experience enough snow accumulation to cover the terminals. If a blunt-tipped, spring-mounted terminal is stepped on, the possibility of impalement should be reduced. Blunt tips are available for retrofitting existing sharp-tipped terminals.

Guidelines

Three standards (all of which are quite similar) provide information regarding the design and installation of lightning-protection systems with respect to dissipation of lightning energy: LPI-175, "Standard of Practice for the Design-Installation-Inspection of Lightning Protection Systems," issued by the Lightning Protection Institute; NFPA 780, "Standard for the Installation of Lightning Protection Systems," issued by the National Fire Protection Association; and UL 96A, "Standard for Installation Requirements for Lightning Protection Systems," issued by Underwriters Laboratories (UL) Inc. However, little guidance pertaining to integration of a lightning-protection system into a roof system is provided in these standards or from lightning-protection system equipment and roofing material manufacturers.

LPI-175 advises that copper lightning-protection materials are not to be used on aluminum roofing materials and aluminum lightning-protection materials are not to be used on copper roofing materials. This requirement pertains to separation of dissimilar materials to avoid corrosion problems. LPI-175 also specifies the maximum spacing of conductor connectors and notes that galvanized or plated steel nails, screws or bolts are not acceptable for securing conductor connectors or air terminal bases.

UL 96A states air terminals should be secured to membrane roofs with adhesives compatible with the roofing material or, if prescribed by the membrane manufacturer, secured to "concrete blocks" (provided they are not porous). Conductors should be secured by looping strips of the roof membrane material around the conductors and securing the strips to the roof with a compatible adhesive. Or if prescribed by the membrane manufacturer, the conductors should be secured to "nonporous bricks." It should be noted that through its Master Label Program, UL requires the use of UL-listed lightning-protection components installed by authorized contractors.

A guide specification by a major lightning-protection equipment manufacturer states, "The roofing contractor will be responsible for sealing and flashing all lightning protection roof penetrations as per the roof manufacturer's recommendations."

It also states, "Lightning protection penetrations and/or attachment procedures should be addressed in the roofing section of the specifications."

When I asked the manufacturer about specific recommendations for attachment of air terminal bases and conductor clips to a roof membrane, the manufacturer advised that when attaching with adhesive, the adhesive should be compatible with the roof membrane being used.

Another lightning-protection equipment manufacturer's specification states air terminal bases shall be "fastened to the structure in accordance with code requirements." However, there are no code requirements that specifically address attachment of air terminal bases. When I asked the manufacturer about this issue, the manufacturer's representative stated that for adhesive attachment, the roof membrane manufacturer should be contacted for recommendations.

Many roofing material manufacturers also provide some guidance pertaining to the interface between a roof membrane and lightning-protection system. Some examples of membrane manufacturers' recommended details follow:

• An EPDM membrane manufacturer states a sufficient number of conductor connectors must be installed to "ensure" the conductor does not contact the membrane. If the conductor contacts the membrane, the manufacturer suggests installing a strip of batten cover material under the conductor. For mechanically attached systems, a conductor should be placed parallel and within 8 inches (200 mm) of the batten strip. Where the conductor falls between or is perpendicular to batten strips, the manufacturer says to install an additional batten and cover strip under the conductor. The membrane manufacturer's detail shows splice adhesive on the membrane and air terminal base sitting in pourable sealer. The detail notes the manufacturer "assumes no responsibility for attachment of lightning rods or damage to the roof system because of failure or detachment."
  
  
• A PVC membrane manufacturer says half the unspecified length of a minimum 2-inch- (50-mm-) wide strip of roof membrane material should be welded to the membrane. The conductor then should be placed over the strip and the strip folded over the conductor and welded to the previously welded portion of the strip. The strips are to be installed 12 inches (300 mm) on center.
  
  
• An SBS-modified bitumen membrane manufacturer recommends installing a pad of cap sheet (3 inches [75 mm] wider in all directions than the air terminal base or conductor connector) in hot asphalt or asphalt roof cement or by torch. Then, the air terminals and conductor connectors should be attached in asphalt roof cement.
  
  
• A TPO membrane manufacturer offers two details. One detail specifies a 6- by 6-inch (150- by 150-mm) pad of membrane heat-welded with the air terminals and conductor connectors sitting on the pad. The manufacturer makes no recommendation regarding attachment of the air terminals and connectors to the pads.

Standard roof details produced by the Metal Building Manufacturers Association (MBMA), NRCA and Spray Polyurethane Foam Alliance (SPFA) do not include details pertaining to integration of lightning-protection systems into roof systems.

Observed problems

When inadequately integrated into a roof system, a lightning-protection system can damage the roof and/or no longer be capable of providing lightning protection.

For example, bitumen displacement can occur in hot climates when conductors rest directly on smooth- or mineral-surfaced built-up or modified bituminous membranes. The conductors can sink into the membrane, thereby displacing the bitumen above the reinforcement. Bitumen displacement has the potential to shorten a membrane's service life.

Roof surface abrasion also is a concern. Conductors resting directly on a roof surface can abrade it. On metal roofs, abrasion can result in loss of corrosion protection and subsequent corrosion-induced penetration of the metal. Loss of protective granules or wearing away of polymer matrices can shorten a roof's service life. Surface abrasion can be accelerated if the distance between conductor connectors is excessive because of a lack of connectors or a connector was not attached to or becomes detached from the roof.

