Minimum OSB3 thickness

[PatsPatter]

Hi all
Could any of the Flat Roof Society out there advise me on the following:
I'm building a 5m x 4m log cabin and don't want to use felt/felt tiles on the roof. Having read quite a lot on here about EPDM, I'm very much inclined down that route - no pun intended.
The thing comes supplied with tounge-and-groove boards for the roof, 11mm thick. I'm assuming it would be best to up that a bit with OSB3, or similar, before applying (gluing on) the EPDM.
The question is this: What thickness of OSB would suffice.
I don't really see the point of loading the purlins - or my wallet - more the necessary by putting thicker than required boarding on it.

Willum
ps We've just bought a 1970s WRECK and this will be my workshop whilst it's in progress - then I'll sit on the verandah and drink beer when it's done Until then, expect to hear a lot more from me.............

 

[catlad]

I have only seen osb3 in 18mm

 

[oldbutnotdead]

I'd say go with felt as long as the roof is fairly accessible (ie you're not doing what I did- build a shed, roof it and then make like the Egyptians and roll it on poles to its final location).
Built my shed in 2006, the felt blew off in 2012 which cost me about £30 and an hour to replace, roof substrate was timber so it got a bit damp and then it dried, contents of shed still dry.
Unless the T & G supplied is chipboard------

 

[PatsPatter]

Thanks guys
The suppled roof is T & G spruce - not chipboard, thank heavens - but a bit thin (11mm).
A bit of research later and as catlad said, OSB3 T&G only seems to be available in 18mm.
I REALLY don't want felt, too many bad memories, so EPDM it is with 9mm WBP, or shuttering ply to top the planks. I've a mind to bond the boards with waterproof PVA, as well, I think.
Willum