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Zip R with corrugated siding rainscreen- need gap or furring?

blamus20 | Posted in Green Building Techniques on

I searched and saw a post here about using corrugated metal siding over rockwool insulation and it was said that a rainscreen gap is not needed because a drainage plane is already there.

Would it still be true for corrugated metal (vertical or horizontal) over a zip wall system?

And if so, can the siding (metal) be screwed to the studs and sheathing directly without furring strips? i.e. framing/cavity insulation (R19) – Zip R5 wall sheathing – vertical corrugated metal siding.

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  1. GBA Editor
    Martin Holladay | | #1

    The answer depends on a couple of factors.

    1. If the corrugated metal panels are sold as siding, you have to install the siding according to the installation instructions provided by the siding manufacturer.

    2. If you are using metal panels that are usually sold as roofing, the ability of the panels to drain depends on the configuration of the corrugations. Old-fashioned "sine-wave" roofing panels drain well. Newer types of metal roofing have wide flat panels and only minor corrugations, so they don't drain as well.

    Make sure you've thought through how your window flashing will integrate with your choice of siding.

  2. blamus20 | | #2

    Thank you Martin,

    The siding will be the same as the 7/8" corrugated used in roofing.

    Would I still need a rainscreen gap and or furring strips on the zip walls?

    In terms of windows flashing - I was thinking of this:

    drawing in the first post but without the J channel piece. I assume its only there for aesthetics but acts as a water collector and channels it to the sides. I imagined that I just have to seal the windows to the sheathing well with straight flash (top and sides) with a flex wrap sill

  3. GBA Editor
    Martin Holladay | | #4

    It looks like you are on the right track. That type of metal panel has corrugations that will drain well, so you don't need an additional rainscreen gap. The the Q&A thread you linked to has good information on flashing details -- how to do it right, and how it's possible to screw things up.

    It's also encouraging that the panel manufacturer sells the panels as siding. If any technical questions arise during the project, the panel manufacturer should stand behind the use of their panels for siding.

  4. Expert Member


    7/8" corrugated panels are accepted as a rain-screen by our building code here in BC. As Martin said, it drains well, but because it is impermeable, it's important to substitute a perforated flashing at the bottom for the typically used base flashing, to allow for some air movement and drying.

    Around openings a U-shaped flashing with one extended leg is typically used at the jambs and sill. At the head, an L flashing with a hemmed drip-edge works well. be sure to include end-dams in this flashing to stop water making its way behind the siding panels.

    I use a lot of corrugated Galvalum on my projects, and after fighting carpenter ants, frequent seasonal maintenance, and mold, I replaced much of the ceder siding on my own house with it a few years ago. It was a great decision.

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