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Community and Q&A

Is aluminum head flashing necessary with Zip System and Home Slicker Classic?

accentcraig | Posted in Green Building Techniques on

From exterior to interior:
2 coat eastern white cedar shakes
Home Slicker Classic
7/16″ Zip System wall sheathing
2×4 walls with 2″ closed cell spray polyurethane foam (ccSPF)

I’ve yet to have a call back since 2006 using this system but the architect is arguing that I should have used head flashing over the windows especially over the combination windows on the sides of the rear hinged patio doors with windows on each side. All the doors and windows are flanged new construction vinyl clad Andersen Series 400. Each were caulked and applied directly to the face of the Zip panel per Andersen’s recommendations. The trim is 1×4 Boral with a PVC backband. None of the trim was caulked to the window or doors so any moisture that gets behind the PVC backband on top of the window will either drain out via gravity or evaporate so why waste the time getting custom Z metal made to accommodate the wide backband when you’ve got a rainscreen system allowing for drainage, not to mention the drying of the backside of the cedar shakes? Any help with this would be greatly appreciated because he’s suggesting that we go back and retrofit the flashing over all the windows. Since we back caulked the cedar shakes around the windows I’m afraid it would just damage the Homeslicker not to mention a serious waste of time, labor and money. Please advise.

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  1. accentcraig | | #1

    Oh, both the Zip System and Home Slicker technical departments say head flashing isn't required with their respective systems. Each were aware that I was using the two together and that the window trim was Boral/PVC.

  2. Expert Member

    To me it's a two part question. If any of the following say you need flashing then you are hooped: Zip, the window manufacturer, your building code.
    The second part of the inquiry is what the architect showed on their plans. If flashing is shown on the drawings or specified, then that's what governs, regardless of others opinions. You can't deviate from the contract drawings without their approval.

  3. Expert Member

    Putting aside whether you will you have to retrofit the flashing, I think including it is good practice. Rain screen walls work on the principle of first and second layers of protection. The idea is that the cladding should stop bulk water intrusion, and the second, inner layer of WRB or in your case Zip sheathing acts as a back-up. The inner layer should be detailed to return the moisture to the exterior wherever possible, not remain in the cavity. Head flashings with appropriate end-dams do just that. Taped head flashing rely on the window frame itself to channel water to each end of the opening where it travels down to the base of the rain screen cavity to drain.
    For many climates where the rain screen's main benefit is allowing the wall to dry to the exterior, rather than stopping bulk water intrusion, your detail is probably just fine. But it isn't as resilient an assembly as it would be with flashing.

  4. GBA Editor
    Martin Holladay | | #4

    There are several issues here, and Malcolm has touched on some of them. Of course the architectural drawings and specifications govern what needs to be done.

    Z-flashing is installed over the exterior head casing of windows for several reasons. One is to prevent water from dribbling down between the head casing and the sheathing, as you point out. Another is to keep water off the top of the casing so that the casing doesn't rot.

    You are assuming that Boral can't rot or deteriorate, but we don't know if that is true. Boral is made of fly ash, a by-product of coal combustion. This is a relatively new product with a limited track record. We don't really know how it's going to hold up when exposed to the weather for years. So I vote with the architect -- exterior head casing needs to be protected with Z-flashing.

  5. EthanT | | #5

    Martin... in 2015 you wrote "[Boral] is a relatively new product with a limited track record. We don't really know how it's going to hold up when exposed to the weather for years."

    What would you say in 2018?

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