A recent project involved hoisting some lumber through an upper window. The lumber brushed the sill and it came apart like wet leaves. I already knew it was bad but hadn’t realized how dire the situation was.

I ended up ripping the thing off, but I’m a bit confounded about what to do for the repair. There doesn’t appear to be any flashing beyond a perfunctory caulking of joints. The tar paper appears to be just trimmed around the edges of the rough opening. I’m not really sure how to integrate any solution for adding a new sill that doesn’t either ignore the problem or potentially make things worse.

The underlying problem seems to be that the sill catches too much water and doesn’t get enough sun to dry out. See the picture attached for how slim the overhang is. An additional problem is that the head flashing could be nonexistent, which means water is probably leaking down behind the WRB.

I see a few options here but I’m curious what others think:
1. Remove all of the trim and see just how rotten the framing is and do a proper job of re-framing, flashing, etc. the cheap, single pane window. – I don’t have a ton of experience, so this feels really daunting.
2. Put in a piece of solid lumber, try to work something like a sill flashing with a tape or fluid applied product, caulk the joints, and come back on another day with more time and money. – This feels like the right thing to do, but I’m not sure if the extra effort will make a difference.
3. Don’t worry about the flashing, just shove a board in there and caulk it, without a proper flashing around the whole window and a better overhang, a bang-up flashing on the bottom of the window is just lipstick on a pig.

Let me know what you think!