Covered bridges protected exposed wooden superstructures from rot caused by rain, sleet, snow, and sun, extending a wooden bridge's life span from fifteen to twenty years uncovered to as long as one hundred years with a roof and sides. Common misconceptions held that they kept heavy snows from blocking passage, soothed animals with a barn-like appearance, or protected travelers from the sun. Often built by farmers in a barn-raising style, they looked like barns but used several different truss systems. A truss is a triangular system of timbers in which each timber helps support the others, distributing weight and stress across the span. In 1805, the first American covered bridge was built in Philadelphia, Pennsylvania, across the Schuylkill River, with three spans and a length of five hundred fifty feet. The first covered bridge in North Carolina was built across the Yadkin River near Salisbury in 1818 by a local plantation owner using Ithiel Town's lattice truss. During the nineteenth century, there were thousands of covered bridges throughout the United States and hundreds in North Carolina.