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Housing - Renovation: No eaves to ventilate a vaulted roof...

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  1.  
    • CommentAuthorjemhayward
    • CommentTimeJun 16th 2009
     
    I've just discovered that I have no eaves ventilation gap at the top of my walls, the wall plate is flush with the bottom edge of the slates, so there isn't really any gap at all (you's find that hard to believe on a windy night). To comply with building regs i need to have 50mm below the felt, and a ridge vent (relatively easy) and eaves vents.

    Is there an obvious tried and tested solution?

    I'm thinking of:
    1. lifting the bottom edge of the lowest row of slates by 50mm and inserting spacers on each rafter.
    2. some form of forced ventilation system (will building regs buy that?)
    3. ventilation from one gable, along the top of the wall (can't really see that working for the far end 9m away)

    help! I wasn't expecting this and can't afford to re-roof.
    • CommentAuthorDavipon
    • CommentTimeJun 16th 2009
     
    Doesn't sound like a wall plate up to slates,the rafters would just be sat on plate if that was so & not birdsmouthed over. Do you have a soffitt ? It's possible there's a noggin between rafters to fix facia to, can you get a photo or do a sketch ?
    •  
      CommentAuthorali.gill
    • CommentTimeJun 16th 2009
     
    • CommentAuthortony
    • CommentTimeJun 16th 2009
     
    I would try if possible to fit over-fascia ventilators gently and carefully under the bottom slate.
    • CommentAuthorsinnerboy
    • CommentTimeJun 17th 2009
     
    not sure i am following your description fully

    this cataelouge will most likely give you a solution - possible the FV250 vent

    http://www.glidevale.com/downloads/Roof%20Ventilation.pdf
  6.  
    Surely there's a contradiction here between the ideal gap you need between the back of the slates and anything else, and the ventilation space you need at the eaves? The former is 50mm. The latter is roughly equal to a 15mm slot across the width of the building. You should therefore have only to cant your bottom row of slates up appr 15mm (not 50mm) unless the potential 50mm air-gap is severely compromised further up.
    • CommentAuthorjemhayward
    • CommentTimeJun 17th 2009
     
    To clarify: The wall plate is rather deep - probably 200mm and runs to the outside edge of the wall. the rafters are let into the plate, so that they end at the outside edge of the plate, and then the battens are above, so the gap is, at best, the thickness of the batten and I haven't got sprocket pieces. The eaves ventilation in the building regs seems to be 25mm not 50mm as I stated, but the reality is that the gap is not even a battens width. the Sprocket detailed in the link above is what I'd guessed at, and I think I can do that without too much aggravation. I'm waiting to hear the opinion of the BCO.
    • CommentAuthorDavipon
    • CommentTimeJun 17th 2009
     
    If you intend to sprocket the eaves you'll need to remove the two eaves slates plus two courses above,this leaves a really horrid job of replacing full rows of slates,you cant really do the favourite diy job of using a "tingle" all the way. If roof is sound you could take off facia & use a reciprocating saw to cut outside corner of plate off alongways as it were.Then replace facia.Otherwise you need to take a lot of roof out to reslate properly.
    •  
      CommentAuthorali.gill
    • CommentTimeJun 17th 2009 edited
     
    jem would recommend doing a search on here for sarking to get info on regs, products, etc.
    the 50mm vent gap is basically 25mm for the roofing membrane to sag between rafters allowing rivulets to form away from the timber and the membranes fixing punctures; then 25mm ventilation gap between membrane and insulation.
    the eaves detail is called clipped or flush eaves.
    take a look at the diagram fig.91 in this.
    looks feasible to just fix a dummy fascia as a spacer then the actual fascia over the face, so long as your tiles end up projecting over the gutter enough. or alternatively rebate a vent void into the fascia timber.
    either way there should be some form of continuous grille at the void. (with specific vent opening dimensions)

    http://www.vencel.co.uk/assets/pdf/16.pdf
    • CommentAuthorjemhayward
    • CommentTimeJun 17th 2009
     
    Having had a better look, its really even worse than I imagined. the wall plate extends to the outer edge of the walls, but the rafters are about 5cm in from the edge, so the last row of tiles rests on the edge of the wall plate. I think my only solution is to box in a void at the top of the wall (see pink on attached drawing) and ventilate either through the tiles (may have listed building issues) or from the gable end. I could cut away the wall plate, but its a long job, and I feel it may weaken the structure. Quite how the ivy and howling winds get in really defeats me...
    • CommentAuthorjemhayward
    • CommentTimeJun 18th 2009
     
