Here's what I'd do.
Use roll roofing about the same thickness as the original.
Apply a good coat of roofing cement on the 2" you slide under the old
Roofing nails every 4" along the 20', at the overlap and the bottom.
Oops, I had to post the above reply from a different account and I just
realized that it showed up as being from "Jay-B" -- but it is actually from
me, the OP, a.k.a. "Ron".
Also, I just met with the person that I have doing the work and he thought
your idea of just using roll roofing, roofing cement, and roofing nails at
the overlap makes sense. So, we are skipping the idea of using the Liberty
self-adhesive roll roofing system etc. Your suggestion should work fine and
be much easier and less expensive to do.
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