Abstract: A hydraulic winch system for use in erecting clear span pole type buildings including two rows of spaced apart, vertically placed poles defining opposite sides of the building, and a plurality of roof supporting truss assemblies arranged side-by-side and each assembly supported by cooperating pairs of opposed poles. The system includes a hydraulic motor-driven winch; a portable base mounting the winch and transportable from pole to pole; pulleys mounted at the top of each pole; a cable mounted on the winch, reeved about the pulleys and extending downwardly with its end adjacent the truss assembly preliminary to hoisting the same to its erected position; and connecting means on the end of the cable for connection to the truss assembly. Using the winch system, the component truss assemblies of the building are hoisted one at a time from a low level building-assembly position to an elevated building-erected position where they are affixed to the tops of the poles, preparatory to mounting the roof thereon.