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Community and Q&A

Vented “over-roof” sheathing

user-5453257 | Posted in Green Building Techniques on

I’m building an unconditioned workshop with a cathedral ceiling (i.e., no attic) under a shed roof.
The planned roof stackup, from inside to outside:

Exposed TJI Rafters @ 24″ OC
1/2″ plywood sheathing w/ h-clips
fully adhered WRB, continuous with wall WRB
2 layers of 3″ of Roxul ComfortBoard (R-24 total)
2×4 purlins parallel to ridge, connected to rafters via long screws (these purlins provide a ventilation channel to prevent ice dams)
Snap Lock metal roof (e.g., attached to purlins

My question is whether I need an additional roof deck + underlayment on top of the insulation.
Drawings of “vented over-roof” assemblies:

show this layer, but I don’t understand why it was added.

Is it an air barrier on top of the insulation to prevent windwashing?
Assemblies using rockwool as exterior wall insulation w/ a rainscreen gap don’t seem to have windwashing problems (e.g.,

Is it a water barrier?
Why isn’t the fully-adhered WRB underneath the insulation sufficient?

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  1. GBA Editor
    Martin Holladay | | #1

    The use of mineral wool insulation on the exterior side of roof sheathing for a sloped roof assembly is an experimental, thinking-outside-the-box approach. I've heard that it can work, but don't be surprised to discover that you are the first person on your block (or conceivably, in your state or province) to try this approach.

    If you have a ventilation channel above the ComfortBoard, you'll get some windwashing. According to most reports, the effect of windwashing on mineral wool insulation (especially denser types of mineral wool) is less of a problem than the effect of windwashing on fiberglass insulation.

    Are you the roofer? If you are hiring a roofing contractor, verify that the contractor is OK about your plan to install the roofing over purlins instead of solid sheathing.

  2. Jon_R | | #2

    I wonder what a purlin over Comforboard is rated at in terms of PSI. Walls see less horizontal pressure while roofs gets walked on, snow loads, etc.

  3. Expert Member

    Just a couple of minor things:
    - You might think about omitting the H clips. They aren't required by code and as your sheathing will be separated by the insulation from the purlins, it doesn't experience any point loading between the roof framing.
    - How are the ventilation channels parallel to the ridge ventilated, or are they really more there as a capillary break?

  4. Expert Member
    Michael Maines | | #4

    Comfortboard 80 has compressive strength of 439 psf at 10% compression and 1065 psf at 25% compression. The relationship appears to be linear. 50 psf is a reasonable design load; with 3 1/2" furring 16" o.c., that's about 1.2 psi, which translates to about 4% compression. With your 6" roof insulation, that's about 1/4" deflection. I probably wouldn't want to risk that on a client's house, but if it was an outbuilding at my own house I would give it a try.

  5. user-5453257 | | #5


    I'm leaning towards mineral wool over foam because of wildfire concerns.
    If it works for the walls, I don't see why it wouldn't work for the roof too.

    I plan on doing the roofing myself, which is one of the reasons I'm doing a snap-lock standing seam instead of a mechanically seamed one.
    The "snap clad" panel I linked to can be installed over purlins.

    Jon + Michael,

    The roof loads should be transferred to purlins, which then transfer them to the sheathing via screws --- in theory there shouldn't be much load on the insulation whatsoever.
    That's the theory, anyway.

    Do you think a second layer of sheathing on top of the insulation is needed simply to provide a flat surface for roofing? It doesn't seem to me like it'd necessarily be any easier to flatten plywood than purlins.


    Good point about the h-clips, I'll bring that up with the structural engineer.
    TBD on ventilation details --- probably just screened openings at the sides of the roof.
    If you're wondering whether the air needs channels that go from the lower side to higher side (rather than parallel to the ridge), I'm relying on Martin's answer to a previous question that parallel-to-ridge channels are sufficient (

  6. Expert Member

    The screws securing the purlins don't carry any load, except for a some shear to prevent the roof from sliding down and off onto your backyard, and occasional uplift due to wind. So Michael's calculation of 1.2 psi on the mineral wool is pretty accurate. Given that he suggests you could be looking at a 1/4" compression, which would leave the screw heads poking up into your metal snap-lack panels, it might be an idea to countersink them 1/2" as a precaution.

  7. Expert Member
    Michael Maines | | #7

    Kevin, your roofing manufacturer may or may not require continuous sheathing as underlayment. Again, on my own house I probably wouldn't worry about it, but if you want the manufacturer's warranty to be valid, you need to follow their instructions. I've used both double-lock standing seam and rib-panel "agricultural" metal roofing on skip sheathing but I don't know what the snap-lock companies require.

  8. GBA Editor
    Martin Holladay | | #8

    One more point: building codes require the use of roofing underlayment (for example, asphalt felt or synthetic roofing underlayment) under roofing. I'm not sure whether your local code enforcement official would care whether the roofing underlayment was directly under the roofing -- but it's possible that the official might.

  9. user-5453257 | | #9

    Thanks for the help everyone!

    I just realized that since the workshop isn't going to be heated, the chances of ice daming are low.

    I can alleviate the wildfire concerns by simply not venting, which would allow me to use cheaper (possibly recycled) rigid foam.

    In this case, the stackup is the same as second option described here:

    My only question is that, with a stackup of:

    plywood sheathing, 5+ inches of rigid foam, plywood, underlayment, metal roof

    isn't the outer layer of plywood unable to dry in either direction?

  10. GBA Editor
    Martin Holladay | | #10

    Q. "With a stackup of plywood sheathing, 5+ inches of rigid foam, plywood, underlayment, metal roof, isn't the outer layer of plywood unable to dry in either direction?"

    A. Yes. The traditional phrase to describe this type of roof is a "hot roof." The idea is to install the roofing underlayment and roofing on a sunny day when the plywood is dry.

  11. user-5453257 | | #11

    Got it, thanks Martin!

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