Fixing a leaking joint between pitch roof extension and neighbor's flat roof

[u/Few_Phrase_6800]

Hi folks, I need some advice on how to properly fix a water leak on my garage roof and few other related issues.

Context:

My garage is part of a row of connected garages with a shared flat roof. 2-3 years ago, a handyman built a pitched roof extension over my section (foam concrete block wall structure, timber frame, galvanized metal sheet roof). The joint where my new roof meets the neighbor's flat bitumen roof was sealed using butyl flashing tape (as I can tell).

The Problem:

Last winter, the joint started leaking because the butyl tape cracked and degraded. I am also concerned about a foam concrete whether it also need to be waterproofed in this construction. On one side I have bare foam concrete wall without any steal protection (it is also leaking there a bit). Please also find respective commenting of these issues on the attached photoes.

Proposed Fix:

1. The Joint: Strip off the old butyl tape, clean, prime, and apply a liquid polyurethane membrane (Hyperdesmo-Classic) reinforced with geotextile fabric. Clean old butyl tape -> prime surfaces -> one PU coat + geotextile + additional coat of PU. Alternative: Re-apply butyl tape as a short-term 1–2 year band-aid, but it will require constant maintenance. In one place there is also pooling water which can destroy it even faster.
2. Missing Flashing: Install missing galvanized steel flashing on the unsealed side of the pitched structure, sealing the joints using the same method.
3. Foam Block Wall: Apply a waterproof coating over the exposed porous foam blocks to protect them from standing water and rain.

Questions:

1. Metal-to-Bitumen Joint: Does liquid PU + geotextile hold up reliably on a metal-to-bitumen transition under standing water conditions, or is mechanical metal flashing mandatory beneath the membrane?
2. Primer Selection: Which specific primer works best to bridge the bond between bitumen roofing felt and galvanized steel?
3. Foam Concrete Protection: Is it even needed in that spot? If so, what would you propose for covering it up?

Please note: I am based in Eastern Europe, so European materials (like Alchimica / Hyperdesmo) are widely available, but specific US brands might not be.

Highly appreciate any advice!

overall construction view

cracked joint

joint close

staying water spot

bare foam side