a beam supporting a roof的用法和样例：
- A rigid framework, as of wooden beams or metal bars, designed to support a structure, such as a roof.
- A joist is any of a series of small parallel beams supporting a floor or roof.
- An arbor or a passageway of columns supporting a roof of trelliswork on which climbing plants are trained to grow.
- A beam or rod that joins parts and gives support.