Accelerated abrasion also can occur on roofs located in areas prone to frequent or prolonged wind events that are sufficient to cause conductors to move between connector points. Surface abrasion typically is not a problem when conductors bear on pavers or over aggregate surfacing.

In addition, when conductors rest directly on a roof surface, frayed conductor strands, protrusions from splice plates and bolts protruding from cross-run clips can puncture membranes. Air terminals and conductors that become dislodged during wind storms also can puncture a membrane. (For further information regarding displacement of lightning-protection systems during wind storms, access www.fema.gov/fima/mat/mat_rprts.shtm.)

Membrane tearing and blow-off is another issue. Conductors that become dislodged during wind storms can tear membranes. During prolonged high winds, repeated slashing of the membrane by loose conductors and puncturing by air terminals can result in the membrane lifting and peeling.

Finally, loss of lightning-protection system integrity can occur when air terminals are blown toward a roof's center or blown off a roof.

Avoiding problems

The integration of lightning-protection systems into roof systems has received inadequate attention from the lightning-protection and roofing industries. The lightning-protection industry, in general, provides little guidance for integration issues. Many roofing material manufacturers provide inadequate guidance or do not stand behind their guidelines. In addition, some roofing manufacturers refer to the equipment manufacturer, which results in neither party providing guidance.

Typically, there is a lack of coordination between roof system and lightning-protection system specifications. There also is a lack of detailed direction from roof system designers regarding protection of roof systems from damage by lightning-protection systems and adequate anchorage of lightning-protection systems. These design issues are exacerbated by a lack of specific guidance for use by designers. Lack of coordination particularly is illustrated by projects where a lightning-protection system is installed without the roofing contractor's knowledge.

Bitumen displacement, roof surface abrasion, and membrane puncture from frayed conductors and various types of connectors easily are mitigated by incorporating a continuous strip of extra membrane material underneath the conductors or, in the case of metal roof systems, adequately elevating the conductor above the surface.

However, to avoid wind-induced detachment of conductors, research is necessary to further understand the loads induced on conductors, which then are transferred to connectors and air terminal bases. A test method needs to be developed to evaluate attachment strength and long-term effectiveness.

Recommendations

Standard details pertaining to integrating a lightning-protection system into a roof system should be developed by MBMA, NRCA and SPFA. In the interim, I recommend the following:

• To avoid abrasion and displacement into a roof surface, provide a continuous strip of extra membrane material underneath the conductors or, in the case of metal roof systems, elevate the conductor above the surface of the roof.
  
  
• For enhanced wind resistance of lightning-protection systems in areas with a basic wind speed greater than 90 mph (40 m/sec) and for buildings in excess of 100 feet (30 m) high, refer to the Federal Emergency Management Agency's Hurricane Katrina Recovery Advisory. (At press time, the advisory still was under development. It is expected to be available this spring).
  
  
• Specify that when air terminals and conductor connectors are attached to a roof with membrane strips, the application of the strips be performed by a roofing contractor.
  
  
• Specify that penetrations through a roof (such as through-roof connectors) be flashed by a roofing contractor. In my opinion, fasteners through vertical faces of base flashings and copings should be permitted to be installed by a lightning-protection contractor, but provisions for ensuring long-term watertightness of the fastener penetration should be specified by a designer. When attaching through copings, the roof system designer also should evaluate thermal movement ramifications.
  
  
• Specify blunt-tipped terminals be used. In addition to the advantages discussed previously, if an air terminal becomes dislodged, it is less likely to cause damage if it has a blunt tip.
  
  
• Specify a lightning-protection contractor attend the preroofing conference.
  
  
• After completing the lightning-protection system installation, I recommend the roof system designer and/or roofing contractor inspect the roof. The purpose of the inspection is to look for membrane damage caused during installation and visually attempt to assess whether the attachment and/or penetration details compromise the roof system's integrity.

When reroofing a building that has an existing lightning-protection system, you should do the following:

• Notify the building owner in writing that the system will be disconnected and nonfunctional during the reroofing work.
• Recommend a lightning-protection contractor remove the existing lightning-protection system. (Some roofing contractors also perform lightning-protection system work.)
  
  
• Inform the owner that if the existing system is UL Master-labeled, the reinstallation must be performed by a contractor listed by UL. (Several roofing contractors are listed for lightning-protection system work.)
  
  
• Conduct research to further understand the wind loads induced on the conductor connectors and air terminal bases, and develop a test method to evaluate the strength and long-term effectiveness of attachments. The test method should include provisions to evaluate dynamic loading and aging influences.
  
  
• Recommend the building owner have a lightning-protection system contractor visually inspect the system each year to verify air terminal base plates and conductor connectors still are attached to the roof surface and connectors still are engaged to conductors.

Hope for the future

Although current guidance is limited, if you pay attention to issues discussed in this article, problems associated with lightning-protection systems can be minimized. I also hope a meaningful dialogue between the roofing industry and lightning-protection industry ensues.

Thomas L. Smith is president of TLSmith Consulting Inc., Rockton, Ill.