As an architect, I perform periodic site observations during construction. On one project I noted that the PVC flashing was sagging slightly into the cavity. When I brought this up to the contractor, he stated that because the flashing was flexible, he could not prevent this from happening. Is this a problem? If so, how could it be avoided?
Flashing that sags into the cavity can be a problem. Water will sit in the gutter created. Over time this water will find its way through any defects in seals at flashing laps or any other defects in the flashing. The faster you can shed water away, the fewer problems you will have. The problem can be solved in two ways. One is to fill in the cavity space below the flashing with mortar or grout to support the flashing. The surface should be smooth, so that the flashing will not be punctured if any mortar droppings hit the top surface of the flashing. A second way to avoid this problem would be to use rigid metal flashing or a rigid metal end piece to which plastic flashing can be bonded. The rigid metal will support the flashing and keep it from sagging into the cavity.