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Community and Q&A

Rigid insulation on unvented roof

useful_girl | Posted in Energy Efficiency and Durability on

Location: Alberta, Canada. (Think: North Pole)
Project: started as an underground vegetable cellar – not sure now. A well-insulated shed anyways.
The building has foundations, floor, wall frames and now trusses. I had designed and built the trusses with no overhang because I was going to cover it with 22mil poly (pond liner) and cover it with earth. Can’t do earth. No equipment. But I want it well insulated. Putting rigid on top of the roof sounds great. I myself am the designer and most of the work crew.
The detail (found on this website) that is titled: “Details for a thick roof” is what I am going off of but I have some questions. I made my own detail to illustrate my questions. My trusses have a 3:12 roof slope.
1. Will a 21” overhang be too much snow load for the OSB cantilever to hold? Since I want to have 6” of Insulation, I am already going to need 4-2×4’s to sit on the edge of the OSB Plus the weight of the shingles…Do I need to worry?
2. Is it structurally important to add the soffit? I don’t need the ventilation and I won’t need to fill in the space because it will already have OSB under the roof surface – Would adding a soffit add integrity to the overhang (solving question #1)?
3. What about wind forces on the roof overhang? Is OSB strong enough to resist strong winds which might want to rip my insulated roof off?
4. Since the 2×4 furring on the roof is needed for an air space under the shingles, the furring actually sits right on the rigid insulation. With the OSB on top of it. Is this correct?
5. There is not a ridgecap on the furring so the air space goes all the way up one side and all the way down the other? And since the purpose is for air, I don’t close up the 1.5” space on the edges?
6. I can use long screws (10” –although that seems mighty long) down through all the Insulation & OSB layers to the trusses that are underneath, but rafters are only every 24” oc. In pictures of furring, it looks to be every 12”, do I install them at 12″, even though they only have OSB to secure them? How many screws – Every 6”?

Thank-you so much for your advice. Great website!

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  1. GBA Editor
    Martin Holladay | | #1

    OSB roof sheathing cannot be cantilivered. A roof needs roof framing. If you want an overhang, you will need to first install framing.

    One simple way to accomplish this is to build a series of triangles out of 2x4s with securely fastened corners. The horizontal base of the triangle is the soffit; the short vertical leg of the triangle is fastened to the top of your wall; and the hypotenuse (in your case) is 21 inches long. These triangles are installed 16 inches on center along the top of your wall.

  2. useful_girl | | #2

    Thank you Martin, I figured as much...but it was GreenBuildingAdvisor's "Thick Roof Detail" that made me think this. It says "Visit GBA to hear Mike Guertin & Peter Yost discuss these energy smart details" Could one of them comment on this?

  3. GBA Editor
    Martin Holladay | | #3

    That gray triangle under the OSB (in the soffit area) is a solid-lumber rafter. The OSB above the soffit is not unsupported.

  4. useful_girl | | #4

    Okay, good. Is there further framing around the edges of the insulation, other than the furring, that isn't shown in the picture? Is the insulation held down by other than the 10" screws?

  5. GBA Editor
    Martin Holladay | | #5

    The detail shows a stack of two or three 2x4s at the eave. (I am posting a similar detail that differs slightly from the one you posted.) These 2x4s frame the edges of the rigid insulation.

    The insulation is held in place by the top layer of roof sheathing (OSB or plywood), which is screwed through the rigid foam with long screws.


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