A Shed Roof is a roof with a single slope.  Shed Roofs are used in simple agriculture buildings (below) and state of the art modern buildings (right).  A Shed Roof can be made with trusses or rafters.  The length of the rafters are a function of the pitch (rise over run) and the span, which includes the two overhangs (front and back).  The number of trusses or rafters is based on the spacing and ridge length.  

In all cases, confirm your estimates with a professional estimate and adhering to local building codes.