Walls and Ceilings - Rotten Wall Under Window

We live in an 80 year old house in Los Angeles with plaster walls and stucco on the outside. We have a stained glass window on the second floor which is decorative and does not open. During last year's heavy storms water leaked in under the window and the plaster bulged and a small amount of mold grew on the plaster. We were able to find a hole on the outside stucco which we sealed (I hope!).

I finally got around to busting open the plaster and found, as expected, that the plaster keys were broken, and the wood lathe was crumbling.

I have been able to repair plaster before without difficulty (including replacing bad lathe), but I am concerned about the the wall studs and the cross beams that are holding the window in place. The ones I exposed felt soft to the touch, but were not crumbling.

I think this is a job that might be too much for me to handle, but want to get an idea of what a proper repair would entail. When I busted open the plaster I found three different types of plaster/compound, so it looks like this is an old leak that was not properly repaired in the past. What needs to be done? Thanks in advance!

I don't have much 'plaster' experience, but it seems to me that the real issue is the integrity of the framing. If the lumber is just soft from being wet it might be ok once it dries. My big concern would be whether or not the lumber is still sound. It makes no sense to cover up compromized framing.

Thanks for your reply. Like you, I am less concerned with the plaster repair than with the lumber framing. I guess my question is if there is any way to repair/reinforce the framing without actually reinstalling the window. It is old stained glass and looks very fragile. It would be irreplaceable if it broke. Is it possible to sister new studs to the old? What if the rotten studs are only in the area under the window where water had come in, but not in the side members of the framing?

It is a little hard to say for certain without being there to inspect the framing. I would assume that the only framing members involved would be those under the glass/leak. It may be possible to replace or add framing without disturbing the window, as you noted great care would need to be used to protect the window.

Sistering a 2x by unto another is only effective if the weak 2x is still fairly sound. If the lumber is rotten it should be removed.

I did a little more exploring and have found that most of the framing members appear to be sound. There are still some 2x's which feel soft, but I don't think they're wet. What about the epoxy wood hardeners I've seen? Are they effective? I'm thinking about applying some of this wood hardener to the two softer 2x's under the window which seem to be partially supporting it and then sistering new 2x's to these.

I've never used the wood restorers but it sounds like a plan :)