Gurus:

This summer we plan to replace our 24-year-old shingled roof with
long-lasting dimensional shingles. The old shingles are still in pretty
good shape and the house has had no ice damming problems.

Contractors are pushing two schools of thought for the old roof.
1. Tear it off and put down ice and water shield before putting on new
roof.
2. Shingle over the old roof and utilize the old flashing.

Which process would you recommend for the longest-lasting roof?

Thanks for your thoughts.

Tom
Milwaukee, WI