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Importance of venting underside of metal roof in hot/humid climates

user-6729986 | Posted in General Questions on

I am renovating a wood framed home in Jacksonville, FL built in 1904. It currently has 5V crimp metal panels over 2×6 rafters and skip sheathing. I plan on creating an unvented assembly with exterior foam (min R-5 in this zone). I understand that a good detail in cold climates is to install the metal panels over purlins or a second raised deck to provide airflow under the metal and help avoid ice dams. Are there any benefits to doing this is a hot/humid climate? I imagine it would help reduce the high temperatures below the metal (reducing convection) and act much like a vented rainscreen for a wall. I am inclined to install a new 5/8″ plywood deck over the skip sheathing, lay down an air barrier underlayment, then install 1-2 layers of foam with 1x4s strapped across the top, but if the benefit of venting under the metal is negligible, nailbase over the skip sheathing would be a much simpler system to install. Also, should additional load be a serious concern with these 2×6 rafters, 24″ o.c and no ridge board or beam? I appreciate your feedback.

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Replies

  1. jackofalltrades777 | | #1

    I am in the same scenario as you except my application is with SIPS. My options have narrowed to: metal sub-girts which raise the metal roof either 1/2" or 1" from the substrate OR 1x3 wood purlins which raise the metal roof 3/4" from the substrate.

    The steel sub-girts provide a strength advantage since the standing seam metal roof clips onto the sub-girts. The 1x3's don't really provide any strength but provide the air channel needed.

    The question is HOW MUCH of an air gap is needed under the metal roof? Is 1/2" enough or is 1" required?

  2. GBA Editor
    Martin Holladay | | #2

    Jacob,
    In Florida, it's quite common to install so-called "5-V crimp" metal roofing over solid OSB or plywood sheathing (with a layer of roofing underlayment between the sheathing and roofing, of course, but without any purlins or strapping). In your climate, this approach works fine.

    -- Martin Holladay

  3. GBA Editor
    Martin Holladay | | #3

    Peter,
    In the case of SIPs, it's essential (in my view) to have an air channel between the roofing underlayment and the metal roofing. The colder the climate, the more essential the air gap.

    If I had to choose between a 1/2-inch gap and a 1-inch gap, I would be inclined to go with the 1-inch gap if I were building in Climate Zone 5 or anywhere colder. In Zone 4, the 1/2-inch gap would probably allow me to sleep at night.

    -- Martin Holladay

  4. user-6729986 | | #4

    Thanks Martin for the excellent responses, as always. Peter, I'm glad you brought up SIPs because I have to confess I, I'm not sure whether I should treat a nailbase panel similarly. In other words, are there any fundamental differences between a true SIP and a nailbase panel over sheathing when in neither case the attic space is vented? If not, that implies I should treat a nailbase panel like a SIP and vent over it, even though installing metal roofing directly to a solid deck is acceptable practice in my area. Perhaps the answer is entirely dependent on the particulars, and in this case I would probably go with a 2.5"-3" nailbase panel yielding R-12-15 with about R-23 of fluffy in the rafter bays.

  5. jackofalltrades777 | | #5

    Martin,

    I am in Zone 4B.

    If it were your home, would you go with the 1" spacing or the 1/2" spacing?

    The con to the 1" is that supposedly the 24 gauge panel can flex slightly under heavy snow loads since the OSB substrate is not sitting directly underneath the metal roof panel.

  6. GBA Editor
    Martin Holladay | | #6

    Peter,
    I think (a) the 1/2 inch spacing will work in your case, and (b) you don't have to worry about heavy snow loads in Florida.

    -- Martin Holladay

  7. jackofalltrades777 | | #7

    Martin,

    I'm actually in the desert southwest (Zone 4B).

    With a 1" gap, that should also help with sound transfer from the metal roof, right?

    Metal roofs tend to transfer sound and SIPS can transfer sound due to the lack of air space. Would the larger 1" gap help with sound transfer?

  8. GBA Editor
    Martin Holladay | | #8

    Peter,
    I'm sorry that I got your case confused with Jacob's. (That's always a risk when you piggyback on an existing thread.)

    You decision to use SIPs obviously raises the stakes. I'm not familiar with the metal sub-girts you are talking about. Do these metal sub-girts allow soffit-to-ridge venting, or only left-to-right venting (rake-to-rake venting)?

    -- Martin Holladay

  9. jackofalltrades777 | | #9

    Martin,

    It's mainly left to right venting (rake) since the sub-girts run from rake to rake.

    The metal roof is designed to install over roofs that don't have sheathing so the sub-girts raise the metal roof and the sub-girts allow the metal roof to clip into the sub-girt for retention.

    MBCI is the roofing company.

  10. GBA Editor
    Martin Holladay | | #10

    Peter,
    SIP roofs make me nervous. I've heard too may failure stories.

    If this were my house, I wouldn't try to cut corners. I would make sure that there was soffit-to-ridge venting above the SIPs.

    -- Martin Holladay

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