Thanks Ramon.
I have obtained slightly different results, maybe because I am using a shed roof of 0 slope instead of a slab for the flat part (because can’t extend to slabs as per Trimming Walls to Roof and Slabs, and Trimming Roof)
Problem 1 is 2 fold a) the rooves have a notch where they intersect. b) the walls don’t clean up.
Things get really out of whack when I extend a nearby (but unrelated) wall to the roof:
This GIF also shows that there are a couple of shards of wall shooting up at the intersection of the flat and sloped rooves. Perhaps this is because the top of the roof is what roof objects are designed to be aligned with, whereas on my project, which is a set design, only the ceiling plane is important so those are the intersections to which I snap the intersecting rooves (which leaves a gap in the top of the roof surface through which it seems the wall protrudes). Maybe this explains the notches in problem 1 Can I change this performance to attend to the underside of the rooves rather than the top plane?