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Parallel chord trusses for vaulted roof

joenorm | Posted in General Questions on

New build, shed roof, and am planning to use 18″ deep parallel chord trusses for the the shed roof.

One of the reasons to use parallel chord is to get more depth for insulation but I am wondering if all the voids in the trusses will cause issues (air gaps)?

I suppose this would be a good candidate for blown in fiberglass or cellulose


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  1. Expert Member
    Michael Maines | | #1

    Joe, trusses and batts are not the best mix--it's better to use a blown-in product of some sort. My preference is cellulose, but fiberglass or mineral wool can also work. All of those require a venting strategy of some sort--do you have something in mind?

    You can get around the venting requirement by using open-cell foam, which meets code, but is not good practice in cold climates because moisture can accumulate at the sheathing. Or closed cell foam (preferably HFO-blown) but it doesn't fill voids nearly as well as open-cell foam.

    1. joenorm | | #2

      I'd like to do cellulose, but was a little concerned about the weight and expense.

      I think blown in fiberglass would be second.

      I was trying to avoid spray foam because of expense and I just don't really like the stuff.

      But it is a 2/12 pitch and as I read more here people are saying that it won't vent very well. At 18" i'd have room for a nice air gap.

      As you can see still trying to figure out the details.

      1. GBA Editor
        Martin Holladay | | #3

        A 2-in-12 roof is a low-slope roof. If you want to vent this type of roof, there are very specific requirements (include the need for one or more cupolas) that you probably haven't met. My guess is that you will need to make this roof unvented -- so you'll need either (a) rigid foam above the roof sheathing, or (b) closed-cell spray foam against the underside of the roof sheathing.

        More information here: "Insulating Low-Slope Residential Roofs."

        1. joenorm | | #4

          thanks Martin,

          This is news to me, I probably would have designed differently knowing this. Good news is the house is not built yet.

          But.....If climate and orientation are a factor, I think both are on my side. I will have a pretty consistent sea breeze pushing air into the low side of the assembly. Does being in climate 4c make any difference here?

          I see 2/12 vented assemblies here quite often, does this mean all of these roofs are likely wet? Or are we just playing it too close with these low slope numbers?

          I was really hoping to avoid spray foam for various reasons


          1. GBA Editor
            Martin Holladay | | #8

            Lots of cathedral ceilings have moisture problems and rot.

            Lots of low-slope roofs have moisture problems and rot.

            I'm conservative when it comes to designing these roof assemblies. Why break the rules? You want a robust, trouble-free roof.

            If your house isn't built yet, the obvious solution is to include an adequately thick layer of rigid foam on the exterior side of your roof sheathing.

  2. Expert Member
    ARMANDO COBO | | #5

    I would first ask your reason to use parallel trusses. Is it because needed depth of insulation? Aesthetics? Clear span distance? Would you be better served with TJI's? What's your climate zone and roofing material? I use both, depending on the above reasons.

    1. joenorm | | #6


      I went back and forth with this. I am using parallel chord mainly for aesthetics and depth to insulate. I do not like the look of a thick TGI roof overhang. I know there are ways to sister onto them but parallel chord, with a 2x6 top chord as a rafter tail seemed like a good alternative to TGI. I want to leave the underside of soffit exposed and painted, rather than covered with plywood or T&G. I have never used parallel chords before, but know people who have with good success. Even if they might be slightly more money than the alternatives.

      1. Andy_ | | #14

        Joe, The overhangs with I-joists are really not that hard to make. Strips of 3/4 ply or OSB as backer and web stiffener, then attach your 2x6.
        I did a 30" overhang with 2x6 since it will be a closed soffit, but if you wanted open you could use 2x8 or even 4x6 for a timber look.

