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Lightning Risk Assessment Guide

For Your Home of Other Buildings

Lightning loss risk assessment for your home or other buildings involves the evaluation of various criteria to determine the risk of loss due to a lightning strike. This guide will assist you in determining if your building is at risk for a lightning strike. It's not possible to cover each special design element in this guide that may render a structure more or less susceptible to lightning damage. In those special cases it is recommended that you seek advice from a lighting assessment expert. Personal and economic factors are often very important and should be considered in addition to the assessment obtained by use of the following guide.


The following variables should be considered to determine the need for protection against lightning for a given building or structure:

Please record the number value that is associated to the closest match in each section of the chart above. Select only one per section. Add sections 1 thru 5. Then divide by the number selected in section 6. Compare the result at the end of the page.

how to use the risk assessment guide

Type of Structure - INDEX A

(determine one type from Index A)

Index StructureIndex
Value
Single family residence less than 5,000 sq. ft. 1
Single family residence over 5,000 sq. ft. 2
Residential, office, or factory building less than 50 ft. in height
Covering less than 25,000 sq. ft. of ground area
3
Covering over 25,000 sq. ft. of ground area
5
Residential, office, or factory building from 50 ft. to 75 ft high. 4
Residential, office, or factory building from 75 ft. to 150 ft high 5
Residential, office, or factory building from 150 ft or higher 8
Municipal services buildings, fire, police, water, sewer, etc. Hangars 7
Power generating stations, central telephone exchanges 7
Water towers and cooling towers 8
Libraries, museums, historical structures 8
Farm Buildings 9
Golf shelters and other recreational shelters 9
Places of public assembly such as schools, churches, theaters, stadiums 9
Slender structures such as smokestacks, church steeples, spires, control towers, lighthouses, etc. 10
Hospitals, nursing homes, housing for the elderly or handicapped 10
Buildings housing the manufacture, handling or storage of hazardous materials 10

Type of Construction - INDEX B

(determine one type from Index B)

index b

Relative Location - INDEX C

(determine one type from Index C)

index c

Topography - INDEX D

(determine one type from Index D)

index d

Occupancy and Contents - INDEX E

(determine one type from Index E)

index e

Lightning Frequency Isoceraunic Level - INDEX F

(determine one type from Index F)


Few areas in the United States are free from thunderstorms and their attendant hazards, but some areas have more storms than others. The map shows the incidence of thunderstorm days - days on which thunderstorms are observed - for the United States. Select one. index f

Determining the Risk - INDEX R

The Risk Index "R" is obtained by dividing the sum of the values given in Index A - E by the lightning frequency index value obtained from Index F.

Value Risk Value
0 - 2 Light
2 - 3 Light to Moderate
3 - 4 Moderate
4 - 7 Moderated to Severe
Over 7 Severe

To the Contsruction Professional

This reprint of the NFPS's Risk Assessment Guide is in answer to several requests from architects, engineers and others for guidance. If there is any doubt opt to protect the structure. In most cases the costs of doing so are tiny in contrast to the value of the building, it's contents and occupants. We recommend, if based on the use of this guide, you specify lightning protection for a particular property and it is rejected, that you require that the rejection is put in writing for your files.

To the Property Owner

We recommend that whatever the risk value of your building that you also consider three outside factors that affect advisability of protecting your structure:
  1. Rising costs - A proper system can protect your property for a life-time.
  2. Increasingly vulnerable contents - with electronic equipment susceptible to tiny blips of current, a lightning protection system provides low-resistance paths to minimize the effect of direct strikes to your building.
  3. Changing views of liability - If there is a liability question today, changes are it will grow in the future.

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