Lightning is a flow of electrical current between earth and sky. The result of this electrical flow can be millions of volts There is still much that is not known about lightning. but most experts agree that a path of ionization begins from the cloud and extends earthward. This path of ionization is the beginning of, and establishes the route for a lightning strike. The more dramatic part of the strike occurs when electrons race upward from earth-to-sky along this route. This is the flash of light with which we are all familiar. It is also the phase of the strike which poses the threat of damage. As is the case with any flow of electrical current. lightning will follow the path of least resistance. If the electrical charge is strong enough. it will also arc from one substance to another It is the resistance to the flow of this electricity which generates heat energy and can cause explosions, fires, and other damage.

In assessing the risk involved with lightning striking any structure, two different subject areas should be analyzed. The first has to do with probability of a strike: the second has to do with consequence of the strike. There is no measure known which can lessen the probability of a lightning strike, except perhaps physically moving the location of a structure.

The use of lightning protection systems may, however, lessen consequence of a strike should one occur.

The probability of lightning strike is determined by a number of factors:

1. The topography in the area of the subject facility. Probability of strike is higher if the project is located on a mountain top or bill top as opposed to a field

2. Size and height of the subject structure. A tall building or one covering more ground area is more likely to be struck than a short or small building. A tall, slender structure (such as a steeple or lighthouse) is also a more likely candidate for a strike.

3. Relative location of the subject structure with respect to nearby larger and taller structures. Presence of a very tall structure in proximity to a small, short —building will tend to further reduce the likelihood of a strike to the small building.

4. Frequency and severity of thunderstorm activity in the geographic area of the project.

As can be appreciated by reviewing the above factors, the probabilities of a strike to a metal roofed structure are no more or less than any other kind of structure, as these probabilities have to do with height and size of the structure and its surroundings, rather than its construction material.

In order to adequately assess risks involved with lightning events, the consequence of a strike must also be studied. In other words, what if lightning does strike a subject building - what will happen? Obviously, there is a potential threat to

human life associated with a lightning strike in addition to the threat of damage to either the contents of the building, or the building itself, or both. These threats are affected by the following factors:

1. The construction materials used for both framing and roof coverings. If these materials are (electrically) conductive, the threat of fire and explosion are both reduced, also reducing the threat to human life. If these materials are noncombustible the threat of damage to them is reduced, and they will not contribute a fuel source to any fire resulting from a lightning strike.

2. Physical contents of a building. If contents are flammable, or explosive, risks of the perils of fire are obviously increased. If contents are highly sensitive electronic or other equipment. highly valuable or irreplaceable items, then the consequence of loss is intensified.

3. Human occupancy. Buildings which are heavily occupied are considered to be at higher risk than unoccupied or sparsely occupied buildings. Also, the type of occupancy has a bearing. If a fire results from a lightning strike, the risk to human life is greater if occupants are handicapped or non-ambulatory and cannot be quickly evacuated.

4. Remoteness of the building. If the building is remote with respect to fire fighting and medical emergency response, the risks of physical loss due to fire as well as human perils are increased.

Because metal roofing is both an electrical conductor, and a noncombustible material, the risks associated with its use and behavior during a lightning event make it the most desirable construction material available.

This fact notwithstanding, and in view of the many variables which contribute to lightning risk, it may. in some cases, be prudent to consider lightning protection. A lightning protection system provides for a continuous conductor from earth to sky (and vice-versa) so that the electrical charge is furnished an obvious path through which to flow, thereby reducing the risk to (electrically) resistive construction materials and human life.

For additional information see:

NFPA 780 StaudardfortheInsta1Lad~m of Lightening Protection Systems.

1995 edition. National Fire Protection Association, Quincy~ Mass.