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  1.  
    I am planning to build a 30deg cathedral style sedum roof with breathable natural insulation. There will be a 150mm ventilated cavity with vents only at the eaves.
    I am quite sure this is suitable but cant find any way of "proving" it to building control. I cant find any suitable manufacturers recommended details. It would be nice to have something from BS or NBC saying that 150mm ventilation at eaves is as good as 50mm ventilation at eaves and ridge.
    Any help would be appreciated.

    Roof build up:
    1.Sedum
    2.50mm soil
    3.Oldroyd Xv20 green roof membrane
    4.roofing felt
    5.18mm OSB
    6.150x50mm secondary roof rafters - forms overhang and provides ventilated cavity - no ridge vents, only eaves vents
    7.Breather membrane
    8.15mm panelvent sheathing
    9.245mm timber I joists with innotherm insulation (breathable)
    10.low grade vapor barrier
    11.plasterboard
    • CommentAuthorringi
    • CommentTimeOct 6th 2016
     
    Will two opposite eaves be vented, so that wind to blow between them?
    • CommentAuthortony
    • CommentTimeOct 6th 2016
     
    dont use osb, use 25 or 28 wpd ply

    why low grade vb?
  2.  
    ''It would be nice to have something from BS or NBC saying that 150mm ventilation at eaves is as good as 50mm ventilation at eaves and ridge.''

    I think you won't have found it said because it probably isn't true. What mechanism will make 'entry only' ventilation 'scavenge' the closed ends at the top of the roof?

    I agree 150 is a marvellous ventilation gap, but I still feel you need openings at the top, unless there is a good breathable membrane, which of course there isn't, in this case.
    • CommentAuthortony
    • CommentTimeOct 7th 2016
     
    it cant have much of a pitch so as long as there is through or cross ventilation it ought to OK
  3.  
    That's my point, though, Tony - there is no through- or cross-ventilation:

    ''I am planning to build a 30deg cathedral style sedum roof with breathable natural insulation. There will be a 150mm ventilated cavity with vents *only at the eaves*.'' (my emphasis).
    • CommentAuthorEd Davies
    • CommentTimeOct 7th 2016
     
    Posted By: Nick ParsonsThat's my point, though, Tony - there is no through- or cross-ventilation:
    AFAICS, the OP doesn't make it clear whether the ventilation from one side reaches to the other, i.e., whether there's a ridge beam blocking the path across from one side of the roof to the other.
  4.  
    <blockquote><cite>Posted By: ringi</cite>Will two opposite eaves be vented, so that wind to blow between them?</blockquote>

    Yes it will be ventilated at both sides.
  5.  
    <blockquote><cite>Posted By: tony</cite>dont use osb, use 25 or 28 wpd ply

    why low grade vb?</blockquote>

    We were thinking of using low grade to encourage breathabilty/ vapor transfer. Using natural breathable insulation, so trying to make the whole construction make-up breathable.

    Do you think the OSB will not last even though it will be very much ventilated? I am a little concerned about the general moisture in the outside air and its effect on timber even when sheltered from rain and other direct sources of moisture.
  6.  
    Nick Parsons - "What mechanism will make 'entry only' ventilation 'scavenge' the closed ends at the top of the roof?"

    I guess I will probably end up with dwangs at the ridge for the secondary roof structure. I will have to dwang them in a way that allow air flow, perhaps 75mm alternating dwangs or herring bone style dwangs.

    I assume "scavenge" means "go around"?
    • CommentAuthortony
    • CommentTimeOct 7th 2016
     
    The weight of the green roof will bend osb, what are your joist centres?
  7.  
    Tony - 600mm centres
  8.  
    Nick Parsons - "What mechanism will make 'entry only' ventilation 'scavenge' the closed ends at the top of the roof?"

    I guess I will probably end up with dwangs at the ridge for the secondary roof structure. I will have to dwang them in a way that allow air flow, perhaps 75mm alternating dwangs or herring bone style dwangs.

    I assume "scavenge" means "go around"?

    Julienbrown, Sorry for my misunderstanding. I took it to be 'ventilation in' at the eaves, with no 'ventilation out'. If you are venting eaves to eaves, that should work in the way that a vast number of Victorian rooms-in-the roof work - a good howling gale over the top of the insulated roof lay-up.

    By 'scavenge' I mean pull (push) any water-vapour-laden air out of the space before it can condense. With closed ends scavenging won't happen. With 'throughput' as you describe, it will.
    • CommentAuthortony
    • CommentTimeOct 8th 2016
     
    600 centres is wat too far apart we use 300 centres for green roofs

    Definitely don't try to use osb and 600 centres.
  9.  
    Nick - thanks. Can you think of any kind of documentary evidence that I could use to convince building control that my ventilation arrangement will work? I cant find anything and i've been searching a lot.
  10.  
    Tony - thanks for flagging this issue up I will investigate further - my wife used to work for an architect's practice that did quite a few green roofs.
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