You'll do ok so long as you are NOT going over existing shingles. Those strip shingles need to lay pretty much perfectly flat to get a good seal. I noticed that on most of the barns in our area with strip shingles, the wind has managed to get ahold of more than a few shingles. T-lok was the best for ag barns, but that option is no longer available. Six nailing over a good solid deck is your best bet.