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Green Building Bible, Fourth Edition
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    • CommentAuthorTC15
    • CommentTimeMay 1st 2021
     
    Would really love some advice on detailing the eaves on a warm roof.

    The build up (from inside out) is 100mm rafters, 9mm OSB, 135mm woodfibre, battens, purlins, steel roofing sheets.

    I've built up the ends of the rafters at the eaves where they overhang the wall insulation, to save on woodfibre. The first row of woodfibre boards on the roof lie along the top edge of these short rafter buildups, about 350mm up from the ends of the rafters (I copied something I saw in an EBS video, here: https://www.youtube.com/watch?v=RIBnzAlK7wM&t=235s).

    This leaves the edges of the woodfibre boards open at the bottom between the rafters - with only the fascia to reduce air movement.

    What would be the best thing to do here?

    I'm wondering about taping breather membrane to the underside of the eaves tray and continuing it around the front ends of the rafter, under the fascia, then beneath the rafter overhang and lapping the membrane on the wall (I'll be fitting a sofit). Would this be the usual thing to do? Is there a better approach?


    Thanks for any advice,
    Tom
    •  
      CommentAuthordjh
    • CommentTimeMay 1st 2021
     
    Posted By: TC15The build up (from inside out) is 100mm rafters, 9mm OSB, 135mm woodfibre, battens, purlins, steel roofing sheets.

    Something sounds odd there. Purlins normally support rafters. In the position you mention, I'd expect maybe to find counter battens, though they would normally be underneath the battens to provide a drainage path. What type of roofing sheets do you have? Maybe a drawing would help.

    I haven't watched the 45 min video I'm afraid, so I don't understand what your problem is either.
    • CommentAuthorTC15
    • CommentTimeMay 2nd 2021
     
    Hi DJH,

    Thanks for replying, and sorry not to have been clearer.

    I think I may have been using the word purlins incorrectly. I am installing vertical battens parallel to the rafters, fixed to the rafters through the woodfibre with helical fastenings. I'm then installing 4x2 horizontal timbers (what I was calling purlins) across the battens, at right angles, to take the corrugated steel roofing sheets.

    I took a picture earlier this afternoon. I'm new to this forum and unsure how best to share pictures - here's a Dropbox link, but do let me know if there is a better way to do this.

    https://www.dropbox.com/s/60yfez6c83rh03l/Eaves%20detail.jpg?dl=0

    You can see the bottom edge of the first row of woodfibre boards in the shadow. Obviously this void will be closed by the fascia, but this will not be airtight, and I am concerned about air movement here.

    I'm wondering whether to tape a further piece of membrane to the underside of the eaves tray, to run across the front of this void (behind the fascia), and around the bottom of the rafters to lap the membrane on the wall. But I imagine that there is a best practice to follow here, and wonder what that might be.


    I hope that makes more sense than the first time!

    Thanks. Tom
  1.  
    I think the void between the profile sheets and the woodfibre ought to be very well ventilated with lots of air movement, to remove moisture vapour which permeates through the lower layers? Otherwise it will condense underneath the steel sheets? So no need to block the air movement with extra membrane, in fact you need ventilation, just exclude birds and wasps.

    I also think that 'purlin' is a good description of the horizontal 4x2s !

    Edit: as it happens I was on our garage roof today trying to fix the leaks in the steel profile roof. It seems many of the heads of the fixing screws have rusted, releasing the compression of the rubber washer and allowing rain through, which has rotted the purlins, so the screws pull out, further releasing the the compression. I think it is less than 10 years old (looks like previous owner's handiwork). I might replace all the fixings with more rust proof ones, or maybe use a roofing membrane instead. Contributing factors are that it is under trees so covered in dead leaves, and is too shallow a slope.
    •  
      CommentAuthordjh
    • CommentTimeMay 3rd 2021
     
    Posted By: TC15I think I may have been using the word purlins incorrectly. I am installing vertical battens parallel to the rafters, fixed to the rafters through the woodfibre with helical fastenings. I'm then installing 4x2 horizontal timbers (what I was calling purlins) across the battens, at right angles, to take the corrugated steel roofing sheets.

    The vertical battens are usually called counterbattens, and the horizontal ones on top are called battens. But they are not normally as chunky as the timbers you are using, so you could call them common purlins I believe.

    I'm new to this forum and unsure how best to share pictures - here's a Dropbox link, but do let me know if there is a better way to do this.

    Dropbox is fine. You can attach pictures to your posts here, which at least means they tend to stay together. You just click on the Browse button underneath the word Attachments and choose an image file. The software has a size limit, which sometimes confuses people. I don't remember whether the limit is on file size or image size or what the numerical value is, but if you hit a problem then just reduce the size of the image (lower resolution) and you should succeed.

