A system for blocking an edge of a building roof structure, e.g., roof structures of cementitious material poured over a corrugated deck or roof structures of rigid insulative panels supported on a corrugated deck, comprises an elongate U-shaped base member for affixation to the deck along the roof edge, an elongate cover member of inverted U-shape for affixation over the base member, the base and cover members being made of galvanized steel or other suitable non-wood material and, as assembled together, forming an auxiliary perimeter structure on the roof structure for border containment of the roofing material.