Wind Damage >> Withstanding Wind And Lightning

Rooftop Attachment of Lightning Protection Systems in High-Wind Regions Hurricane Katrina Recovery AdvisoryFEMA May 2006, Revised July 2006 Purpose: To recommend practices for installing lightning protection systems (LPS) that will enhance Withstanding Wind And Lightning 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, Withstanding Wind And Lightning thus allowing water to leak into buildings (see Figures 1 and 2). 

Also, when displaced, Withstanding Wind And Lightning 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 Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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, Withstanding Wind And Lightning during high winds the conductor frequently bends back the malleable connector prongs. 

Conductor connectors have also debonded from roof surfaces during high winds. Based on observations after Hurricane Katrina and other hurricanes, Withstanding Wind And Lightning 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. 

Construction Guidance Parapet attachment: When the parapet is 12-inch high or greater, Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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 Withstanding Wind And Lightning 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, Withstanding Wind And Lightning 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 Withstanding Wind And Lightning 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 Withstanding Wind And Lightning 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.

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