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Community and Q&A

Cantilevered exterior wall rainscreen detail

user-6955756 | Posted in General Questions on

Hi everyone,

I’ve got a cantilevered exterior wall with a rainscreen and am wondering about how to finish the bottom of the wall. I’ll have the typical 3/4″ ventilated strip between the wood siding and sheathing, but I’m wondering whether I need flashing to run along the bottom of the wall as well?

I worry that without this flashing water that gets behind the rain screen will drip down and end up soaking into the bottom of the OSB sheathing, eventually rotting it out. As the same time I wonder whether this is overkill as the cantilever will have extensive ventilation for this OSB and perhaps simply extending the WRB down a 1/4″ or so would allow any water to drip off without being sucked up by the OSB.

I’ve attached a sketch of what I’m planning to do (leaving out the ventilation strip as it was getting cluttered in the sketch) and was wondering what people thought – is it overkill or a good idea?


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  1. rockies63 | | #1

    Not sure about the flashing, but are you planning on having an insect barrier wrapped around the furring at the tops and bottoms of the walls (and openings)?

    How are you finishing the underside of the cantilever and its connection to the lower and upper wall?

    1. user-6955756 | | #4

      Hi Scott, yes I've purchased what is essentially screen door repair tape which I will line the ventilation strip with. I've testing it out and it seems to stick well enough to the aluminum and should do the job without blocking airflow into the rainscreen.

      As for the underside of the cantilever, the cantilever exists on the floor of the structure. Due to some unique conditions (a.k.a poor planning) I've plopped the building onto two laminate beams which run the length of the structure. The siding will run to the bottom of the rim joist, covering the cantilever. The underside of the cantilever is open to the environment - I've not insulated it. I'm aware I'll loose a lot of heat through it but efficiency is not my priority on this project, durability is where I am focused most. I've attached a sketch which show the condition a bit better.

  2. Expert Member
    Akos | | #2

    You need continuity in your air barrier. Underneath your joists you put plywood with taped seam to the wall sheathing and to your foundation. Put a layer of WRB over the plywood and trim out as you like (T&G, aluminium or cement board). This layer of WRB does not get taped to the one on the wall, the one on the wall laps your bottom flashing.

    The rest of the details are fine, I would drop down the flashing so the bottom of it is flush with your finished underside surface.

    1. user-6955756 | | #5

      Hi Akos, thanks for your reply. I've attached a sketch in my reply to Scott above which I think shines a bit more light on my situation. I'm building this out in a forest with no power and no water, so I've had to make some compromises in the build to allow me to get it done. The joists connect to a 3-ply 2x12x16 laminate beam running the length of the building. I've attached a photo from the winter that shows the condition before I put the bracing between joists and laid 3/4" PT plywood atop as my subfloor.

      The floor does not have a WRB, the plywood is the only thing between by subfloor and environment. I know this will lead to air/vapour through the floor but I'm a bit handcuffed in how to achieve air barrier continuity - perhaps taping the seams of the plywood? I'll be caulking along the bottom plate of every wall before I lift them into place.

      1. Expert Member
        Akos | | #7


        Looks like you are building in snowy forested area, don't know how similar that is to northern Ontario.

        Here one of the big challenges is critters. Getting critter barrier right is more important than air barriers. You want to block access to any insulation area and edges of plywood. For the bottom of your rain screen, go with a perforated J molding.

        Since your floor joist are already down, it is hard to get that sealed up. What you can do is put hardware cloth or metal lath trays stapled to the bottom side of the floor joists, install fluffy insulation in between than put on your plywood subfloor with taped seams.

        Watch the bottom/side of your door/window and exterior trim. You want to put some hardware cloth over over the gap between the rough opening and jambs. Spay foam does nothing for keeping critters out.

        Watch transition from your wall sheathing to roof deck with the rafter tails poking through. Lot of gaps that need proper critter proofing.

        For weather protection, looks like you have decent overhangs, you don't need anything on the underside. The only thing there you might get is a bit of splashback during a rain storm, that will not cause any issues.

        1. user-6955756 | | #8

          I'm actually just a bit north of Huntsville Akos. Fully agree on the critters - have had them in the walls before and it's one thing I truly cannot stomach.

          I've got perforated j molding for the bottom of the rain screen and I'll be lining that with screen door repair tape to keep out insects as well. I've not seen that tried anywhere before but in my mind it's a pretty good solution. I'll post a photo or two once I've installed it for others to see how it turned out.

          Before I put the subfloor down I installed a 1/4" galv mesh across the entire thing - it's sandwiched between the top of the joists and bottom of the plywood. I'm hoping that will keep them from getting through there. I've done the same thing along the walls but just a 3' strip along the bottom of the wall lapping over the rim joist. I've attached picture of the wall before I put the sheathing on it. I couldn't find one of the floor unfortunately.

          I'll definitely give the other areas you mention some attention as well - I hadn't given much thought yet to the rafter/wall joint... thanks!

          Appreciate your feedback on everything, helps a lot to pick peoples' brains here.

          1. Expert Member
            Akos | | #9

            Sounds like you are on a good path, best of luck on the rest of the build.

            If you are going to use this in the winter, try to get at least some insulation on top the subfloor before you install your flooring. Without a basement, that floor will be mighty cold on -30C days.

  3. Expert Member


    The flashing doesn't need to be shaped as you have drawn it. Hemmed flat stock protruding a quarter inch or so will work and be less obtrusive.

    1. user-6955756 | | #6

      Thanks for the suggestion Malcolm, I really like that idea! It will help keep the rain screen open and make the flashing itself less visible.

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