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Base of wall flashing

Nat_T | Posted in General Questions on

I’m trying to develop/draw the detail at the top of my foundation wall.

I see a lot of details (including in the details section here at gba, at jlc, etc) that show running wall sheathing to overlap the basement wall. I’ve also found articles specifically recommending this as a good way to air seal the junction. Why would I be allowed to let my sheathing touch the concrete – isn’t that going to quickly be a problem?

In my case I am using zip-r and a horizontal lap type siding (over a rainscreen). My current plan is to have the insulation of the zip-r be primarily over the concrete with the back face of osb about 1/2″ proud of the concrete (so front face ~1″ proud). It seems to me I should protect that bottom face of the osb.

I’ve seen some details for using a tape from the concrete to the osb. Is a tape solution going to be durable long term? It would not be an easy fix if that tape gets beat up (errant weed wacker) or starts to come off over time. I’ve also seen details for liquid flashing it – again I wonder if that is durable long term? There wouldn’t be anything protecting this flashing against the concrete detail so it must get beat up.

I assume this is a common detail done every day but I’m not finding anything that looks durable and makes sense to me.

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  1. Expert Member


    Why not align your stud walls so the face of the Zip-R is just proud of the outside of the foundation, and the bottom edge held up 1/4". You then don't have to worry about capillary moisture on the OSB, and have the choice of sealing the gap between the sheathing and foundation with either a bead of caulking or tape. The siding can extend down an inch to cover the gap against mechanical damage.

  2. Nat_T | | #2

    Thanks, Malcolm - always the first to answer my questions.

    That was what I was planning to do (my description may not have been clear). I'm just not that confident how much longevity a tape has in this condition given it's basically unprotected. If I caulk is the edge of the osb protected? Or maybe I dont' need to worry about that, is it ok to leave it unprotected.

    I find it strange that so many details show the sheathing touching the concrete. If you go to the details section here on gba and look at foundation details it's even shown that way here.

  3. Expert Member


    The caulk will be filling the 1/4" gap between the OSB and concrete below. The edge of the OSB will be behind the caulk and held up off the foundation the same 1/4" . The tape would need to extend down the face of the concrete 1" for adhesion, but would be protected by the cladding, which should extend down 1/2" below the edge of the tape.

    All that said, we do it slightly differently here. We use a 1"x2" as a pour-strip on the exterior face of the forms. That lets you align everything with the inside of that 3/4" x 1 1/2" gap, and then you run the cladding down to the concrete 1 1/2" below, closing everything off. The detail also gives you some wiggle-room if your foundation isn't perfectly straight.

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