Should flashing be installed over the head of arched windows in cavity walls? If so, how should it be done?
Yes. The masonry over the head of an arched window is just as susceptible to leakage as that over any window. Although water will run off the window head quickly because of the slope, it will leak at joints or unsealed penetrations in the window head. So flashing should be installed to direct water in the masonry above the window back out before it reaches the window head. Flashing over an arched window head is very difficult, however. Unlike windows with horizontal heads, arched windows do not have steel lintels with straight surfaces that make flashing easier. In an arched window, the flashing must be curved, and there is nothing to support the flashing before the window is installed. For these reasons, such flashings should be made from a metal that can be soldered, such as 16-ounce copper or 26-gauge stainless steel. Fabricate the flashings from two pieces to conform to the curve of the arch, soldering the curved joint between two pieces watertight. Another approach is to make the window head act as a flashing. The head and jambs must be watertight in this approach. The inside edge of the window head must be upturned and sealed to the backup to prevent water from flowing behind it into the interior.