If you want to add window flashing installation to the surface of a stucco wall, then you need to be prepared to do some extra work. Stucco is not like the ordinary walls, which are usually firmly positioned, but could be crumbly or problematical. When you are ready to start your window flashing installation, you first need to check that the stucco is suitable for use.
Checking the Stucco
You should be able to test the stucco easily by running your hand over the surface of the wall. If the stucco is in a poor condition, then it will flake and crumble. In this case, you will have to remove the stucco from the wall before you add the flashing, and then fit new stucco over the top.
Add the Flashing
Add some boards around the stucco, at 16 inch spaces. Then, start your window flashing installation by sliding the flashing down between the edges of the stucco and the wall behind it. The boards should keep the flashing from crumbing, and good stucco will hold together enough for you to push the flashing in between the edges.