Our 1890 house has slate asbestos siding, which I think was installed over the original wood lap siding, but possibly that was torn off first.  The house has three  roofs that are only over first-floor porches and a bay window, so these roofs intersect the walls between the first and second floors.  These roofs need to be replaced.  One detail is puzzling me — how to do the flashing?

The flashing needs to go up behind the slate (and building paper, I think) and over the top row of shingles.  There are some straight sections and some sloped (where step-flashing would normally be used).  But the slate siding is very brittle, it can’t be easily pried up on the bottom edge like lap siding, and can’t be removed unless you start at the top of the second floor and work all the way down.  I don’t want to just rely on caulk as flashing!  Is anyone aware of techniques or materials that address this type of problem?