Yes 100mm insulation between the joists with osb top and bottom. I used plain osb screwed down. The grade of the osb is structural I think. It was the one with the better preservative treatment but even so I did give it a couple of coats of creosote or what passes for creosote these days. Seeing as the roof is covered by another waterproof layer you should only need protection from dampness.
My last workshop roof had bitumen coated fiber sheets on it and I really didnt like them. If it is the fiber ones they go soft in really hot weather and hard in really cold weather and after a few years they get very brittle and if you walk on them they break. You need to be really careful when screwing or nailing them down to get the right tension on the fixing so that its not too loose or too tight that it starts to crush the profle and makes it prone to leaks. Also used to have a large tree behind the workshop and in high winds some of the broken off branches punctured the bitumen sheets. You dont get these problems with the metal sheets. Metal stuff can also be repainted after a while to tidy it up again.