OSB2 or 3 for flat roof

Hi

I've just about finished my double skin brick shed and I'm about to do the roof.

My plan is for a warm roof so I'm going to lay the 18mm OSB3, then a Vapour barrier followed by 100mm celotex insulation boards. I'm thinking about putting another layer of OSB on top of the insulation before I finally lay a single sheet EPDM on top.

Can I use OSB2 11mm just to cover the insulation and put the EPDM on top? I can get hold of some OSB2 for a lot less than OSB3 which is why I'm considering it. My local merchant only carries the 11mm OSB2 and 18mm OSB3 but i think the 18mm is overkill just for a covering of the insulation.

Can someone offer some advice please? I would imagine the OSB2 would be fine but I'm no expert.

Thank you..

When I did such a roof I used OSB3, but only because from my supplier it was the same price as OSB2. I think the only difference is in how much it swells if it absorbs water (off the top of my head, 15% for OSB2 and 20% for OSB3, but someone can correct me on that). If you do the EPDM properly, the OSB shouldn't be getting wet. I don't see a problem with OSB2. You should definitely have a sheet of it on top of the Celotex, to stick the EPDM to, and 11mm is fine. If it comes to it 11mm probably fine underneath as well, if you've also got a layer on top.

Cheers
Richard

Thank you Richard,

I already have the OSB3 18mm and 100mm insulation ready, I was just wondering if OSB2 would be fine to go on top of the insulation as that's all my supplier has at the minute.

I've put the order in for the OSB2, thank you for your input.