Hey everyone!  Another doozy question here I cannot find any reference for. I have a rock faced block house. We are enclosing a second story back deck (partially to prevent leaking due to sideways rains). Turning it into a conditioned space.

The two side walls of the addition will butt the existing masonry wall and serve as inside corners for the addition. There is no possibility of removing the masonry blocks to tie everything together. They aren’t siding, they are the actual solid walls of the house.

The blocks are also not at all flat. They were cast to look like stone and have irregular ridges +/- 1/2″.  I know I need to put the flashing strip/tape/sheet behind the siding of the wood addition. That’s no problem. The question is, what flashing material would I use, and how would I attach it to the masonry block?

Is it possible (resonable?) to do 9 feet (times two) of vertical counterflashing (into the grout lines) like they do for chimneys with lead? I can’t even imagine what that would look like.

Caulk is right out, of course. Is there any kind of elastometric and/or epoxy mortar that I can put behind a mechanically fastened sheet of flashing? Something that won’t degrade in 10 years?

For the crevice between the two materials, is there an outdoor rated backer rod/rubber gasket that I could squish in and then seal with a liquid waterproofing membrane (like the Prosoco Fast Flash)? Or should I just pack it with mortar? Or is there a waterproof joint sealer that can go 1/2″ wide?

Any engineering advice and any specific products you can recommend, would be most  appreciated.

Thank you
Luba

Pic of the wall. Now imagine attaching wood framing where the gutter is in the left corner (but on the second floor…That’s what we’ve got.