A roll-up door has a bottom bar attached to the flexible curtain of the door. The bottom bar has transverse openings at each end adjacent to the jambs of the door frame. A breakaway shaft travels in a track in each jamb and has one end normally seated in a respective transverse opening by the interaction of spring loaded ball detents and detent recesses in the shaft and bar. The opposite ends of the shafts are connected to a counterforce mechanism that urges the shaft downwardly and are also connected to a strap wound with the curtain drum to urge the shafts upwardly against the counterforce. The breakaway shafts will separate from the bar upon impact to free the bar from engagement with the jamb and prevent damage to the door. The breakaway shafts may then be reconnected with the bottom bar and reset by unwinding the curtain to an overtravel position that is beyond the normal closed position of the door. A counterforce mechanism is also disclosed in which a counterweight balances the weight of the curtain on the drum and a spring attached between the counterweight and a tension line connected to the breakaway shaft holds the curtain taut.