How do you install flashing at an arched opening over a window? This situation is a particular problem if the window is installed after the masonry work is completed.
How can flashing be installed in this case?
Flashing above an arched opening at a window can be difficult. If a true masonry arch is used and the windows will be installed after the masonry work is completed, I recommend using a stainless steel or copper sheet metal flashing that is rigid enough to support itself immediately over the wood centering on which the arch is built.
The flashing fabrication is constructed by soldering the curved horizontal leg of the flashing to a back plate. This fabrication would have end dams. The flashing would not have a drip on the front edge, but instead would be hemmed so that it is easily formed into a curve. In this way, when the centering is removed the flashing is rigid enough to remain in place. The window can then be installed beneath the arch and the sealant placed between the top of the window and the bottom of the flashing.
As is the case with any flashing over the head of a window, anchors for the window must not penetrate the flashing. Instead, the window must be anchored to the backup wall using straps or some other method.
I have seen many projects where a flashing is installed in the horizontal joint located several courses above the top of the arch. I do not recommend this approach because it is more noticeable and the masonry directly below this flashing is not flashed. Water penetrating through the masonry directly above the head of the window can flow down the wall and onto the head of the window, which potentially creates an interior water leakage problem.
In some projects, curved steel angles are installed above the window to support the masonry instead of using a brick arch. In these cases, the flashing can be installed on the top surface of the angle. This flashing must be specially fabricated.
Because the lintel supports the flashing, it may be possible to use a lighter gauge metal. In this case, the sealant joint can be placed between the window and the underside of the steel angle, or between the window and the underside of the metal flashing.