George wrote in :

Our house is built of more-or-less solid cement blocks (c 1920, we
think.) It has a lower roof that butts against the main wall, which
joint I need to flash. My plan for the counterflashing is to cut a slot
along the blocks, to accept the 'top' of some rake edge, with the other
face of the rake laying down over the step flashing.

A single kerf fits the rake edge, but snuggly. I can go with that, and
bed the rake in (silicon?) caulking. Or, I can cut a wider slot
(1/4-1/2"), and bed the rake edge in mortar.

Any thoughts on the pros/cons would be appreciated.

Thanks,
George

Sounds similar to flashing a chimney with flashing & counterflashing.
Your task is much simplier is my guess but cutting the groove is
discussed.

http://www.familyhandyman.com/DIY-Pr...g/Step-By-Step

Mortar will eat up aluminum flashing. Use polyurethane caulk. Not
silicone.