        1. Expert Member
          ARMANDO COBO | | #15

          I like the looks of the TJI overhang because its proportional to the roof. I don't like when I see houses with shed roofs and a thin overhang, it looks to me like a guy with a pork pie hat, you want to smash it down some more... maybe there is reason that down here we use 10 gal. hats.
          Here is a detail for CZ3... Sorry, I don't know why it shows sideways! I guess is fixed

          1. Expert Member
            MALCOLM TAYLOR | | #16


            I agree about flimsy overhangs, but the downside of TJ's is you can't expose them as rafters. You have to have soffits, which for some styles of houses doesn't look great.

          2. joenorm | | #18

            If I went the foam rout I would just use 2x12 with foam over it. I was really trying to avoid foam, but I may have shot myself in the foot with a low sloped roof

  3. joenorm | | #7

    Climate 4c

    Roofing is metal standing seam

  4. Expert Member
    ARMANDO COBO | | #9

    IMO, parallel chord trusses are a great choice, but I would recommend to insulate with rigid foam on top of the roof decking, 3" R15 min., and 10" R35 min. ocSF under the roof decking.
    As much as I like cellulose insulation, its almost impossible to "hang" support 10" R35 cellulose on open web trusses, creating voids on the webs plus gravity will pull down the insulation creating voids that reduces considerably the Rvalue of the cellulose. Even if you try to fill all 18" cavity with cellulose, is very hard to achieve high density with a suspended cloth to avoid gravity's issues, thus having voids under the roof sheathing.
    We tried and failed miserably with that technique in New Mexico, with flat trusses, 15-20 years a go.

  5. joenorm | | #10

    With foam on top, do you need a secondary sheathing coarse, or can you put metal right onto the foam and fasten through?

    1. GBA Editor
      Martin Holladay | | #11

      Your metal roofing needs either purlins (for example, 1x4s or 2x4s, usually 24 inches on center, parallel to the ridge) or a continuous layer of OSB or plywood above the rigid foam. Contact the manufacturer of your metal roofing to find out what the metal roofing manufacturer requires.

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

      1. joenorm | | #17

        Curious why you suggest "parallel to the ridge"?

        Wouldn't that just damn any water that gets in there? I see the other method of the strapping on a 45 deg angle, that seems to make more sense.

        1. GBA Editor
          Martin Holladay | | #19

          Purlins parallel to the ridge work fine. Of course you need roofing underlayment between the top of the rigid foam layer and the underside of the purlins. Any condensation that forms in this air space evaporates harmlessly when the sun comes out.

          Most roofers hate the idea of working above purlins installed at a 45 degree angle, because it's a little tricky to determine where to put the fasteners.

  6. Expert Member
    ARMANDO COBO | | #12

    To install metal roof over foam, I would nail 2-2x4s "3" all way around the perimeter of the shed roof to use as nail base for the fascia and metal flashing, it matches the 3" rigid foam. Then I "encapsulate" the foam, nailers and 1" of the fascia with peel & stick WRB. Then I install 1x4's at 45° angle (not Horizontal) to allow for better venting and drainage, and metal screen or Cor-a-vent bug shield at the ends. The metal edge will cover the 1" WRB.
    You could install the 1x4's horizontal, but you need to create gaps for proper air flow and drainage; with the battens at 45°, you don't need to create gaps, so your battens are continuous.

  7. Expert Member
    ARMANDO COBO | | #13

    Check this video by Matt Risinger of Austin. This is what I'm talking about:

    1. joenorm | | #20

      I am curious how the line got drawn between the 2/12 and 3/12. One being "low slope" and the other just another pitched roof.

      I'm not really arguing about the points made here, just wondering how the conclusions are frown here out of curiosity.


      1. Expert Member
        MALCOLM TAYLOR | | #21

        The short answer is centuries of experience.

        Below 3/12 the mechanisms for draining water through gravity can no longer be trusted to work against the competing forces like capillary action, wind driven infiltration, and debris accumulation when using most roofing materials, and most methods of flashing which rely on laps. Similarly the slope below 3/12 makes ventilation ineffective.

        It's worth remembering that the difference between a 2/12 and a 3/12 roof is significant, and represents a much a much larger proportional increase in pitch than an increase on a roof going from say 8/12 to 9/12.

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