of vertically disposed channels 42 are shown welded to the face of channel 18 these serving as spacers so that channel 10 is inward of the position in which it would be if the spacers were not present and channel 10 were against channel 18 for stair assemblies wider than the assembly herein described the spacer channel 42 can be ommited or made narrower and for stair assemblies narrower than the assembly herein described the spacer channels can be made wider other structures than channels may be used to adjust the spacing between channels 18 19 to manufacture a stair assembly 10a through use of the apparatus shown in fig1 the two stringer channels are placed across channels 30 31 and channels 40 41 at opposite sides of the space between channels 18 19 tread pan riser elements are placed one at a time between the channels 18 19 adjusted in position and clamped in place by operation of cylinders 33 34 to drive the cylinder shafts inwardly thereby pressing channel 18 or spacers affixed thereto against the outer side of the stringer 10 to clamp the tread pan riser element firmly in place between channels 18 19 the clamped element is next welded at opposite ends to the channels it being preferred to first connect a tread pan riser element at one end of the stringers another element is then placed against the first element clamped and welded not only at its ends but also to the first installed tread pan riser element this procedure is repeated for each tread pan riser element until all of these elements have been welded in place to the stringers and to one another a.