What is Z flashing?
Friday, July 27, 2018
Although commonly called “Z” flashing, the Z has squared corners and it could as easily have been named “stair-step flashing.” We see it most often used at the horizontal joint between two sheets of plywood exterior siding, to provide protection from water seeping in at the opening. Because the end-grain wood at the top and bottom of plywood sheets easily absorbs and holds water, it is especially important to protect it, and avoid wood rot developing around the joints.
In the photo at the top of the page, it has been used between the bottom small piece of new plywood—installed because the end of the original panel had developed wood rot and delamination from the splash-back of rain at the ground from the roof overhang above—and the remaining part of the original sheet of plywood siding. When making a plywood siding repair like this, it is imperative to seal (preferably with primer and two coats of paint) the cut-off end of the original plywood sheet and both ends of the new one, unless you don’t mind doing the repair again soon.
The walls of a home have to be able to repel water just like a roof and, once you think of the exterior walls as a vertical roof—with lots of openings in it—that has rain beating against it and water draining down it, you have the correct mind-set to evaluate the installation of siding, and the trim that frames windows, doors and corners. The Z-flashing provides similar protection to a drip strip at the edge of a roof.
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To learn more about exterior walls and structures, see these other blog posts:
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