I’m replacing some knackered 10mm ish T&G that’s had a rought time under some felt with holes in.

I’d thought OSB might be cheaper, but wickes have got ply sheets for £23.

Will standard ply be ok? hopefully the new felt shingles will keep it water proof, so do I need to consider external grade ply (which is plenty more money).

how long do you want it to last? osb isn’t suitable for a shed as you won’t be able to completely waterproof it, it will get damp and turn to mush within a couple of years. ply will be much better, 9mm wbp would be sufficient. you’ll get it for about 17 quid a sheet at Selco if you’re near one, or 12mm would be about 21 a sheet

That’s the answer I was looking for. WBP ply it is then. thanks.

WBP Ply is much more suited, will cost more but will last longer. If you can stretch to it then Marine ply is even better. OSB won’t last very long

Needs to be 18mm if your nailing felt shingles on.

OSB3 is just as good as regular (shuttering) ply; I roofed my stable with 11mm sheets, covered with cheap shed/kennel felt three years ago; still as good as day #1. If you’ve not got a decent circular saw or saw table, it’s worth getting it cut to size at the merchant.

zoiks, 18mm WBP sheets are quite pricey. Got a really annoying roof size which means lots of wastage. (1.6m x 3.8m)

OSB on mine. The kid’s playhouse is still fine after 14 years and 2 moves. To the point of walking on it. Staple the felt on rather than nail it on.

Wickes used to do bitumnen faced plywood, otherwise use shuutering ply, wear gloves and eye protection when cutting it spits quite bad, as for OSB, horible stuff, swells up goes soggy, and has really sharp edges and will cut you easily.

I hadn’t realised there were different grades of OSB. Looks like OSB3 is designed for humid use. I understand it will probably still swell if left out in the rain but if it is designed to withstand humid conditions then it should be fine as long as it is properly covered.
Felt shingles overlap the nails so should remain waterproof much longer than regular felt. So less worry about the roof material actually getting wet.