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R-25 rigid insulated roof deck with cold metal roof above question

Dirk Gently | Posted in GBA Pro Help on

Zone 6 remodel in design phase. If anyone has links to similar design pics/blog building a cold/insulated roof it would be much appreciated.
Insulating roof deck to R-25 or greater with combo of reclaimed rigid foam.
I would like to then create a vent space (my question), followed by Plywood, then metal roofing.
What is the best way to create the vent space and how deep does it need to be (roof pitch varies 7 to 9).
is a criss cross pattern of 1×3 screwed to rafters followed by another horizontal to the ridge the best approach?

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  1. Expert Member
    Dana Dorsett | | #1

    For a cross-ventilated system it's easier and more rugged to use segmented 2x4s with 1' gaps through-screwed to the rafters. With a metal roof they perpendicular to the rafters, but you have to space the gaps carefully to account for for the roofing fasteners.

    If you go with 1x strapping instead, use 1x4 not 1x3s, since the latter will be more twisted & bent, and is FAR more likely to split.

  2. Dirk Gently | | #2

    OR would 2x4 screwed to rafters (thru foam) followed by plywood be a better approach?

  3. Lucy Foxworth | | #3

    This is the way we built our cold roof. We did not ventilate as we are in SC where there is essentially no risk of ice dams. I wonder if it would be useful for heat dissipation though. I will probably do it on the next time we have to re-roof.

    Here's the reference to my blog on how we did our roof. We did put a full layer of plywood and felt over the polyiso as well:

    This is an excellent article in JLC "Retrofitting an Insulated Cold Roof" Nov. 2008. It shows how they built a ventilation space in a cold climate.

    Another good reference on a cold roof:

  4. Sean Cotter | | #4

    Not to hijack, but Lucy, in your example do you feel the horizontal nailers set down in the second layer foam have any effect on the r value in those particular areas? I have a steep pitch cape style house with no overhang at all. I would like to add some overhang when we add a small addition but I have been puzzled on how that would work.

    In your case, you then added a gap and a cold roof roof over the top?

    In regards to the OP (and perhaps others), when you create the vented secondary roof deck, what does that final detail look like at the edges of the roof? I assume a standard ridge vent finished the peaks?

  5. Dirk Gently | | #5

    I do not plan to use the jlc idea of 2x4 purlins sandwiched in the foam layers for the loss of r value. My roof is small enough that driving 8" screws thru all layers of foam does not pose a problem.
    Adding overhang is a major goal for me also. the new layer of plywood roof deck o top of installed 2x4air gap should make this easy. the plywood can be overhung off gable ends and beefed up from. underside with "outrigger" type of blocking returned to sidewall.
    Not sure about soffit extentions yet.
    Thanks for the JLC LINKS. they hav a much better seach funcction on their site......r u listening Fhb?

  6. Lucy Foxworth | | #6

    I wouldn't have done it that way, but my carpenter is an old style carpenter who loves wood. If he can manage to squeeze in more blocking, more pieces of 2 x 4 he will (unless you are there to argue with him about it's necessity). He also has been a roofer in the past and felt strongly that we needed to have wood to put the next layer of sheathing on.

    Again, I'm in SC where a >R-50 roof is unheard of (rafters dense-packed with 10 inches of cellulose). So I was content with the way he did. The house is performing beautifully, with temperatures dropping 1-2 degrees in the winter and going up on a couple of degrees when it's seriously hot out.

    We did not do a gap. I was not aware of how that might be beneficial in this area. I think I would do it now because I think it might mitigate some heat. I have to look into that.

  7. GBA Editor
    Martin Holladay | | #7

    Most builders would find it easier to install 2x4s perpendicular to the ridge, with one 2x4 above each rafter, than to install two layers of 1x3s. I can't see any advantage to your suggestion of using two layers of 1x3s.

    For more information, see How to Install Rigid Foam On Top of Roof Sheathing.

  8. Dirk Gently | | #8

    Had read that article some time ago....but could not find it again. Thanks for link. Very helpful.
    Going with the 2x4 screwed to rafters with new layer of 5/8 plywood method. Metal roof on top of that.
    There are some good drawing details on how to construct the vented ridge cap on Cora-vents website.

  9. Howard Gentler | | #9

    Dirk: I just wonder why the extra decking. Why not go with strapping across the 2x4's for screwing in the metal sheets? The decking just seems to add expense, weight, and possibly reduce general air movement. What are the benefits?

  10. Dirk Gently | | #10

    I waffled back and forth on that.
    Main reason is that I feel it will be easier to add gable end overhangs by hanging the plywood out the desired amount. If I can be shown otherwise I am all for it.
    I have not installed much metal roofing so I am not sure how sturdy it will be for walking with only strapping below. I hope to never have to walk on it but want to be safe.
    The footprint for this house is small....but the walk out basement makes the roof very tall and I really dread lugging 22 sheets up there.

  11. Jason Hyde, Peterborough 6A | | #11

    Dirk, check with your roof panel installation guidelines. Some manufacturers allow an install over purlins, others require "solid substrate" - sheathing.


  12. Howard Gentler | | #12

    I thought the issue might involve the overhangs, and decking may provide added support, although using 2x4's as strapping would be more rugged than typical furring strips. I've never had to add overhang (yet and thankfully), but is surely a surmountable challenge for real carpenters. Depending how large an overhang, how steep a roof and how much snow load surely factors into how much support is needed for such add ons.

    If your roof is pretty steep you won't be able to walk on it anyway without some sort of assist (roof hook/ladder, etc). Much more slippery than asphalt shingles. But, if not too steep you can still identify where the purlins are, -by the screw heads and can walk there.

    What Jason mentions is correct. The most commonly used stuff up here has five 3/4" high ribs, and 3' coverage. It is rigid enough not to need decking: literature says either/or.

  13. Dirk Gently | | #13

    So I got some pricing for metal roofing and also verified that BOTH suppliers of the standing seam roofs REQUIRE plywood underlayment. I never asked on the exposed fastener metal roof. In case someone is wondering about pricing for materials only:
    Exposed Fastener metal is about $200 more than Asphalt shingles
    Standing seam metal is about $1,300 more than asphalt shingles
    Also doing the math on purchasing decent 1x4 purlins instead of plywood is only about $325 savings
    My roof is only 7.5 square.

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