A tensioning apparatus for a flexible covering mountable to a frame structure having at least one frame member. The tensioning apparatus includes a sleeve having opposed end bores which are oppositely threaded. First and second stubs are fixedly mountable in opposed portions of the frame member with threaded ends extending axially outward into engagement with the bores in the tubular sleeve. Rotation of the tubular sleeve causes axial adjustment of the portions of the frame member relative to each other. In one aspect, a stabilizer rod is fixed to one of the first and second stubs and extensibily projects through the tubular sleeve and the other of the first and second stubs. A set screw may be optionally mounted in the tubular sleeve for fixed engagement with one of the first and second stubs to lock the tubular sleeve in a fixed position.