The following info is from "Covered Bridges Today" by Brenda Krekeler, copyright 1989: "Wooden bridges were covered with a roof and siding to protect them from the weather. Bridges left unprotected, deteriorated. The superstructure would last only a few years if exposed to the ravages of nature. The importance of the covering of the truss was recognized in 1805. The contract for the first covered bridge in the United States, built by Timothy Palmer over the Schuykill River in Philadelphia, required that it be covered to protect the wooden beams. Covered bridges have been discovered as far back as the 16th century in Europe." "At one time the United States had as many as 12,000 covered bridges. Today, an estimated 840 remain." Lois D