"Bruce L. Bergman (munged human readable)"
wrote in message

If you want to get fancy you cut holes sideways at the root of each
blocked channel and weld in two or three chunks of mild steel tube
to
cross over to the clear channels on each side. Water goes down, then
takes the detour to the sides, then out. And even if one or both
clogs with leaves it still can't get over the sleeve.

-- Bruce --

I'd put it at the ridge where water drains away from the added
flashing. Then you need only seal around the flue pipe.

jsw