[Answer] [misc.consumers.house] [Newsgroups]
None of the cities I work in, would allow two layers cemented together,
for
a substitute. How would two layers stop ice back-up? It would just
back-up
under the layer closest to the sheathing, and/or back-up on top of the
layer
closest to the shingles, which is perforated by the nail shanks. Granite,
it wouldn't back up between the layers. But, the damage would be the same
if
there wasn't any underlayment.
As a licensed & registered GC in the cities, I'm unfamiliar with State
Licensing which is required for plumbing/HVAC/electrical.
I have to take something back, when I said I "never heard of two layers
cemented together", I was mistaken. It seems to me, about 30+ years ago, I
recall troweling Karnak 19 to adhere two layers together. Had I had a clue
back then, I would've questioned how that would stop ice from backing
underneath the top/btm side of the two layers.
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