Properly tying roofing materials in at curbed transitions is an important application method. This detail is often encountered at area dividers, firewalls, and expansion joints. It is required when one section of roofing is being completed and the adjoining area is not being worked on.
The flashing application in the completed roof area should be in accordance with the manufacturers requirements using the manufacturers approved materials. The point of contention is the tie-in point at the opposite end of the curbed transition. Application varies based on the covering material. If a metal cover is applied over the curb, most manufacturers allow their material to be applied to the top of the curb and terminated.
If there is no metal covering, the best practice is to apply (fully adhere) the new flashing over the top horizontal point of the curbed material. The existing flashing material can then be set over the new flashing down the face of the curb at the existing roof area.
In remedial construction it is best not to disturb the existing flashing in the area that is not being replaced unless maintenance or repair work is required. Please note the manufacturers typically only warrant their materials to the top of the flashing. However, most leaks occur at the horizontal top of the curb so it is essential proper waterproofing is applied at this point.