A plank bridge over a stream; a ferry-gate. A roofing-board is called a clap-board.
A little low and lonesome shed, With a roof of clap-boards overhead.
Alice Cary: Settlers' Christmas Eve.
Probably a corruption of clath-board, a covering board, from Anglo-Saxon, clath, a covering, whence our clothes.
Boards for making casks are also called “clap-boards.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Clapper from Infoplease: