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Protecting Buildings

Structures that are 50' high or less can be protected using a set of comparatively simple guidelines. Lightning rods are set at high points to intercept discharges, and wires conduct the electricity to the ground. As long as the rod, down conductor and ground are sufficiently alluring to the lightning bolt, the charge will be safely diverted to the earth.

That's the theory. In actual practice, even if the main charge is diverted to the ground, the tremendous voltage of the lightning strike and the incredible rate at which the voltage rises induce currents in anything around that's conductive. Though a lightning protection system will probably prevent fire in your home, there still may be serious damage to electronic equipment and some hazard to persons using telephones (who account for about 1% of lightning-related deaths each year).

Lightning rods:Each lightning rod ( air terminal in lightning lingo) protects a volume of matter shaped like a cone with its point at the tip of the rod. This is usually called the cone of protection. For houses 25' tall or less, the imaginary surface of the cone has a slope of 1:2. That is, it goes outward 2' for each 1 ' downward. Thus a lightning rod 25' above the ground protects a ground area described by a circle with a 50' radius as well as everything inside the cone. Any projection above the imaginary surface of the cone must have its own lightning rod. For buildings between 25' and 50' tall, the slope of the cone changes to 1:1, so the radius of the circle where the cone intersects the ground is 30' for a 30'-tall house.

It isn't necessary to cover the entire ridge of the roof under these imaginary cones—creating the residential equivalent of a porcupine. Rods may be spaced every 20' if they're between 10" and 24" tall, or every 25' if they're taller than 24". (However, other parts of the roof—including dormers and intersecting ridges—must fall within the cones.) Most rods are 24" tall so they can be spaced at 25' intervals but don't have to have the supports required for rods taller than 24". They should be located within 2' of the ridge or edge of the roof.

Neither television antennae nor wind vanes make good lightning rods. To offer adequate protection, rods must be at least 3/8"-solid or 5/8"-tubular copper, or 1/2"-solid or 5/8"-tubular aluminum. Likewise, no metal roof has low enough resistance to serve as an adequate rod or conductor-for which an object must consist of at least 3/16"-thick metal. What's more, it's suggested that any ornament on a lightning rod should have less than 20 square inches of surface area to prevent the rod from being blown down in the wind.

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