To provide a method for reinforcing an external facing, capable of reinforcing an external facing of a building at a low cost and in a short construction period.

A method for reinforcing an external facing is used for reinforcing a slate roof 1 by forming a waterproof resin layer 3 on the surface of the slate roof 1 with a plurality of slate materials 1a disposed side by side. A first step of forming a foam adhesive layer 2 for bonding a waterproof resin layer 3 on the surfaces of the slate materials 1a by spraying a foamable resin material onto the surfaces of the slate materials 1a and foaming it and a second step of forming a waterproof resin layer 3 by spraying a non-foamable resin material onto the surface of the foam adhesive layer 2 are executed in this order to reinforce the slate roof 1.