Flashing must be provided at the junction between a roof and a masonry wall or chimney. The base flashing should extend at least 150 mm (6 in.) up the side of the chimney or masonry veneer. A common approach is to bend the top of the base flashing and imbed it at least 25 mm (1 in.) into the horizontal masonry joints. The flashing is installed on all sides of a chimney where it penetrates a roof. This flashing method does not direct any water that is inside the masonry to the exterior, and this water may leak into the chimney or wall.
Stepped base flashing should be installed in pieces, following the roof slope. The flashing pieces (also called “flashing squares”) are overlapped like the roof shingles, and extend at least 100 mm (4 in.) under the shingles (Figure 124). One square is used at each shingle course and is bent up along the wall under the sheathing membrane.
Note: Flashing should also be installed at the intersection of shingle roofs and walls clad with materials other than masonry.