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Vinyl windows with synthetic stone

user-6957254 | Posted in General Questions on

Hoping this isn’t a bad question…..any advice for detailing Synthetic Stone around vinyl windows (some have integrated J channel, some have brick mounding with integrated J channel).

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  1. GBA Editor
    Martin Holladay | | #1

    The first place to go for this detail is the manufacturer of the synthetic stone you will be using. Stone thickness varies from manufacturer to manufacturer, but I can't imagine any instance when you would need J channel (which was developed for vinyl siding).

    An example of a detail from the GBA detail library is shown below. More details for synthetic stone (for example, details at head, jamb, and sill) are available to GBA Prime subscribers. It's possible that many synthetic stone would have a different detail -- one that includes a drainage gap (rainscreen gap).


  2. user-6957254 | | #2

    We have vinyl windows installed, some with integral J Channel and some with Brick Molding and integral J Channel. On one wall of the home, we plan to instead install synthetic stone. I have been researching how best to use these windows with the stone.

    I have contacted the stone manufacturer, and they had zero advice to offer. I also contacted the window manufacturer, and they have been no help either.

    I have seen this topic posted on a few building sites, but no clear answer as to how to move forward. The window manufacturer doesn't offer a J Channel filler that snaps in, as some other companies offer.

    I am thinking it may be best to install a drip cap at the head, and go over the J Channel. The stone would abut against the window on the sides, but not sure what to do with the gap there and also the sill.

    Any insight is appreciated. I have buried myself in this topic for a while, without any clear advice or answers.

  3. GBA Editor
    Martin Holladay | | #3

    Is this a new house?

    Why did you order two types of windows -- some with J-channel and some with brickmold?

    Did you install new windows before installing your cladding -- and before your cladding details were finalized?

  4. user-6957254 | | #4

    Yes, new construction.

    The windows with integral J channel are the slider type windows (that fold inside to help clean, etc). The windows with Brick Molding and integral J Channel do not open. Not 100%, but I believe these are the storm windows.

    Windows are already installed. Yes, our plans changed. I realize not ideal, but clueless how to move forward. Changing the windows wouldn't be an option for us. And husband is set on using the stone.

  5. GBA Editor
    Martin Holladay | | #5

    There are several issues here. Issue 1 is that synthetic stone over OSB or plywood is quite risky. That's not to say that it's impossible to do correctly; merely to say that there have been many failures and lawsuits resulting from poorly detailed synthetic stone installed on wood-framed walls. It's a type of cladding that keeps lawyers who specialize in construction defect litigation very busy.

    So: Ideally, synthetic stone is installed with a rainscreen gap between the cladding and the sheathing. This advice mirrors the best practice advice for stucco over OSB or plywood. For more information on the topic, see “To Install Stucco Right, Include an Air Gap.”

    Good flashing details, multiple layers of WRB, and a rainscreen gap add thickness to a wall. That's why these details need to be finalized before windows are installed.

    If you feel confident that you can flash your wall in a way to make the synthetic stone veneer safe, you can probably come up with details for your windows. But you'll need a solid grounding in water management principles and flashing to make it work.

  6. user-6957254 | | #6

    Martin, yes. It is in the plans, and already purchased Slicker Max as our rainscreen, along with the 2 layers of WRB, and a weep screed at the bottom.

    The only thing I cannot wrap my head around is the detail around the windows. Install a drip cap at the head? Or allow the J Channel to work as it would with vinyl siding. Also read conflicting info regarding the backer rod and sealant. Husband agrees that sealant isn't ideal on the exterior of the home, due to maintenance issues and how it can trap water and not let it drain freely.

  7. GBA Editor
    Martin Holladay | | #7

    The detail at the windows will depend on how you flashed your window rough openings. Since the windows are already installed, it's too late to change these details.

    Usually, the water-resistive barrier (WRB) -- for example, housewrap -- laps the top flange of a flanged window. Note, however, that window flanges aren't the same as flashing. These flanges are aids to window attachment, not flashing elements.

    Water that gets past the head may dribble down the rough jambs and end up on the rough sill. That's why your rough opening has jamb flashing and a sill pan. The sill pan should direct water to the outside of the WRB under the window.

    Sometimes the installation of stucco or manufactured stone veneer requires the installation of two WRBs. If you install a second WRB, it makes sense for this second WRB to lap Z-flashing at the window head.

  8. user-6957254 | | #8

    May I ask your opinion on using backer rod and sealant around the window? Conflicting opinions on this topic. Let it drain freely?

  9. GBA Editor
    Martin Holladay | | #9

    If you are talking about the gap between the window frame and the rough opening, it's perfectly appropriate to seal these gaps to prevent air leakage.

    The gaps at the jambs and head can be sealed with canned spray foam, caulk (with or without backer rod), or high-quality tape.

    You have to be careful at the sill, though, since the sill needs to drain to the exterior. To air seal the gap at the sill, the best approach is to use high quality tape on the interior side of the window. You can also use backer rod and caulk if you prefer, instead of tape -- but just make sure that the work is performed from the interior, and that the backer rod is placed carefully, near the interior side of the window frame, so as not to interfere with the drainage of liquid water that ends up on the sill pan flashing.

  10. user-6957254 | | #10

    Martin, I was actually referring to the exterior window, the small gap between the stone and the window.

  11. GBA Editor
    Martin Holladay | | #11

    Vinyl windows have a high rate of thermal expansion and contraction. Don't caulk the gap between the window and the stone veneer.

  12. user-6957254 | | #12

    What I plan to discuss is, have z flashing at the head of the window (on both the integral j channel and the brick molding windows). Any water that hits the head should go over to the outside of the window. Abut the stone up to the sides of the windows, and any water than happens to get into the channel on the sides will reach the sill and be flashed down the front of the stone. If water happens to get behind the stone at the corners, in theory the rainscreen will catch that and carry it down to the weep screed at the bottom.

  13. GBA Editor
    Martin Holladay | | #13

    Assuming the flashing is installed as you say, your logic is good.

    Wall rot behind adhered manufactured stone veneer can happen even if the flashing is correct, however. The problems occur when there is no air gap between the stone and the water-resistive barrier (WRB). Even if the flashing is properly lapped, and designed to direct water to the exterior, water can be trapped and linger if there is no air gap. You need ventilation drying to keep the wall safe.

    Here are some photos of rotten OSB, reproduced as a reminder of the vulnerability of OSB sheathing: All About Wall Rot.

  14. user-6957254 | | #14

    Martin, wouldn't the slicker max rainscreen provide that air gap that is needed? that is what we purchased, and that is what will be installed between stone and the wrb.

  15. GBA Editor
    Martin Holladay | | #15

    Yes, Slicker Max is a good idea. It's a drainage mat + filter fabric product that provides an air space.

  16. user-6957254 | | #16

    Perfect! That is what my research came up with also, and that is why we purchased it.

  17. JC72 | | #17


    Per the install instructions SlickerMax provides sufficient gap and can be installed behind Manufactured Stone/Stucco.

    The wall mock up goes in this order: Sheathing, WRB, Slicker Max, Lathe (i.e. wire mesh), scratch coat, then stone cladding.

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