Have lived San Diego, Chgo, and now far North East US. In
our experieince: Two layers shingles SOP all places.
Warranties, materials AND labor AND installation identical
whether one or two layers. Only dependency I know of is a
deck inspection; if there is excessive rot, past leakage,
past fire/water/storm damage, etc., the roof must be
stripped. Inspections must include fascia and ?? (forget
the word) mounts, etc. for integrity of attachment. Usually
for a 30 yr warranty to be any good someone has to guarantee
the integrity of the deck or the 30 yrs drops to twenty or
even ten. Be sure to read all the small print.
If the contractor's warranties (not just one contractor)
differ based on two layers vs one, then it'd matter. If
not, assuming it's a good warranty, plus the warranty on the
materials, well ... why spend the extra money?
HOWever, after 50-60 years (two layers) there can be a
VERY expensive project coming up. Won't matter whether it's
one or two layers - so watch the warranties AND contractor
reputation! Do NOT let a newbie do it! You get what you
pay for usually, so you must pay attention to the details.

Pop


"Gustov" wrote in message
...
It may shorten the life of the new shingles, add a lot of
extra weight to
the roof, and there will be the extra cost to bear when
the roof has to be
reshingled again, because it will cost more to strip two
layers instead of
one layer. If I had a vote, I would vote to remove the
old ones before
doing the job. Hope this helps.


"Jeff R." wrote in message
...
Our Condo Association is in the process of reviewing
bids to have our
units
reshingled
with 30-year fiberglass (GAF or Tamko) over a single
layer of shingles.
One
of the
bidders strongly suggested that we strip the existing
shingles first,
claiming that if we
didn't we would decrease the useful life of the new
shingles by as much as
50%! Other
people I've spoken to say that 2-4 years is more likely.

Obviously we would like to avoid the additional cost of
removing the
shingles but not if
it means we would only get 15 years out of the new
shingles. Can anyone
provide more
insight as to the actual impact of NOT stripping first?

Thanks in advance, Jeff R.

--- Other Info ---
We are in the Midwest.
Believe the existing shingles are asphalt organic felt
(does this
matter?)
Some of the existing shingles have started to curl
slightly.