roofing over board porch roof; sheathing necessary?

Our old farmhouse has an unenclosed porch that is roofed with what looks like 2x6 from underneath, then has a metal roof on top. The metal roof is in bad shape so we are going to put on a shingled roof.
Some of the baords need to be replaced, but after that, is it necessary to use OSB, since the roof will be solid, or can we use 30# felt to hide any possible small uneven places and then do the shingles? I even wonder about using a double layer of felt. It just seems like overkill to use the OSB, especially on an unenclosed porch, but I don't know how well the felt can level things out.
Thanks!
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you may need to install plywood or OSB if the existing boards are NOT 1 1/2" thick (usually it is only 3/4" - 1" t&g boards). otherwise the roofing nails will be sticking out from the inside of the roof boards. you will also need drip edge around the perimeter to cover the edge of the plywood or OSB.
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