Windows : Z-Flashing

The Importance and proper use of Z-Flashing

Time and time again I get calls to go out to a client’s home to take a look at water damage caused by water seeping in around windows. Sometimes people catch this early on and only the window trim has begun to deteriorate, and sometimes there is major water damage to walls, floors, and ceilings.

When water damage is occurring around windows, the FIRST thing I look for is if there is an adequate amount of high-quality sealant around the window and trim. The SECOND thing I look for is Z-flashing. Quite often, missing Z-flashing is the culprit.

If there is a window in your home that is installed in a wall with lap siding, AND the over-hang of your eaves don’t adequately keep the top of the window dry when it rains, then Z-flashing MUST be installed to protect from water leaks over time. 

Z-flashing is a metal flashing that takes the shape of the letter ‘Z’ when it is viewed from the side profile. It can come in various sizes to fit exactly as you need it for your specific application. I often order a piece or two from my local sheet metal shop specific to fit the needs of the job I’m doing, but you can also find it at Lowes, Home Depot, or your local lumber company.

Here is a link for Z-flashing at Lowes :


This flashing should be installed directly onto the top edge ofthe top piece of window trim. The bottom lip of the flashing should hang over the outside of the top trim board with the horizontal – or the middle – of the flashing resting on the window trim board. The very top fin of the flashing should be installed behind the house wrap. If there is no house wrap on your wall, the top fin of the flashing should be taped to the wall sheathing with a high quality window tape. Also, make sure the Z-flashing is slightly longer than the trim board. The flashing should hang past the trim board on each side by at least 1/4 of an inch.

Here is a link for an example of window tape from The Home Depot :

Finally, keep the piece of siding approximately 1/8 of an inch from resting on the lip of the flashing once its installed; this will allow you to get a good bead of sealant in th gap and keep the siding from wicking moisture. I recommend a high quality sealant around windows. SherMax from Sherwin Williams is a great product. Forget the cheap stuff. Quality matters. 

Here is a link to SherMax : 


James Wells
Project Manager
HRR – Home Repairs and Remodels


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