    I'm wondering whether to tape a further piece of membrane to the underside of the eaves tray, to run across the front of this void (behind the fascia), and around the bottom of the rafters to lap the membrane on the wall. But I imagine that there is a best practice to follow here, and wonder what that might be.

    It depends on where your principal airtightness layer is? You want a single continuous airtight layer all around the envelope of the heated space. If, as is often the case, this layer is at the inside of the construction (or near it - e.g. a VCL behind plasterboard) then you'll often have another layer near the outside of the construction, a breather membrane or suchlike, that keeps water out, lets water vapour through, and stops bulk air movement but doesn't need to be perfectly airtight. If your main airtightness layer is somewhere else, things will look a bit different.

    As Will says, other things to think about are making sure there is plenty of ventilation directly underneath the roofing sheets (above the breather membrane) and making sure that the fasteners for the roofing sheets don't corrode.
    • CommentAuthorTC15
    • CommentTimeMay 3rd 2021 edited
     
    Thanks again, Will and DJH.

    I feel that my flimsy grasp of the right terms may be getting in the way here. I've belatedly followed DJH's original suggestion and drawn all this out (attached).

    Will - The void I'm concerned about isn't between the woodfibre and the roofing sheets (which will be well ventilated) but between the ends of the woodfibre and the back of the fascia.

    DJH - Thanks for clarifying the situation with airtightness. I have a continuous vapour barrier inside the building. I've also been paying careful attention to airtightness outside the insulation layer.

    The thing is, I've been going to some effort to tape my breather membrane over the woodfibre, but there'll be a break in this between the roof, where it is taped to the top of the eaves tray, and the top of the breather membrane on the walls. Maybe this is fine, but it will definitely be drafty behind the fascia, and that seems to undo lots of the good work I have done in detailing the breather membrane elsewhere.

    The membrane is shown in green in the attached image.

    Thanks!
    Tom
      Eaves detail - drawing2 .jpg
    • CommentAuthorTC15
    • CommentTimeMay 3rd 2021
     
    The resolution of the drawing isn't very high when it's uploaded. The original is here:
    https://www.dropbox.com/s/dx4gtfluwoqzkdy/Eaves%20detail%20-%20drawing2%20.jpg?dl=0
    •  
      CommentAuthordjh
    • CommentTimeMay 3rd 2021
     
    Posted By: TC15The thing is, I've been going to some effort to tape my breather membrane over the woodfibre, but there'll be a break in this between the roof, where it is taped to the top of the eaves tray, and the top of the breather membrane on the walls. Maybe this is fine, but it will definitely be drafty behind the fascia, and that seems to undo lots of the good work I have done in detailing the breather membrane elsewhere.

    Thanks for the drawing; that helps me a lot.

    You could fit a piece of breather membrane between that at the top of the wall and the eaves tray. It would need to wrap around the ends of the rafters (or be carefully cut and taped around the rafters) but that should be possible, I think?

    Do you need some fire protection in the wall somewhere? I'm not sure of the regs but it looks like everything in there is potentially flammable.
  2.  
    Your description in the first post does not include sheep's wool between rafters, where the dwg does. Does this mean you have dropped the sheep's wool? Is it domestic? I can't see it achieving a U value of 0.18 for English Bldg Regs without 75-100mm between rafters (and then you have to think whether there's an interstitial condensation risk. Rule of thumb for hybrid warm roof is 1/3 of the R value below the 'deck' (OSB) and 2/3 above.
    • CommentAuthorTC15
    • CommentTimeMay 3rd 2021
     
    Thanks for sticking with this, DJH.

    Putting breather membrane from the underside of the eaves tray, around the ends of the rafters and lapping the membrane of the wall sounds like a good solution.

    This must be a frequently encountered problem, and I wonder whether I'm overthinking it. Is the industry standard just to ignore it (leaving out an external air tightness layer in this area)?

    Thanks for flagging the fire protection issue - I need to think more about that.

    Also, thanks Nick for your careful reading of my description of the build-up! There is sheep's wool between the studwork and rafters, yes. The insulation suppliers did their analysis, and I'm taking their word for it that I don't need to worry about interstitial condensation - though it's not analysis that I saw, or can do myself...
    •  
      CommentAuthordjh
    • CommentTimeMay 4th 2021
     
    Posted By: TC15This must be a frequently encountered problem, and I wonder whether I'm overthinking it. Is the industry standard just to ignore it (leaving out an external air tightness layer in this area)?

    I don't know what the 'industry standard' is I'm afraid, but generally ignoring the industry standard is sensible and sometimes even discounting the 'best practice' of normal trades too. :cry:

    I suspect that whatever is used to keep rodents and insects out of the nice warm cavity that has the added bonus of nice chewable nest material insulation is usually viewed as sufficient to stop draughts.
    • CommentAuthorTC15
    • CommentTimeMay 4th 2021
     
    Ha! Thanks!
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