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How to install fiber-cement siding over mineral wool?

TimGodshall | Posted in Green Building Techniques on

I am planning to install 2″ thick Rockboard 80 over Zip Sheathing using vertical furring strips screwed to the 24″ OC studs. I am planning to install fiber cement siding on the outside of this assembly. I have two questions:

Are 1×4 furring strips thick enough to provide adequate material for the fiber cement fasteners to attach to?

What type of fasteners should I use? It seems like smooth-shank roofing nails wouldn’t hold well enough. I’ve used screws before and had to pre-drill for each screw to penetrate the fiber cement. Is there a type of screw that will self-tap through fiber cement?

Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Timothy,
    Yes, 1x4 furring strips are adequate to hold the siding. For more information on this issue, see Fastening Furring Strips to a Foam-Sheathed Wall.

    In that article, I wrote, "Until recently, many siding manufacturers recommended that siding nails penetrate 1 in. or 1¼ in. into wood. That’s beginning to change, however. The Vinyl Siding Institute requires only ¾ in. of fastener penetration for vinyl siding; James Hardie Corp. accepts only 7/16 in. of penetration for fiber-cement lap siding. According to building scientist Joseph Lstiburek, if you have any doubts arising from the fact that your siding nails penetrate into only ¾ in. of wood, just switch from smooth-shank nails to ring-shank nails. Unless you’re building near the coast in south Florida, ring-shank nails will be more than adequate, even when penetration into wood is only ¾ in."

    In the case of any siding product, including a fiber-cement siding product, it never hurts to consult the siding manufacturer's instructions. You didn't tell us whether you are installing lap siding or panel siding; nor did you mention the manufacturer's name.

    As an example, I looked into HardiePanel fiber-cement siding. Here's what I learned.

    According to the manufacturer's installation instructions, "James Hardie recommends installing HardiePanel on a capillary break (rainscreen/furring) as a best practice." So far, so good.

    These installation instructions also note, "Fasteners must be corrosion resistant, galvanized, or stainless steel. ... Consult applicable product evaluation or listing for correct fastener type and placement to achieve specific design wind loads."

    So the next step is to look up the HardiePanel Evaluation Service Report. In that report, you'll find a table listing appropriate fasteners for different wind speeds and exposure categories.

  2. TimGodshall | | #2

    Thanks, Martin. I did inquire with two manufacturers. James Hardie replied in the affirmative that a 1x4 is fine using 1 1/2" roofing nails. The representative from Nichiha, said that while "do-able", installing their fiber cement siding over 24" OC furring is not ideal. He listed these drawbacks:

    1. Having 24” OC stud spacing creates more space between fasteners, which then results in the board never being as securely on the wall as with 16” OC stud spacing.

    2. Having furring strips pushing the boards away from the wall results in a void area behind the boards, allowing them to flex inwards towards the wall and thus making the boards more susceptible to breakage due to impact. This also compounds with #1 in not being able to get the boards super tight on the wall.

    He recommended using 5/4 x 4 or even 2x4 furring and using #8 screws to install the siding.

    It seems odd that one manufacturer would say that a rainscreen gap is the ideal installation, where another would say that you need to keep the siding tight against the plywood in order to prevent breakage and waviness. Is it possible that their products are really that different? Or is one company just more cautious about moisture management (JH) and the other is more cautious about the forces of wind and gravity (Nichiha)?

  3. Expert Member
    MALCOLM TAYLOR | | #3

    Timothy,
    On small jobs I've had very good success with the flat screws usually used to install metal snap-lock panels. They really pull down the boards leaving them far tighter than nailing. However the labour involved is appreciably greater than using a roofing nailer, and you may find you have to pre-drill and the ends to avoid tear-out.
    All residential projects here in coastal BC have their cement board siding installed on furring strips. I haven't heard of any problems with impact damage. Do your neighbours seem like the types to chuck stuff at your house?

  4. GBA Editor
    Martin Holladay | | #4

    Timothy,
    I'm not familiar enough with Nichiha siding to know how it differs from James Hardie products, but it's certainly possible for one brand of siding to be more flexible than another. If Hardie says that your plan is fine, and Nichiha says it isn't, those facts indicate that you should specify Hardie siding.

    Note to GBA readers: Framing choices can affect siding choices. As I wrote in my article, The Pros and Cons of Advanced Framing, "Some siding types ... require 16-in.-on center fastening, and are therefore incompatible with 24-in.-on-center framing." It's good to plan ahead.

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