A multi-purpose, shaped, prefabricated, structurally-enhancing support block conforms to the side profiles of conventional I-joists. The support block is preferably installed by fastening it to the I-joist web, rather than to the I-joist flanges, thus preserving the integrity of the I-joist flanges. Further, support blocks may be used in mid-span where additional point-load support is needed, or at joist ends for additional load bearing and/or enhanced rim board, attachment and nailing capabilities. The preferred support block includes a load-jack portion for bearing compressive load and a web and flange support portion reinforcing the thin web of an I-joist to help prevent buckling, rolling, or twisting of the I-joist under peak load conditions.