"BETA-2K" writes:

I am about to have a new asphalt shingle roof put on my house. It now has
one asphalt shingle roof on it and the decking underneath is in good shape.

Other than it costing a little less by saving on the tear-off process, is
there any real difference between ending up with one roof or two?

Even if it costs more, if there is an advantage to having the present roof
taken off first, I would do that.

Where do you live? Does it get below freezing in the winter much?

I'm in nothern Illinois, USA. One reason I did a tear off vs nailover
was to get the iceguard sheeting that provides a lot better protection
against ice damming than otherwise. It's code here now, but wasn't
done on roofs 10 years ago. Another reason is that there were some
small signs of water penetration on the house when we purchsed it, and
given I wanna stay here awhile, I wanted to make certain there was no
rotting or anything more going on with the roof decking.

There are other modest advantages to a tear off which are hard to
quantify. One of which is that you'll have a lot less weight on your
roof. Not a big deal, it's surely designed to hold it though. The
second is that the new shingles are working on a much flatter surface,
so they'll look better in the case where you have any curling
occurring on your first layer. Finally additional layers can harbor
additional moisture. Whether either of these "soft" peace of mind
issues will amount to any real increased risk of water penetration
over the life of your new roof is probably debateable at best.

Personally I feel it's just technically a lot cleaner to do a tearoff.
However, if you don't plan to stay in the house very long and can put
the delta in $ to better use elsewhere, a 2nd layer isn't necessarily
a bad thing thing, though buyers may look slighly more favorably on a
tearoff vs a 2nd layer.

Best Regards,
--
Todd H.
http://www.toddh.net/