If you already have a wood / felt roof, personally I'd just re-felt it and not worry. There will always be nooks & crannies in a wooden shed that mites can hide in, so whilst a felt roof isn't great from that point of view, neither is it always the source of the problem.
Felt is cheap and cheerful, there's a good chance you'll never get red mite anyway, and if you do, THEN you can replace it with something else for only the same effort you're contemplating now[size=78%].[/size]
Just my 2p worth.