Lightning striking a house could cause structure fires as well as electrical damage to appliances and home electronics. In areas of the U.S. where thunderstorms are seasonal problems, lightning rods on the house could be practical safety measures. Not every home needs lightning rod protection. Isolated houses and houses in elevated, exposed areas run a greater risk of lightning strikes, but tall structures and tall terrain features nearby could give a home good passive protection.
Lightning Rod Theory
Lightning strikes when an electrical pathway is completed between polarized electrical charges in clouds and grounded sources. The current always seeks the path of least resistance, often to the tallest object in the area. Lightning hits projecting objects such as lightning rods more often than flat surfaces, such as rooftops. The electrical potential builds to a higher level on the projecting rod, according to the University of Missouri Extension. A wooden steeple with high electrical resistance bursts into flames when struck, but a lightning rod on the steeple conducts the bolt safely to earth through a grounding system. The metal lightning rod and ground network don't heat up enough to ignite a fire.
If thunderstorms hit your area frequently, consider other factors before investing in a lightning rod system. A site on the peak of a hill increases risk in comparison to a safer location on a valley floor. If you own a farm, protecting a tall barn or silo near your home with lightning rods draws lightning strikes away from the house, even though no rods were installed on the house itself. A building with lightning rods protects buildings within a radius equal to the distance from the tip of the rod to the ground, according to Michigan State University. Buildings twice that distance from the silo run a reduced risk of strikes.
In a naturally protected location or in a home protected by lightning rods, lightning could still cause indirect damage. Nearby strikes to power lines could travel into the home through the wiring. Surge protectors prevent damage by shunting excess voltage to ground. To protect home appliances, hire a professional electrician to install surge protection where power lines connect to the home wiring. Individual surge protectors safeguard sensitive home electronic devices. Any method of surge protection only works in a home with a proper electrical ground system.
A properly installed lightning rod system reduces the damage threat from lightning strikes by 90 percent. Air terminals -- the rods themselves -- must project at least 10 inches above the building's highest point. Rod spacing 25 feet apart along the roof peak offers the best protection. Grounding cables connect to any metal structure on the building exterior, including roof vents and gutters. A typical system terminates in two pairs of grounding rods 8 feet long, driven full length into damp soil. In less-conductive sandy soil, the best ground network is a continuous wire loop buried around the house, says the University of Florida.
- Michigan State University; Lightning Protection for Homes and Farm Buildings; Jon Althouse
- University of Missouri Extension; Lightning Protection for Missouri Farms and Homes; Kenneth L. McFate; October 1993
- University of Florida Extension; Protecting Homes from Lightning -- What to Do before Lightning Strikes
- University of Florida News; In Southeast, Ground Rods May Not Protect Homes Against Lightning; Aaron Hoover; August 15, 2002