When a roof intersects with a wall or another roof penetration, such as a plumbing vent, the roofing system should be turned up to make the junction watertight. The part of the roofing that is turned up is generally known as a “base flashing.” It may be made of the same material as the main roofing membrane or of a compatible material (see Figures 117 and 118).
Source : Canada Mortgage and Housing Corporation (CMHC)
Note : Where siding is used as cladding material, the base flashing is turned up behind the siding. Where siding abuts a roof, a space of at least 50 mm (2 in.) must be maintained between the siding and the roof surface. A cant strip should be installed where a built-up roof abuts a vertical surface to avoid a right-angle bend, which might rupture the roofing membrane (Figure 118). Membrane roofing is more flexible than built-up roofing, and usually does not require a cant strip. The roofing membrane should extend at least 150 mm (6 in.) up the wall and over the sheathing. Lap the sheathing paper at least 100 mm (4 in.) over the membrane.