A flashing assembly for reinforcing an interface defined between a roof and a portion of a building wall, either a flat roof or a flat roof with parapet wall. The flashing assembly has a first member configured for securing along the interface between the roof and the building wall and a second member for covering an upper part of the first member. The first member has an inner vertical part adapted for extending along the building wall, a locking flange extending at a right angle to the inner vertical part, and an outer vertical part extending in a substantially parallel orientation to the inner vertical part. In one aspect, the first member has an inclined bottom flange for diverting water away from the building while the top locking flange is secured to the roof. In another aspect, the flashing assembly has a top locking flange that is sealed in a groove formed in a parapet wall above the roof and a securing bottom flange, which is attached to the roof.