The Hancock-Greenfield Bridge (aka County Bridge) has offered a picturesque passage over the Contoocook River in southern New Hampshire since 1937. It was built to replace another covered bridge that had been destroyed in a flood. But why even build a covered bridge? In a word, longevity. The roof and walls help protect the timber supports from rot. For comparison, an uncovered wooden bridge lasts an average of 20 years, while the covered variety can reach 100 years or more.