Rooftop Attachment of Lightning Protection Systems in High-Wind Regions

Hurricane Katrina Recovery Advisory
FEMA 
May 2006, Revised July 2006

Purpose: To recommend practices for installing lightning protection systems 
(LPS) that will enhance wind resistance in high-wind regions.

Key Issues
 Lightning protection systems frequently become disconnected from rooftops 
during hurricanes. Displaced LPS components can puncture and tear roof 
coverings, thus allowing water to leak into buildings (see Figures 1 and 2). 
Also, when displaced, the LPS is no longer capable of providing lightning 
protection in the vicinity of the displaced conductors (cables) and air 
terminals (lightning rods). 

Figure 1 caption: This displaced air terminal punctured the membrane in several 
locations. [end of Figure 1 caption]

Figure 2 caption: View of an abraded end of a conductor that became 
disconnected. [end of Figure 2 caption]

 Lightning protection standards such as NFPA 780 and UL 96A currently provide 
inadequate guidance for attachment of LPS to rooftops in high-wind regions.
 Some LPS manufacturers provide guidance for attachment, while other 
manufacturers refer to the roofing material manufacturer for attachment 
guidance. Some roofing material manufacturers provide guidance for attachment, 
while other manufacturers refer to the LPS manufacturer for attachment guidance. 
In most cases, the attachment guidance provided by LPS and roofing material 
manufacturers is inadequate for hurricane-prone regions.
 LPS conductors are typically attached to the roof at 3 foot intervals. Because 
the conductors are flexible, when they are exposed to high winds, the conductors 
exert dynamic loads on the conductor connectors (clips). Guidance for 
calculating the dynamic loads does not exist. The attachment guidance that 
follows is therefore based on professional judgment.
 LPS conductor connectors typically have prongs to anchor the conductor. When 
the connector is well-attached to the roof surface, during high winds the 
conductor frequently bends back the malleable connector prongs (see Figure 3). 
Conductor connectors have also debonded from roof surfaces during high winds. 
Based on observations after Hurricane Katrina and other hurricanes, it is 
apparent that pronged conductor connectors do not provide reliable attachment.

Figure 3 caption: During high winds, the conductor deformed the prongs and 
pulled away from this conductor connector. [end of Figure 3 caption]

Construction Guidance
Parapet attachment: When the parapet is 12-inch high or greater, it is 
recommended that the air terminal base plates and conductor connectors be 
mechanically attached with #12 screws that have 1.25 inch minimum embedment into 
the inside face of the parapet nailer and properly sealed for watertight 
protection. In lieu of conductor connectors that have prongs, it is recommended 
that mechanically attached looped connectors be installed (see Figure 4). 

Figure 4 caption: This conductor was attached to the coping with a looped 
connector (above). However, a gasket was not installed between the connector and 
the coping. Also, because the connector screws were too short to engage the wood 
nailer, the screws pulled out and the conductor was displaced in some areas 
(figure at left). [end of Figure 4 caption]

Attachment to built-up, modified bitumen and single-ply membranes: For built-up 
and modified bitumen membranes, attach air terminal base plates with asphalt 
roof cement. For single-ply membranes, attach air terminal base plates with 
pourable sealer (type recommended by the membrane manufacturer). 

In lieu of attaching conductors with conductor connectors, it is recommended 
that conductors be attached with strips of membrane installed by the roofing 
contractor. For built-up and modified bitumen membranes, use strips of modified 
bitumen cap sheet, approximately 9-inch wide minimum. For single-ply membranes, 
use self-adhering flashing strips, approximately 9 wide minimum. If strips are 
torch-applied, avoid overheating the conductors. Start the strips approximately 
3 inches from either side of the air terminal base plates. Place strips that are 
approximately 3 long, followed by a gap of approximately 3 inches (see Figure 
5).

Figure 5 caption: Plan showing conductor attachment. [end of Figure 5 caption]

[text box]
Note: As an option to securing the conductors with stripping plies, conductor 
connectors that do not rely on prongs (such as the one shown in Figure 6) could 
be used. However, because the magnitude of the dynamic loads induced by the 
conductor are unknown and because of the lack of data on the resistance provided 
by adhesively-attached connectors, attachment with stripping plies is the 
preferred option because the stripping plies shield the conductor from the wind.

Figure 6 caption: Adhesively-attached conductor connector that does not use 
prongs. [end of Figure 6 caption]

If adhesive-applied conductor connectors are used, it is recommended that they 
be spaced more closely than the 3 foot spacing required by NFPA 780 and UL 96A. 
Depending upon wind loads, spacings in the range of 6 to 12 inches on center may 
be needed in the corner regions of the roof, with spacings in the range of 12 to 
18 inches on center at roof perimeters (see ASCE 7 for size of corner regions).
[end of text box]

Mechanically Attached Single-Ply Membranes: It is recommended that conductors be 
placed parallel and within 8 inches of membrane fastener rows. Where the 
conductor falls between or is perpendicular to membrane fastener rows, install 
an additional row of membrane fasteners where the conductor will be located and 
install a membrane cover-strip over the membrane fasteners. Place the conductor 
over the cover-strip and secure the conductor as recommended above.

Note: By following the above recommendations, additional rows of membrane 
fasteners beyond those needed to attach the membrane may be needed to 
accommodate layout of the conductors. The additional membrane fasteners and 
cover-strip should be coordinated with and installed by the roofing contractor.

Standing Seam Metal Roofs: It is recommended that pre-manufactured mechanically 
attached clips that are commonly used to attach various items to roof panels be 
used. After anchoring the clips to the panel ribs, the air terminal base plates 
and conductor connectors are anchored to the panel clips. In lieu of conductor 
connectors that have prongs, it is recommended that mechanically attached looped 
connectors be installed.

Conductor Splice Connectors: In lieu of pronged splice connectors (see Figure 
7), bolted splice connectors are recommended (see Figure 8). It is recommended 
that strips of flashing membrane (as recommended above) be placed approximately 
3 inches from either side of the splice connector to minimize conductor movement 
and avoid the possibility of the conductors from becoming disconnected. To allow 
for observation during maintenance inspections, do not cover the connectors.

Periodic Inspection and Maintenance: Each spring, it is recommended that the 
lightning protection system be inspected to verify that connectors are still 
attached to the roof surface and still engage the conductors. Also check to 
ensure that splice connectors are still secure. Inspections are also recommended 
after high wind events.

Figure 7 caption: A failed prong-type splice connector. If conductors become 
detached from the roof, they are likely to pull from pronged splice connectors. 
[end of Figure 7 caption]

Figure 8 caption: To avoid free ends of connectors being whipped around by wind, 
bolted splice connectors are recommended because they provide a more reliable 
connection. [end of Figure 8 caption]

Strengthening Attachment of Existing Systems: On critically important buildings 
that use adhesively-attached connectors and pronged splice connectors, it is 
recommended that attachment modifications based on Construction Guidance be made 
in order to provide more reliable securement. 

[End of Recovery Advisory]