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Housing - New Build: Timber frame internal extenal junctions

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  1.  
    • CommentAuthorWC_B
    • CommentTimeSep 22nd 2026 edited
     
    Hi there

    We are designing a small timber framed extension with internal exposed timber frame and warm roof above.
    Rafters above main structure to extend outside to form a canopy.

    I have been going in circles about how to detail the moment where the timber rafter goes from inside to outside.
    Roofing suppliers are insistant on a VCL at the internal side of the insualion.
    Which ideally would form a continues lap from the roof to the curtain walling head.
    But getting this lap is very awkward with rafters going in 2 different directions.
    We want to leave the external rafters exposed from below.

    Has anyone detailed something similar and could advise how they have done or would do it?
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    •  
      CommentAuthordjh
    • CommentTimeSep 22nd 2026
     
    Welcome to the forum. :bigsmile:

    What type of insulation is it? And why does the VCL extend across the top of the window instead of connecting to the inside corner?
    • CommentAuthorWC_B
    • CommentTimeSep 23rd 2026 edited
     
    Thanks!

    Haven't spec'd the insulation yet but some sort of PIR.

    Yes I suppose the VCL does not need to run long the top of the window.
    I am not sure how best to run the VCL upwards from the inside corner to meet the VCL running horizontally under the roof insulation.

    I guess I am not confident in the practicality of installing a continuous lap from A to B and am wondering if I am missing a simpler solution..
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    •  
      CommentAuthordjh
    • CommentTimeSep 23rd 2026
     
    Is PIR not a vapour barrier on its own? So I'm not sure why you need one at all, unless you're not planning to seal the joints between the sheets?

    And what's the point of the VCL, and indeed the insulation, on the projecting overhang?

    You obviously can't run a membrane through the sheet material that the insulation rests on, and you can't run it through a continuous joist, so I think you need to consult whoever designed it to find out exactly what it is supposed to be doing.
    • CommentAuthorGreenPaddy
    • CommentTimeSep 24th 2026
     
    Perhaps I'm missreading your drawing, but it looks like the "roof rafter" which sits on the "ceiling rafter" (I'd call these all joists) are all covered by the acoustic ceiling. Then the exposed timbers sit below this ceiling board. Is that right?

    If that's the case, then could you not put the VCL at the underside of the "ceiling rafters", with the acoustic ceiling onto that? If the acoustic ceiling needs a void behind (for acoustic absorption) which the VCL will interfere with, then run the VCL in a toothed fashion, under the "ceiling rafters" and up their face, to the "roof rafters", across horizontally, and back down the "ceiling rafter" face, under and back up? This is how I normally do flat roof VCL's. Noggins can be an issue, if you have these, but I usually fit protective plywood patches over the VCL, so the noggin ends don't puncture the VCL when hammered in place and nailed.

    You'll never join a VCL on the outside face of the roof deck to a VCL internally, unless you stop the deck board to let the VCL pass down, and then continue the deck board minus the VCL - feels like a water ingress risk and tear point of the VCL.

    I imagine the insulation on the canopy is just a simple way to give the roof fall back to the internal gutter.

    VCL into the window reveal is not uncommon, as that can all be fixed and sealed before the windows arrive. Then seal the window frame to the VCL.
  6.  
    Declaring an interest: we're Build4less, a building materials supplier, but this is a detailing point rather than a product one.

    GreenPaddy is right that you can't take a membrane through the deck, but you may not need to. Over that short distance the deck itself can act as the airtight layer: dress the wall VCL up to the underside of the deck between the rafters, tape it to the deck soffit, and seal it round three sides of each rafter with airtight tape or preformed collars. Then make sure the deck joints over that zone are taped or glued, so the board links airtightly to the VCL on top. Planed, kiln dried or glulam rafters seal far better than sawn ones, and any shakes crossing the line will leak along the grain.

    On djh's point, foil faced PIR is very vapour resistant, but its joints and edges aren't reliably so, which is why roofing suppliers want a VCL on the deck. Their warranty usually depends on it, so I'd keep it.

    For the infill between rafters above the glazing head, I'd avoid PIR. Rigid board rarely fits tightly round timber and leaves gaps that need foaming, whereas a dense mineral wool or wood fibre batt friction fits and closes up around the rafters.

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