A covered bridge is a timber-truss bridge with a roof and siding which, in most covered bridges, create an almost complete enclosure. The purpose of the covering is to protect the wooden structural members from the weather. Uncovered wooden bridges have a lifespan of only 10 to 15 years because of the effects of rain, snow and sun.
Bridges having covers for reasons other than protecting wood trusses, such as for protecting pedestrians and keeping horses from shying away from water, are also sometimes called covered bridges.(Wikipedia)