A clamp for use with metal bar joists (or beams) has one or more steel angled bracket members or plates, adapted to fit under the corrugations of a corrugated steel roof. Each plate of the clamp is adapted to sit horizontally across a top of the bar joist perpendicular to the joist, and extend slightly over an edge of the joist. Each plate contains an elongate slot allowing a bolt to be inserted through the slot. A retainer heel clip is secured to an end of each such bolt. This retainer heel clip is tightened to the underside of the top flange of the bar joist using a washer and a threaded plate or nut, thus providing a clamping action. Each clamp has a vertical attachment tongue section of steel welded to it, including one or more elongate slots which complement perforations that occur along the length of a steel framing angle, allowing bolts to attach the clamp and the framing angle together. A pair of the clamps may be used to secure a pair of support angles for supporting a rod for hanging objects at desired locations within a roof support structure.