PURPOSE: To improve strength entirely for a building on repairing the roof for the building by a method wherein reinforcing materials for reinforcing the existing building frame and the roof bed are arranged and fixed, extending from the roof bed to the building frame.

CONSTITUTION: With an existing roof removed, reinforcing materials D are arranged in the form of diagonal bracing to sheathing roof boards 5 on the roof bed B and fixed continuously to a ridge beam, purlins and pole plates, and thereby strength can be improved for the building A and the roof bed B. Then, asphalt roofing β is laid all over the reinforcing materials D and a backing materials G1 on the lower tier is laid thereon, and a new roofing material F1 is mounted on the backing material and fixed to the roof bed B with fixtures α. Then, one end of a backing material G2 on the upper tier is mounted on the fixing piece 19 of the new roofing material F1 on the lower tier, and an insert edge 14 of a new roofing material F2 on the upper tier is fitted into a fitting groove 17 of the new roofing material F1 on the lower tier, and the new roofing material F2 on the upper tier is mounted on the backing material G2 on the upper tier. Such a process of work is repeated, starting from the eaves side and extending to the ridge side, and thereby a new roof E is formed.