OSB classes and exterior use

I'm confused about OSB grading and appropriate uses for it. Can anyone explain, or post links to useful refs?
There appear to be "classes" of use, including interior and underlying a flat roof (or in this case, a green roof). There also appear to be "classes" as a description of the quality (i.e. OSB2, OSB3) and the weather resistance of the boards themselves. However "class 2 use" (under a roof) seems to equate with "OSB3 class 3 materials". Class 2 materials aren't up to use in a class 2 scenario.
So what are class 2 & class 3 materials? What are they made of, what's the difference, how do I tell them apart?
What am I usually being sold? If the merchant says "Yeah, class 2, sell loads of it, make your roof lovely that. Never heard of class 3 though" do they mean material or usage class? Is this stuff really fit for roofing, or are they just being wrongly optimistic to sell the stuff?
Wickes list these products on-line (class 3 too), but they're only available in-store. Our local store doesn't stock them all, knows nothing of their merits, and they'd smashed the edges off the T&G anyway.
http://www.wickes.co.uk/OSB2/invt/110024
http://www.wickes.co.uk/OSB3/invt/110517
http://www.wickes.co.uk/OSB3-Polycoat/invt/164519
http://www.wickes.co.uk/OSB3-T+G-Roof-Deck/invt/190141
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Andy Dingley wrote:

GIYF
Everything you wanted to know and weren't too afraid to ask: http://www.wpif.org.uk/PanelGuide.asp
Annexe 2B and section 2.5 to start with.
also http://www.osb-info.org/index.php?lang=uk
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Thanks, that's a good site for it.
What still puzzles me though is what our local builder's merchant is selling. They claim that it's "class 2" and "suitable for roofing", which I can't make out to be either OSB2 or OSB3. When I question this, both the merchant and my builder give me the usual "What's patination oil?" look as if I'd asked for it to be studded with rhinestones.
Does _anyone_ actually bother with compliance to regs, or just basically knowing WTF they're doing?
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The sarking I used had 'OSB 3' and the makers name (from TP I think) printed all over it. I also used some offcuts of it to cover wet, unmade ground all winter without it disintegrating. Seems to have at least one side covered with a textured, clear epoxy-like finish.
Phil.
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That's what confuses me. OSB3 that I've seen, and that definitely _was_ OSB3, seems to have visible quantities of resin on it. Our local guy's stuff doesn't, it doesn't have any obvious way to distinguish it from OSB2
So should I trust the merchant's assurance that it's OK for roofing?
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Andy Dingley wrote:

Andy, it seems you're losing the will to live. Less than 3 minutes Googling produces this:
Construction Products Directive (CPD) for Structural Use.
http://www.wpif.org.uk/uploads/PanelGuide/06_%20Section%202_3%20BRE%20V3%2021_04.pdf Panel Guide (as in my last post) section 2.3:
"Service class 1: is characterised by a moisture content in the materials corresponding to a temperature of 20C and the relative humidity of the surrounding air only exceeding 65% for a few weeks per year.
Note: In service class 1 the average moisture content in most panels will not exceed 11%.
Service class 2: is characterised by a moisture content in the materials corresponding to a temperature of 20C and the relative humidity of the surrounding air only exceeding 85% for a few weeks per year.
Note: In service class 2 the average moisture content in most panels will not exceed 15%.
Service class 3: climatic conditions leading to higher moisture contents than in service class 2."
I'm no expert, but it does seem to be the case that OSB3 is suitable for service class 2.
According to this pdf: (top of page 2)
http://www.smartply.com/assets/files/pdfs/roof.pdf
All SmartPly OSB products are compliant with the Construction Products Directive (CPD) for structural use. Every board of SmartPly 3 is marked CE 2+ structural.
When I question

It seems in this case that the average Builder/Builder's Merchant has cottoned on to the fact that you need Service Class 2 product and OSB3 should be marked as such, so you don't need to explicitly know it's OSB3.
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Andy Dingley wrote:

Wickes seem to specifically employ blind fork lift drivers. I don't mind it TBH, often I only need a small piece of Ply/MDF/Chipboard, so I find a sheet thats been mullered on the edge & get it for half price.
--
Dave - The Medway Handyman
www.medwayhandyman.co.uk
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