I have a window installation delayed due to finding the header above it that the window would anchor to being rotted a few inches up due to previously having been the place where screws from tracks for hurricane sliders were penetrated through the exterior stucco wall. What I'm calling a header is actually a layer (from inside working out) of 2x12, plywood, 2x12, plywood (and then the stucco). The inner 2x12 and plywood layer is actually fine, it's the outer 2x12, plywood, and stucco layer that is rotted. However, it is not rotted all the way up, as the shutter screws I mentioned were anchored very near the bottom of this. In terms of the rot and giving the window what it needs to anchor to, it would be possible to remove all of that if I simply cut away 3.5 inches up and inserted a 2x4 of the right length. So in total, I'm talking about replacing like a 3.5x36" corner of what in total is a 12x82-ish inch header board. I'll provide a picture of my idea below, with the red line outlining the 2x4 section to be inserted (to clarify, it will really be the outer 2x12 piece that this applies to, but it's easier for the picture to show it at this inner layer; and then of course redo as much outer stucco work as needed). My question is really, is this a horrible idea for the structural integrity of this wall or for any other reason?
