Home Roofing Elements Issues2022 Spring Lightning Risk Assessment

Lightning Risk Assessment

by Linda Schmid

by Chuck Cooley, Hicks Lightning Protection


Risk management is a method that seeks to eliminate, reduce, and avoid the effects from potential exposures and avoid any future losses.  Any loss due to potential exposures, can impact time, production, and revenue.  We need to identify and possibly control the potential risk.

One of the risks that sometimes is overlooked, is the risk to a structure or a building due to a potential lightning event.  Lightning Protection Systems are the last things to be installed on a project, but it’s also the first thing that is removed or value engineered out of a project.  The cost of a properly installed lightning protection system can be less than 1% of the overall cost of the project.  However, the repercussions/damages of not having a lightning protection system in the event of a lightning strike, can outweigh the initial proposed cost.  

Engineers, architects, and building owners typically rely on the risk assessment that is found in Annex L of NFPA 780 standard, the Installation of Lightning Protection Systems. This annex provides both, a simplified assessment, and a detailed calculation to achieve a more in-depth analysis.  This article will discuss and provide information on the simplified version of the risk assessment.

The NFPA risk assessment takes into consideration the following in determining whether a lightning protection system should be installed, or possibly considered optional.

• Building Environment

• Type of Construction

• Structure Occupancy

• Structure Contents

• Lightning Stroke Consequences

There are some instances where the need for lightning protection should be given serious consideration, regardless of the outcome of the risk assessment. Some examples would be:

• Large crowds

• Structures that contain explosives or flammable material

• Structures containing irreplaceable cultural items

• Continuity of critical services (ie. 911 call centers; hospitals)

• Tall, isolated buildings

• Environmental consequences

• High lightning flash frequency


The first formula that makes up the risk assessment is the Annual Threat of Occurrence.  It is derived from the following equation:

Nd = (Ng)(Ad)(Cd)(10-6) (1)


Ng = lightning cloud to ground flash density flashes/km2/year.  This value is obtained from the U.S. Lightning Flash Density Map (shown on opposite page).

    Ad = is the equivalent collection area of the structure (m2).  This is derived from the length “L”, width “W”, and height “H” of the structure.

Ad = LW + 6H(L+W) + ∏(9H2) (2)

Cd =is the relative structure location.  The value that best suits the location of the structure will be used:

Relative Structure LocationCd
Structure surrounded by taller structures
or trees within a distance of 3H
Structure surrounded by structures or equal
or lesser height within a distance of 3
Isolated structure, with no other structures located within a distance of 3H1
Isolated structure on hilltop2


The second formula that makes up the risk assessment is the Tolerable Lightning Frequency.  It is derived from the following equation:

Nc = (1.5 x 10-3 ) / C (4)


C = (C2)(C3)(C4)(C5) (5)

Construction Coefficient – C2

StructureMetal RoofNonmetallic RoofCombustible Roof


Structure Contents Coefficient – C3

Structure ContentsC3
Low value noncombustible0.5
Standard value noncombustible1
High value, moderate combustibility2
Exceptional value, flammable liquids,
computer, or electronics
Exceptional value, irreplaceable cultural items4


Structure Occupancy Coefficient – C4

Structure OccupancyC4
Normally occupied1
Difficult to evacuate or risk of panic3


Determination of Lightning
Consequence Coefficient – C5

Lightning ConsequenceC5
Continuity of facility services not required,
no environmental impact
Continuity of facility services required,
no environmental impact
Consequences to the environment10


This simplified risk assessment takes the calculated tolerable lightning frequency (Nc) and compares that it to the annual threat of occurrence (Nd). If Nd is less than or equal to Nc, a lightning protection system is optional. If Nd is greater than Nc, it is recommended that a lightning protection system be installed. 


When having a lightning protection system installed, the components being used must comply to UL96 Lightning Protection Components.  Also, make sure the lightning protection contractor is  certified with Underwriters Laboratory and the Lightning Protection Institute.  After the installation has been completed, the installing contractor will apply for a UL Master Label or a LPI Master Installation Certificate.  This inspection is done by a 3rd party to ensure the installed system, complies to the requirements of NFPA 780, UL96A or LPI 175.

It is also important that everyone know the effects and the dangers of lightning.  Lightning Safety Awareness Week was created in 2001 to inform people about the dangers of lightning.  In 2022, Lightning Safety Awareness Week will start the week of June 19th.  For more information on lightning safety, visit the Lightning Protection Institute at www.lightning.org  or the National Lightning Safety Council at www.lightningsafetycouncil.org. l

(1),(2),(3),(4),(5),(6),(7),(8),(9) Risk assessment charts and equations – NFPA 780 – Installation of Lightning Protection Systems – www.nfpa.com  

(A) Cloud-to-ground flash density gridded map 2021 – www.vaisala.com – 2021 Annual Lightning Report

About the Author:

Chuck Cooley is the Business Development Manager of Hicks Lightning Protection, Ponder, Texas.

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