Are there any disadvantages to using a metal drip edge on a flexible flashing?
In most applications, I recommend using a metal drip edge with a flexible flashing. The drip edge provides a clean, straight edge to the flashing. There also are many applications where sealants must be bonded to the underside of flashing. Metal drip edges provide a surface to which these sealants can be bonded. If a metal drip edge is not used, sealants must be bonded to the flashing, which is difficult with many types of flexible flashing. In addition, flexible flashing is likely to be damaged when the sealant is replaced in the future. A rigid metal drip edge, moreover, can be used to direct water away from the wall surface, which isn't possible with flexible flashing alone. There are, however, several considerations that must be addressed when using a metal drip edge. First, lap-joints in the metal drip edge must be watertight. If the joints are not sealed, water leakage can occur even if the lap-joints in the flexible flashing on top of this metal drip edge are well made. It is also important that the flexible flashing be fully bonded to the metal drip edge, even if the metal drip edge has an upturned leg in the back. If adequate bond is not achieved between the flexible flashing and the metal drip edge, water that reaches the surface of the flashing may find its way through the joint between the flexible flashing and the drip edge. Rubberized asphalt flashings are commonly used in conjunction with metal drip edges to facilitate good bonding between the flashing materials. Rubberized asphalt flashings are self-adhering; when the release paper is removed and the surfaces are properly prepared, they will form a tenacious bond. A metal drip edge must have well-sealed corners. I typically recommend prefabricating the corners in metal drip edges with soldered or welded seams. Corners that are not properly sealed will leak.