Ventilation gap behind external cladding on timber frame extension.

Discussion in 'Building' started by RomanAbramovich, 7 Feb 2019.

  1. RomanAbramovich

    RomanAbramovich

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    On a diy basis I'm building a small ground floor single skin timber frame kitchen extension. Flat roof, soffit and facia are in place and have fixed external sheathing of osb, breather membrane and 25x50 vertical battens ready to accept cement board (finish is to be render).

    Plan is to fix mesh along the underside of the external 25mm ventilation void, once cement boarded and rendered, to prevent insects etc nesting, but I can't find any info on how to treat the top of the ventilation void - initial thoughts were to butt the cement boards up against the soffit, which will close off the void - is this correct or should the void be ventilated from the top also, to allow air flow behind the cement board?
     
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  3. tomfe

    tomfe

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    Normally soffits go on after.
    If you're worried about ventilation then drill some holes.
     
  4. ^woody^

    ^woody^

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    The top is more or less the same as the bottom, only you need to detail it to stop rain getting in. Typically this will be an overhanging eaves with soffit ventilation
     
  5. RomanAbramovich

    RomanAbramovich

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    Thanks for the replies.

    So to clarify, I should first remove the soffits, then fix the cement boards, render, fix insect mesh to the top and bottom of the ventilation void, then fix soffits in place?

    This will allow the ventilation gap to 'breathe' into the enclosed soffit void (which makes sense as it allows air flow).
     
  6. ^woody^

    ^woody^

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    Isn't this being built from new? How can you remove soffits that are not there?
     
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  8. RomanAbramovich

    RomanAbramovich

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    Yes it's a new build extension. I got the soffit and fascia on before fixing external vertical battens (at 600 centres), meaning the top of each batten butts up to the underside of the soffit board - if I continue as is, the top edge of the 25mm ventilation gap will be up against the soffit board, effectively sealed. Alternatively I can remove the soffit, fix external cement board, render, then refit the (trimmed) soffit board.
     
  9. ^woody^

    ^woody^

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    I'd just leave you with the principles as above, as it's not something that should be made up at this stage, but designed in at the start as it's quite important. 25mm continuous gap, insect screened, protected from direct or blown rainwater.
     
  10. Notch7

    Notch7

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    So the cavity should be open bottom and top?
     
  11. ^woody^

    ^woody^

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    Drained (bottom) ventilated (top) cavity.

    This should apply window cills, plus door and window heads too.
     
  12. RomanAbramovich

    RomanAbramovich

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    Thanks for the advice. Makes sense to ventilate both top and bottom. I'll get onto removing the soffit at the weekend!
     
  13. DIYnot Local

    DIYnot Local

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