An apparatus for supporting upon an underlying surface has an adjustable backing plate structure. The backing plate structure has a main backing plate, first and second connecting racks extending from respectively opposite sides of the main backing plate, and a supporting member connected to the first and second connecting racks. The support member is in the form of a roller and is movable between a first position extending in parallel spaced relation to an end of the main backing plate and a second position contacting a bottom surface of the main backing plate. Each of the first and second connecting racks has a locating end received in a slot of the main backing plate so as to be adjustably slidable therein.