A purlin bracing system for metal building roof includes braces which have pairs of tabs extending from one end. A brace is arranged perpendicular to a purlins, and its tabs are inserted through slots in the central web of the purlin. Another brace is then applied on the opposite side of the purlin, and headed pin is dropped through aligned holes in the tabs to secure them on opposite side of the purlin's central web. No tools or special fasteners are required.