On Wed, 3 Feb 2016 08:26:07 -0600, philo wrote:

I had a new roof put on 25 years ago that went over the existing
shingles. The new roof did not lie 100% flat. The roofer told me that
because the original shingles did not lie flat the new roof would not
either, but would eventually settle in.


That was common with roll roofing, but not worth doing with shingles.
Penny wise, pound foolish.