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Community and Q&A

Siding butt joints and inside/outside corner flashing

Smith6 | Posted in Green Building Techniques on

I am installing 3/4″ horizontal square edge lap siding using a drainable house wrap rated at 96% drainage without furring strips. I am planning on scarfing butt joints. I do not see the need to use some type of flashing behind the butt joint since the point of the rain screen is to shed water and the flashing would actually seem to interfere with that. Is that correct thinking?

Also, my inside and outside corner detail is vertical 2 x 6 trim with the horizontal siding butted to the vertical trim. Keeping in mind that I am using the drainable house wrap, what type of additional flashing design should I use for each type of corner?

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  1. Expert Member

    The flashing, whether building paper, membrane or metal, really just acts to make the cladding act as though it were continuous with no joints. In the language of our code, this is the "first layer of protection". I can't see how it interferes with the rain screen cavity as it should be lapped over the siding piece below, keeping it away from the drainable house wrap.

    For butt jointed trim or corners I would back-caulk, bedding the siding in a flexible sealant like Big Stretch.

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Malcolm about using small rectangles of flashing at the joints. These piece of flashing are optional, but if you decide to use them, they won't "interfere" with the rainscreen principle.

    I think that siding should be installed without any caulk. Caulk eventually fails, and it usually does more harm than good when installed near siding -- interfering with drainage and drying.

  3. Smith6 | | #3

    Thanks Malcolm and Martin for the response. On the inside corner detail, I have seen recommendations to use metal flashing behind the vertical trim that is wider than the trim>siding joint that then overlaps the bottom sill flashing. I would assume that this should be taped and not nailed since that would pierce the drainable house wrap..... And do you recommend any additional flashing on the outside corners?

    On the butt joint flashing, it almost seems as if it would somehow direct and hold more water into the siding lap and scarf joint instead of just allowing any water that penetrates the scarf joint to drain with the rainscreen..

  4. Expert Member

    That's why you don't see many scarf joints anymore. They expose more end grain and leave a fragile profile that is vulnerable to damage.
    I'd butt joint the cladding and make sure to stain or paint all six sides.

    I mentioned the idea from our code of "layers of protection" in my earlier post and I think it's a good one. The cladding should be designed to function as a barrier to moisture intrusion. The cavity acts as a gap between this and the second layer, which is a backup. In your stack-up the dimpled house-wrap is the only protection keeping the wall dry at the joints. The flashing I'm suggesting provides a second layer at this vulnerable point.

    Apart from flashing, you can use strips of membrane at inside or outside corners to overlap at the butt joints. I prefer caulking as it keeps the moisture from getting back there in the first place. Unlike applying it on the surface, back-bedding means the caulking is protected by the cladding and unlikely to fail for the life of the wall.

  5. Smith6 | | #5

    Malcolm, thanks for the additional detail. I am also going to have some vertical channel rustic siding on a shed bedroom. I have read that with vertical installation, a scarf joint is still recommended. Your thoughts?

  6. Expert Member

    Yes, scarf joints area good idea on vertical applications - or angled ones like joining fascia boards on gables. It is rare to see siding installed vertically with joints though. Usually it spans the full height of the wall.

  7. Chaubenee | | #7

    What kind of siding? What color then? What flashing material do you intend to possibly use?

  8. Smith6 | | #8

    Douglas fir, square edge lap,prefinished, water-based, no VOC stain/waterproof sealer. For butt joint flashing, either asphalt felt, or strips of membrane...still deciding on best material for the metal flashing.

  9. Chaubenee | | #9

    When you say membrane, what do you mean? What color is it? You would ideally want dark colored for when the seams contract. You sure don't want white material back there.

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