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:

• What is the average lifespan of a house foundation?

• What causes vertical cracks in fiber cement siding planks?

• What causes raised white lines of residue on a block wall that are crusty and crumbling? 

• What is the difference between soil subsidence, heave, creep, and settlement? 

• How much ventilation is required for the under-floor crawl space of a home? 

• What causes stair-step cracks in a block or brick wall?

• What causes a horizontal crack in a block or brick wall? 

• How can I tell if a diagonal crack in drywall at the corner of a window or door indicates a structural problem?

• What causes the surface of old bricks to erode away into sandy powder? 

• What are the pros and cons of concrete block versus wood frame construction?

• Should I buy a house with a crawl space? 

• Why is my stucco cracking?

• There's cracks running along the home's concrete tie beam. What's wrong? 

• What would cause long horizontal lines of brick mortar to fall out?

• How do I recognize serious structural problems in a house?

• What is engineered wood siding?

• Should I buy a house that has had foundation repair? 

• What is a "continuous load path”?

• Should I buy a house with asbestos siding?   

• How can I tell if cracks in the garage floor are a problem or not? 

• What do you look for when inspecting vinyl siding?

• Why is housewrap installed on exterior walls under the siding? 

• How do I recognize serious structural problems in a house?

• Why did so many concrete block homes collapse in Mexico Beach during Hurricane Michael? 

• How can I tell if the concrete block walls of my house have vertical steel and concrete reinforcement?

• What are the warning signs of a dangerous deck?

• How can I tell whether my house foundation problems are caused by a sinkhole or expansive clay soil?

        Visit our EXTERIOR WALLS AND STRUCTURE page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.