On Mon, 25 Nov 2019 21:55:02 -0500, micky
wrote:

If one gets a new asphalt? shingle roof with 20-year Monsanto? shingles,
how long should it last? 20 years?

Mine is only 13 years old and has a small leak already. I can't see the
back half of the roof from the ground, but the leak is in the middle,
not by the chimney flashing.

I watched them put it on and they did a good job. They used famous name
shingles, though I can't remember if it was Monsanto, 3M maybe.

My Ex got about 20 years out of her "20 year" roof about 40 miles
south of you. Here in Florida we pretty much plan on about 70-80% of
the guarantee.
The guarantee is pretty useless too. It is the pro rated cost of what
you paid for the shingles against the list price and does not include
labor (the last time I read one).