Ok, Issue The First was the roof. Here are the existing roof joists and where they tie into the original house. You can just see the old rafter ends that formed the soffit. We had to open up the entire run of that masonry wall so we could see and get to every roof joist.
Here's a side view. The roof joists are nailed into a wooden nailer strip that runs across the face of the masonry wall. Yep, it's a 1x6 and the joists are fastened at each end with a single nail. Each and every one. Yeah, that's not going to work for me. At the bottom left you can see the old chimney to the original house. I'd tear it out but it's structural and at this point I'm less concerned with losing that space than I am opening up that can of worms. It's there and structurally sound. That's good enough for me.
What to do? Well, there are a variety of ways to address this but I'm unwilling to just tear the old roof down and start the whole thing from scratch. I decided on a compromise, a new load bearing wall. I started by putting in a double top plate. The first board is nailed directly to the joists and the second to the first. The top plate runs the width of the room and is fastened to every joist.
Then I put in a sill plate which is fastened to the slab with the Hilti gun. 16 inch on center studs then run from top to sill plates.
Here's a better look at the double top plate. I always use a double top plate on all load bearing walls. In fact, most building codes require it as a minimum.
Top plate, sill plate and studs in place. After I address one more roof issue I'll put in the horizontal bracing and fire blocks. This wall will be the North wall of both the new bathroom and closet. There will be both plumbing and electrical runs that will have to go in later and it needs to be complete before that happens.
The view of the old closet space. The roof joists are now supported by a load bearing wall that's fastened at top and bottom securely instead of by a single nail into an inadequately sized nailer board. The wall does eat into my space a little but that's a small price for proper structural stability. Honestly, I had a feeling about what I was going to find here a long time ago. When I walked on the roof I could feel it give a little. Disconcerting is putting it mildly.
The roof joists are 2x6 dimensional lumber over an 18 foot span and are spaced 24 inches on center. That's marginally adequate but certainly not ideal and needs to be modified a little. To address the last of my roof concerns there will be some new 2x6 ceiling joists going in that will tie into the roof joists and that new load bearing wall. When that's done I can begin taking down the interior walls and start framing in earnest.
Yesterday was a big day and by the end of it both Lu and I were fried. This is hard work, if you've never done it before, but totally within any good DIYers reach. Don't be afraid, just have a plan, understand there will be unforeseen issues you must deal with and start tearing stuff out. It's cathartic.