Phisherman wrote:
Having just one and only one layer is best. ...

Of course it is (and where did I say or even imply it wasn't?) , but the
worst problem is still (generally) the more rapid deterioration of the
shingles themselves as opposed to actual risk of a roof failing. If
that weren't the case, considering the number of roofing jobs done every
year which are second- (or even third-layer), we'd hear of roofs
collapsing every where and we don't...

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