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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorlineweight
    • CommentTimeDec 17th 2020
     
    Illustrated is where a warm roof (ply deck on joists, then all insulation above that, with membrane type roofing on top) meets an externally-insulated timber frame wall. So, timber frame (some insulation between studs) then sheathing then a continuous layer of insulation on the outside. In this case the wall is shown with tile hanging as the external face - so there are vertical counter-battens, fixed into the timber frame with through-insulation helical fixings, and then the tiling battens are fixed to that.

    The combination of warm roof and externally-insulated wall is attractive because it (in theory) allows a near continuous layer of insulation to go around the corner.

    In practice - at least, built the way I have shown it, the insulation is bridged by the ply roof deck where it has to run out over the wall insulation. That's not a big deal in itself, I think, but it always raises the question of quite how that cantilevered bit of roof is supported.

    In the past, I've seen metal angle brackets as reinforcement, fixed into the outside of the stud wall sheating, and the underside of the deck. Of course, that's potentially adding some bridging, and I feel is a bit messy from an installation point of view.

    Another option might be to double-up the plywood for a portion around the perimeter of the roof... but that then becomes awkward in laying insulation over the small step it creates.

    In the example illustrated, maybe you could take some support from the counter-battens but that's not really what they are there for.

    Or... you simply take the view that the plywood deck can cantilever 100-200mm without problems and stop worry about it.

    Interested in any thoughts or ideas as to the best solution to this. It crops up in many variations of what is drawn - for example, it could be a blockwork wall with EWI and the same basic question/problem is there.
      Screen Shot 2020-12-17 at 15.56.48.jpg
    •  
      CommentAuthordjh
    • CommentTimeDec 17th 2020
     
    Posted By: lineweightyou simply take the view that the plywood deck can cantilever 100-200mm without problems and stop worry about it.

    Obviously it depends on the thickness of ply! The external walls of my whole house are carried on cantilevered ply. 25 mm ply and not cantilevered for 100-200 mm though.

    I would have thought that the design load would be a person standing on the edge of the roof - which ISTR is 1 kN. We needed some sprockets under our overhangs - the roof surface is 18 mm ply plus metal roof on top, overhang varies from 300-600 mm.

    Doubling ply will probably work, as will inserting sprockets or other brackets, which can be timber rather than metal if you prefer.
    • CommentAuthorlineweight
    • CommentTimeDec 17th 2020
     
    Posted By: djh

    I would have thought that the design load would be a person standing on the edge of the roof - which ISTR is 1 kN.


    That would make sense.

    I wonder if I can persuade a structural engineer to actually calculate this for me some day.

    What did you use for sprockets on your 300+ overhang? Did you manage to do them in a way that didn't bridge the insulation layer?
    • CommentAuthorCranbrook
    • CommentTimeDec 17th 2020
     
    Are your joists running perpendicular to your gutter/overhang?

    I've done similar recently and just ran my joists as far as I wanted the over hang. On the sides that run parallel with my joists, I had no overhang
    • CommentAuthorlineweight
    • CommentTimeDec 17th 2020
     
    Posted By: CranbrookAre your joists running perpendicular to your gutter/overhang?

    I've done similar recently and just ran my joists as far as I wanted the over hang. On the sides that run parallel with my joists, I had no overhang


    Then each joist would be a big thermal bridge across the insulation... which is what this is trying to avoid.
    •  
      CommentAuthordjh
    • CommentTimeDec 17th 2020
     
    Posted By: lineweightWhat did you use for sprockets on your 300+ overhang? Did you manage to do them in a way that didn't bridge the insulation layer?

    I don't exactly remember but they're something like 100 x 100 curved glulam about 1-1.5 m long. They run back into the insulation just under the top surface plywood and are tied to purlins at the inboard end. It's a fairly complex roof structure - 450 mm deep split into three layers of curved rafters and straight purlins and noggins, so there are only isolated points of solid timber from top to bottom. It's all filled with warmcel.

    Timber beams aren't too bad as thermal bridges go.
    • CommentAuthorlineweight
    • CommentTimeDec 17th 2020
     
    I guess with timber beams it depends what the overall insulation is. If it's a thick wall buildup with mineral wool or wood based insulation then in relative terms they are not so bad.

    But if they are bridging 100mm of PIR then I think it's a bit different.
    • CommentAuthorGreenPaddy
    • CommentTimeDec 18th 2020
     
    maybe fix 50x50 (or 75x50 on flat) on top of the ply, say 1m long, into the ceiling joists below. That way your getting a smaller cross section of cold bridging, and 50mm top cover?
    • CommentAuthorlineweight
    • CommentTimeDec 18th 2020
     
    Posted By: GreenPaddymaybe fix 50x50 (or 75x50 on flat) on top of the ply, say 1m long, into the ceiling joists below. That way your getting a smaller cross section of cold bridging, and 50mm top cover?


    Yes, that's an option I've thought of, although unlikely to be popular with builders as it would mean quite a bit of fiddling around with different thicknesses of insulation to do it.

    It would also raise a few issues with how to do the VCL layer that most insulation/roofing suppliers want you to run continuously underneath the insulation and on top of the deck.
  1.  
    The 18mm ply is stiff enough on its own, but it will act together with that timber behind the fascia to form a 'L' beam, which will be very stiff without sprockets etc*. You can try this with a mock-up of scrap bits.

    I have a 1950s racing dinghy built of 5mm ply, the stiffness and strength come from the hull and deck bits of ply acting together in 'L', 'C' or box shape sections. The rigging tension load at the foot of the mast is about 1tonne / 10kN.

    *Except maybe at the ends.
    • CommentAuthorGreenPaddy
    • CommentTimeDec 18th 2020
     
    Not sure about needing different thickness of insulation, just check out the locations of the 50x50 into the 100mm insul.
    Also not sure about the VCL going on top of the ply deck (as opposed to the room side), and how would you get continuity with the wall VCL?
    • CommentAuthorlineweight
    • CommentTimeDec 18th 2020
     
    Posted By: WillInAberdeenThe 18mm ply is stiff enough on its own, but it will act together with that timber behind the fascia to form a 'L' beam, which will be very stiff without sprockets etc*. You can try this with a mock-up of scrap bits.

    I have a 1950s racing dinghy built of 5mm ply, the stiffness and strength come from the hull and deck bits of ply acting together in 'L', 'C' or box shape sections. The rigging tension load at the foot of the mast is about 1tonne / 10kN.

    *Except maybe at the ends.


    I can see that the 'L' beam will be very stiff in the direction along its length - but does the addition of that timber actually make it any stiffer in the direction perpendicular to that?
  2.  
    Yes, it spreads out the weight of the 1kN person, over several metres of ply roof edge.

    (And over several sheets of ply)
    • CommentAuthorlineweight
    • CommentTimeDec 18th 2020
     
    Posted By: WillInAberdeenYes, it spreads out the weight of the 1kN person, over several metres of ply roof edge.

    (And over several sheets of ply)


    Ah right, I see what you mean.
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