Their distinctive shapes form a signature for New Hampshire. Why the first covered bridges were built has been obscured by time. Some say that horses were more comfortable crossing an enclosed span than an open trestle. Others point to the fact that a bridge lasts longer if its heavy timbers are protected.
Covered bridges consist of a roadway supported on each side by a wooden truss and a roof.
The frame, made of heavy timbers called chords, is made of two high walls that span the stream with cross timbers top and bottom for the ceiling, floor and foundation. The chords were connected with trusses. Tree nails, called trunnels, made of hardwood hold the bridge together. Truss designs varied. Many bridges in the White Mountains utilize paddleford trusses, which were originated by Peter Paddleford of Littleton, New Hampshire.