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Green Building Bible, Fourth Edition
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General: 15mm T&G OSB?

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  1.  
    • CommentAuthorShevek
    • CommentTimeMar 16th 2021
     
    Am I interpreting Samrtply's website correctly? Do they offer 15 mm T&G OSB?
    https://mdfosb.com/en/products/smartply-max-t-g
  2.  
    My reading of the web site is that the T&G OSB 3 comes in thickness of 15mm, 18mm and 22mm but not al sizes are available in all 3 thicknesses.

    So yes 15mm T&G OSB is available
    • CommentAuthorowlman
    • CommentTimeMar 16th 2021
     
    I find the name slightly confusing, "Smartply Max": with use of the acronym OSB kind of reduced to an afterthought. Stretching definitions to the limit, that's a little bit naughty, MEDITE, why; what's wrong with just OSB?
    • CommentAuthorShevek
    • CommentTimeMar 17th 2021 edited
     
    Posted By: owlmanI find the name slightly confusing, "Smartply Max": with use of the acronym OSB kind of reduced to an afterthought. Stretching definitions to the limit, that's a little bit naughty, MEDITE, why; what's wrong with just OSB?

    Because they have a number variations:
    https://mdfosb.com/en/products/smartply-osb
    • CommentAuthorShevek
    • CommentTimeMar 17th 2021
     
    Posted By: Peter_in_HungarySo yes 15mm T&G OSB is available

    That's what I thought. Great for roof sarking.
    • CommentAuthorrevor
    • CommentTimeMar 17th 2021
     
    I have never used sarking boards but my understanding is that they are normally treated softwood boards with a 2 to 3 mm gap so don't know why you need T&G boards I would look into it in more detail. Whilst OSB is pretty stable it does absorb moisture and on a roof will be exposed to quite severe climatic conditions from cold to hot particularly under slate so you need to allow for that .
    • CommentAuthorSimonD
    • CommentTimeMar 17th 2021
     
    15mm TG4 OSB3 may very well be available from the manufacturer. The question for most small scale builders is whether it's actually available from the timber or builders merchants. Normally it's 18mm board that's stocked for structural purposes and you may only be able to get the 15mm board if you buy by the pallet.

    Always best to check with the supplier.
    • CommentAuthorShevek
    • CommentTimeMar 21st 2021 edited
     
    Posted By: revorI have never used sarking boards but my understanding is that they are normally treated softwood boards with a 2 to 3 mm gap so don't know why you need T&G boards

    For wind tightness (in a vapour permeable roof build up). 15 mm is better than 18 mm because it's more vapour permeable.
    • CommentAuthorCWatters
    • CommentTimeMar 21st 2021
     
    I'm not an expert on vapour permeability but I was interested to see how 15mm OSB compares to say roofing membrane on its own. Google says..

    A vapour Permeable roofing membrane like VP400 has a vapour permeability of around 0.08 MN.s/g or 0.08 GN.s/kg.

    The SmartPly Smartply max data sheet says "150 (wet cup) / 240 (dry cup)". Google found a page which said a 147 Wet cup was about 11 GN·s/kg

    So 15mm OSB is about a factor of 100 less vapour permeable than permeable membrane.

    Have I made a mistake?
    • CommentAuthorShevek
    • CommentTimeMar 21st 2021
     
    Posted By: CWattersI'm not an expert on vapour permeability but I was interested to see how 15mm OSB compares to say roofing membrane on its own.

    Doesn't compare very well on racking strength ;)
    •  
      CommentAuthorfostertom
    • CommentTimeMar 22nd 2021
     
    (Without checking those figures) membrane @ 0.08 and OSB @ 11 are both very well at the vapour permeable end of the scale compared with something that's vapour impermeable like polythene. In other words both are good enough, subject, as in every modern case with high insulation level, ideally (but frequently not), to a proper WUFI check.
    • CommentAuthorborpin
    • CommentTime6 days ago
     
    Posted By: revorI have never used sarking boards but my understanding is that they are normally treated softwood boards with a 2 to 3 mm gap so don't know why you need T&G boards I would look into it in more detail.


    Sarking traditionally was sawn softwood to take slate nails.

    All houses in Scotland (used to be) boarded with OSB - improves racking strength I believe - with battens and counter battens to ensure airflow. I think the 'English' method of just using battens has become prevalent for large builders as it is cheaper.

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