    Just had another thought... Could I remove the existing felt from underneath and replace it with breathable membrane fitted in strips between each rafter, and then insulate with celotex the full thickness of the rafters, then my 140mm celotex beneath rafters, then VCL then plasterboard? i.e. convert my cold roof to a warm roof without removing the tiles?
    •  
      CommentAuthorali.gill
    • CommentTimeJun 18th 2009 edited
     
    Damn, i hoped that was going to be a photo attached.
    continuity of insulation
    condensation on underside of felt
    possible timber decay
    listed building restrictions

    looks like you need a building control officer with an understanding of historical structures to visit.
    sprockets on the rafters would work but raise other issues through altering the plane of the roof tiles - eg susceptibility to wind uplift, driving rain, etc, the effect will depend on location, exposure and existing roof pitch.
    • CommentAuthorjemhayward
    • CommentTimeJun 18th 2009
     
    I think sprockets would be a problem:
    1. the roof is shared with adjacent cottages so roofline couldn't be altered
    2. pitch is quite shallow (30) so rain could blow up eaves quite well

    BCO is reluctant to advise - happy to tell me I'm wrong, but not forthcoming on what is right - for understandable reasons.

    I'm warming to the idea of converting to warm roof, just would like to know if its been done this way before.
    • CommentAuthorjemhayward
    • CommentTimeJun 18th 2009
     
    I've now discovered Icynene - this may well offer me what i need. Has anyone got any experiences of Icynene?
    • CommentAuthorDavipon
    • CommentTimeJun 18th 2009
     
    Might be worth looking here as to why no to spray on foam!!
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4078
    • CommentAuthorjemhayward
    • CommentTimeJun 19th 2009
     
    The only "problem" that this highlights is the possibility of condensation due to loss of ventilation. We're going to be trying to achieve an airtight build, with managed ventilation. Surely sprayed foam is no different to "warm roof" construction, which is acceptable to building regs. and popular on new green builds?
    • CommentAuthorjemhayward
    • CommentTimeJun 19th 2009
     
    Update: BCO won't allow spray foam without sarking - that's not going to work, so we're back to modifying eaves to provide a bigger gap. This is ok and easy on the front, but the back is difficult to access... may do an experiment with a chisel!
    • CommentAuthormark_s
    • CommentTimeJun 19th 2009
     
    what about those vent tiles? We have some in a slate roof. Gray plastic the size of a slate with a raised hat that provides a covered vent.

    A row of them near the bottom of the tiles?

    Is there existing non-breathable felt?
    Could you remove it from the inside - creating a vented space through the slates and fix breatchable membrane to the inside edge of the rafters and your rigid insulation inside this?
    • CommentAuthorDavipon
    • CommentTimeJun 19th 2009
     
    I know it sounds like a lot of effort but cutting the corner off the outside of plate is a viable solution,removing facia is least disruptive to roofline,mains powered reciprocating saws are very effective in this situation,you cut into plate go as close to rafter as poss,turn saw & go back to other rafter then hack out waste with sharp chisel,you may not have to do that if you create enough air space!Replace facia after a lick of paint or whatever,done.Slates are a pain to refit after removing eaves course,you have to rip out "triangles"above the rows you remove then slate in leaving just one at top of each "triangle" to nail through or tingle.(or glue!)
    • CommentAuthorjemhayward
    • CommentTimeJun 20th 2009
     
    Vent tiles would contravene listed building "rules", and I spoke to company that makes breather membranes and described the idea of cut and patch retrofit, but they find that the membrane gets wet at the eaves and can then rot the rafters, so eves ventilation is still needed. I think its a saw, chisel and lots of foul language - I have my carpenter coming to look on Monday.
    • CommentAuthorjemhayward
    • CommentTimeJun 23rd 2009
     
    Carpenter has confirmed we could remove facia and guttering, cut slots in wall plate (bought B&D powerfile to do the job...) and then fix stainless steel mesh to keep things out. BCO happy with this so we now have a workable solution. Now got 18m of wall plate to file away!
    • CommentAuthorjemhayward
    • CommentTimeJun 30th 2009
     
    But the powerfile hasn't got the reach to the job from inside, so I may look at small chainsaw as alternative.
  23.  
    update: building control visited, and said for us to prop up the bottom slates a bit and put vents at the top, so thats what we've got.

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