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Community and Q&A

SIP roof with roofing sleepers

JackofAllTrades777 | Posted in GBA Pro Help on

I want to create an air space between the SIP roof panels and the finished roof. I am taping the SIP roof seams with SIGA tape and installing a breathable roof membrane (Majcoat).

I want to create an air gap between the finished metal roof and the SIP. I was planing to use 2x’s but the problem is which way to run the boards?

The roof is a simple shed roof design (N/S) so if I install the boards length wise (N/S) that wouldn’t work the best with a metal roof since the are 16″ wide panels. I would have to install the 2x’s running E/W but wouldn’t that prevent the water from draining as the 2x’s would stop the natural flow of any moisture from draining off the roof.

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  1. JackofAllTrades777 | | #1

    I think Google found my answer. Are counter battens the way to go? That way the moisture can drain without getting trapped by the 2x sleepers.

    What is a good spacing? 24 oc ?

  2. Expert Member

    The spacing of the north/south battens depends on the size of the east/west counter battens. The spacing of the east/west counter battens should come from the tables in the manufacturer's installation guide.

  3. GBA Editor
    Martin Holladay | | #3

    The east-west and north-south designations are confusing to me. I prefer to talk about nailers that are "parallel to the ridge" or "perpendicular to the ridge."

    To save money and labor, you can use a single layer of purlins (nailers) -- either 1x4s or 2x4s. These would be parallel to the ridge, and would be spaced according to your roofer's preference (or the requirements of the manufacturer, as Malcolm noted). I've usually spaced them 24 inches on center.

    While this approach doesn't allow drainage of liquid water, it works. The amount of condensation the drips from the underside of the metal roofing is relatively small, and the moisture evaporates harmlessly in the air space above the asphalt felt (or, in your case, Majcoat).

    If you are worried that you'll have lots of condensation, and you prefer free-draining channels that are generously sized, two layers of nailers (sometimes called battens and counterbattens) are fine. It's a great system -- just a little more expensive and time-consuming to install. Of course, the first layer of nailers is installed perpendicular to the ridge; the second layer is parallel to the ridge.

    -- Martin Holladay

  4. SwitchgrassFarmer | | #4

    Metal roofing atop SIPs is an area that seems to lack definitive manufacturer guidance. Of course after listening to Martin's two recent podcasts where SIPs were mentioned, maybe that doesn't matter. It's time to sell SIP manufacturer stock short; no one will buy SIPs anymore!

    For our own SIP roof the stack up was:

    1. Foamed joints, internally SIGA taped, and internally ridge flashed SIP

    2. Somewhat water vapor permeable roofing membrane

    3. Purlins eave to ridge

    4. Sheathing

    5. Less water vapor permeable roofing membrane

    6. Screw down R panel metal roof with ridge vents from manufacturer (Fabral) and vents on fascia (Cor-A-Vent)

    Came to this scenario for these reasons:

    A. Metal roofing alone can be extremely noisy in the rain. The racket is deafening in our post frame barn where the metal roofing was screwed directly to the purlins.

    B. I was worried that a lot of condensation would occur on the underside of the metal roofing, and then drip back down on the SIP. Whether that would have been a problem or not is unclear. I can tell you that on many a rainless night we have so much condensation off the top of our metal roofing that we actually have flow in our gutters. This is probably a local climate (or micro-climate) dependent issue.

  5. user-228058 | | #5

    What if you used something like Benjamin Obdyke cedar breather ripped into strips and tacked under the purlins prior to installing them? This would allow the small bit of moisture a way out, beneath the purlins if you will. It should equate to less labor on the roof....
    I am planning this approach on my rainscreen walls with horizontal purlins and vertical siding.

  6. Expert Member

    Unfortunately Cedar breather has very little compression strength. It is designed to be used as a mat, not in strips. It might work on walls, but I doubt whether it would hold up under the dead and live loads of a roof.

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