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Building Regulations and Standards: Ridge ventilation Details for pitched Green roofs

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    • CommentAuthordailampard
    • CommentTimeFeb 17th 2009
     
    Having searched numerous sites on the green roof issue I have failed miserably in my attempts to find any information / CAD drawings or indeed anything relating to venting pitched green roofs.

    It would appear (though I might be mistaken) that every picture of a pitched green roof I see on the net has no provision for roof ventilation ? Maybe there are provisions for eaves to eaves ventilation but ridge ventilation does not appear to have been addressed in any of the steeper pitched roofs.

    As a building contractor I am well aware of the problems of unvented roofs and have dealt with horrendous problems caused by such practices. The situation worsens in high vaulted/ cathedral ceilings with no loft space and the where requirement to vent each individual rafter space is essential. Maybe I'm wrong but surely ridge ventilation has to be a requirement in such pitched roofs?

    I know how I would go about providing ridge ventilation and tying in a roofing membrane but what I am really interested in finding out is why there is absolutely nothing on these sites about ventilation. Are these roofs being built without adequate provision and has it actually mattered in the end ?

    As a matter of interest , I recently spoke to the technical department of one major UK supplier of green roofing materials regarding ventilation and they didn't know / had never been asked and had no drawings or info. Am I missing something here or is this a subject that has just never been addressed ?

    Any comments or experiences welcome
    •  
      CommentAuthornigel
    • CommentTimeFeb 17th 2009
     
    Interesting question.

    Firstly you would rarely see a green roof with a steep pitch. The growing medium would tend to slide off and also water run off becomes a problem and the roof is too dry.

    Where the building has a loft then that would be ventilated at the eaves.
    Where there is a cathedral ceiling then you would need to allow a continuous 50mm air gap from eaves to eaves.

    I have not heard of anyone that uses ridge vents as you would have problems with the detailing and also preventing the vents becoming blocked over time.
    • CommentAuthorTimber
    • CommentTimeFeb 17th 2009
     
    Why not consider building it as a warm roof.

    That takes away the need for ventillation, problem solved?!

    Timber
    • CommentAuthordailampard
    • CommentTimeFeb 17th 2009
     
    Nigel - yes, I agree there's a limit to pitch but I've seen plenty of green roofs at approx 30 degrees. Eaves to eaves ventilation works ok on trad construction ( but not in all applications) to 35 degrees but that is with a loft space starting at eaves level , the situation is totally different with a sloping ceiling boarded to the ridge and only the 50mm air gap above individual rafter spaces allowing ventilation.

    You could detail ridge vents - lot of faffing about! - I'll send you a pdf or dxf of what I would consider if you send your email.

    Timber - Yes, you would think a warm roof might be the answer but having just spoken to Kingspan and Rockwool neither have information on the structure and neither could definitively answer the question- You could consider a warm pitched roof covered by a totally waterproof membrane the same as a flat roof with a "large bump" in it / Flat roofs used for green roofs remain unvented and seem to work presumably with a very good vapour barrier underneath.Water vapour would still need to be removed elsewhere in the building.

    Not saying that's not the way to do it but not yet convinced - besides which the addition of a substantial layer of insulation on top of the rafters creates yet another awkward detail in fixing on pitched roofs and one which I would prefer to avoid.

    Thanks - Keep the ideas going !
  5.  
    Posted By: dailampardFlat roofs used for green roofs remain unvented and seem to work presumably with a very good vapour barrier underneath.


    Since when...? ...only when the insulation is above the rafters think.

    J
    • CommentAuthordailampard
    • CommentTimeFeb 17th 2009
     
    James / sorry... confusing - replace flat roofs with" parapet flat roofs often seen being used for green roofs remain unventilated...

    Also consider those flat roofs added on terrace extensions where they all abut each other and the only vents are at the front of the roof - are those roofs truly ventilated indeed are they actually ventilated at all?
  7.  
    Has any one had any more thoughts on this discussion.
    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.
    • CommentAuthorgyrogear
    • CommentTimeOct 22nd 2016 edited
     
    Why not build an entirely sealed roof ?

    On the inside, you get an unventilated cathedral ceiling, and on the outside, no moisture could get into the roof structure anyhow (barring accidents / incidents...). Guess you'd just have to make sure that the structure does not contain any elements/materials that need to breathe...

    FWIW, here is a file that I put together from borrowed sources, a while back when I was poking around at my roof for other reasons...


    gg
    •  
      CommentAuthordjh
    • CommentTimeOct 22nd 2016
     
    Posted By: gyrogearWhy not build an entirely sealed roof ?

    The usual reason is that roof structures usually contain timber and perhaps other hygroscopic materials and that the roof and timber contain an unknown quantity of water when it is built (rain etc) so some means has to be provided for that water to escape. In most roof constructions (possibly all) there is also the possibility of a fault at some time allowing more rainwater in that also needs to escape. In the case of faults it is entirely likely that the fault will go undetected and/or unrepaired for a significant length of time.
    • CommentAuthorgyrogear
    • CommentTimeOct 22nd 2016 edited
     
    Posted By: djhroof structures usually contain timber and perhaps other hygroscopic materials and that the roof and timber contain an unknown quantity of water when it is built (rain etc)


    Granted ! However, I *did* say...

    Posted By: gyrogearGuess you'd just have to make sure that the structure does not contain any elements/materials that need to breathe...


    We lived for many years in an apt building roofed with a concrete shell. This was clad with timber decking and weatherproofed with zinc.

    For the OP, I was thinking of something waterproof, such as concrete skinned with polyester; or a glass-brick roof made up with concrete; or even a poured limecrete skin reinforced with bamboo.

    Or even truck windscreens, assembled over a geodesic stainless-steel structure.

    Granted, none of these would necessarily present a totally flat surface, but perhaps that could be turned into a design feature...

    Then all he needs is a heavy-duty rubberized membrane, and just make sure that none of his plants have spiky root systems...

    gg
    •  
      CommentAuthorfostertom
    • CommentTimeOct 24th 2016
     
    Posted By: julienbrown150mm ventilated cavity with vents only at the eaves.
    I am quite sure this is suitable
    Probably, with that voluminous cavity - but this kind of ventilation shouldn't rely only on wind blowing through - better to also enable stack ventilation, so at those times when the cavity is a shade warmer than outside air, buoyancy draws air in at eave and out at a higher level i.e. ridge. Recent research shows that stack ventilation is many times more continuous and effective than wind-driven (and in case of an 'airtight' interior is therefore the overwhelming driver).
  12.  
    Could you add a couple of "chimneys" at the ridge to ventilate the batten cavity and encourage the stack and Bernoulli effects?

    David

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