I'm getting estimates on a new roof. Two companies, recommended by people in the neighborhood, mentioned that they would install shingles on the existing ship lap deck. It's about 17 squares. The deck is in good condition and they would hand nail. Is that a good idea?

Everything I read on the Internet says you need plywood. Three other companies stated they would not do the job unless they sheet with 1/2" plywood.

Is it just a warranty issue?