Dormer cheeks plywood type

Discussion in 'Building' started by Doubies, 14 Aug 2020.

  1. Doubies

    Doubies

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    Hi, my builder is currently building a flat roof dormer for me and has used 11mm OSB plywood instead of 19mm wbp plywood.

    The spec from the architect is below which details k-render but I have discussed with the builder and agreed tiles would be better (not sure if that is relevant information).

    "k-render fixed to expermet mesh to give key, fixed to 19mm wbp plywood sheathing fixed to 100x50mm timber studs (100x100mm corner posts). Include for 100mm thk Kingspan / Celotex insulation slabs fixed between studs with 500g Visqueen vapour barrier over studs & insulation."

    The builder tells me what he is using is better but I'm concerned. Should I be?

    Thanks for replies to all
     
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  3. JohnD

    JohnD

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    "11mm OSB plywood instead of 19mm wbp plywood."

    surely he said "cheaper" not "better?"
     
  4. ted456

    ted456

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    Yes.
    You dont need so called "wbp" ply - using OSB for side and roof sheathing is good enough.
    Dont even contemplate the k-render proposal go with the Tile Hanging option.
     
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  5. it’s good advice. what’s going on the roof?
     
  6. ^woody^

    ^woody^

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    Your architect has over-spec'ed the sheathing.

    But, as the contractor has priced based on the specification he should not be changing it without your consent, and in any case he should refund you for the cheaper cost of OSB.
     
  7. Doubies

    Doubies

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    Thanks for all the replies, a big help. With regards to the roof, so far the builder has put up the same osb plywood on the roof, although when I queried it he said he needed to double up the 11mm plywood that is on there. From what I can tell the architect has two sections in the spec relating to the roof:

    "New Flat Roof Dormer over Shower Room Provide and fix roof structure to new flat roof dormer over shower room consisting 200x50mm C24 flat roof joists at 400crs throughout to bear onto block wall to landing and onto top of stud wall (as shown on roof plan) together with firring pieces to give cross fall 25 - 75mm with 11mm OSB boards over the whole of the flat roof area prior to fixing of insulation & weatherproofing. Provide and fix further layer 100mm Celotex insulation slabs fixed between flat roof joists pushed up tight to u/s OSB board."

    and:

    "Provide and fix new flat roof covering to flat roof dormer over shower room to incorporate GRP fibreglass covering consisting tough weatherproof 'top-coat' slate grey over 2 layers polyester roofing resin with 450gsm fibreglass matting fully bonded to 19mm thk OSB3 T&G board mechanically fixed through to 50mm Celotex insulation board over 500g Visqueen dpm over 11mm OSB board. Include for all edge kerbs, upstands, trims, etc with GRP roofing system."
     
  8. ted456

    ted456

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    OSB is not Ply.
    Are you saying Ply had previously been fixed on the roof and now its proposed to add OSB to give you approx 22mm-24mm sheathing?
    I've never sheathed a roof that way but others on here might know better than me?
     
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  10. would not be happy with 11mm
     
  11. Doubies

    Doubies

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  12. Roystone

    Roystone

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    @Doubies How did you get on with this - we have the same challenge. Building Reg drawings say 25mm thick exterior grade plywood. Builder has used 11mm thick OSB3 board (and left plenty of gaps between boards - you can see daylight).
     
  13. ^woody^

    ^woody^

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    25mm is correct for roofing, especially if joists are at 600mm spacing. Possibly 18mm with 400mm spacing. Preferably plywood too.

    Definitely not 11mm OSB.

    There should not be any gaps either.
     
  14. Roystone

    Roystone

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    @woody I thought as much. The spacing is 400mm. Having checked the drawings again, it says roof at 19mm exterior ply, sides at 25mm exterior ply. The gaps are bad - one at the bottom of one one cheeks that runs for at least 50cm - I can fit my hand through. Called the joiner, who said - "don't worry, it will be covered with membrane, lead soaker and tiles hung".

    Plans also say single layer membrane roof bonded to 19mm ply. They have loads of GRP ready to put on the OSB
     
  15. ^woody^

    ^woody^

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    Gaps should not be there in the first place so as needing to be hidden

    The board supports the covering material (not the covering material hides the gaps), may perform a structural function (secures joists and timbers), and fewer gaps reduces air moment and heat loss.
     
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