Here is a link to a short video in which I address this question of going over the old shingles: https://www.youtube.com/watch?v=bfD-0xvdJGc&index=39&list=PLMw9xVe_yEaVt4yv1i4LoDTlNKNd6boRd&t=35s
That said, you must make sure that you're meeting local building code requirements.
If you're within 5 miles of the coast, I would encourage aluminum rather than steel.
I hope this helps.
Please let me know if you have additional questions.
Todd Miller
2017-05-24 13:00:30.000000
Is it a good practice to apply metal tile or metal shingles over the existing shingles.
Which metal roof is best in this area.