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Community and Q&A

Detailing stoop slab: concrete to wood

ohioandy | Posted in Green Building Techniques on

I am building a new house which will be clad in full-thickness natural stone veneer. A stoop slab just outside the front door will be poured on top of a cold room that’s part of the foundation.

My concrete contractor, of course, wants to pour the slab right against the rim joist. That’s not going to happen. But what SHOULD happen?

With the masonry veneer, the wall above includes a 1″ air space–should I have the guy leave the 1″ also between the slab and the rim? How would I ensure proper water drainage out of that space?

Or perhaps should I just put in a triple layer of 6-mil plastic against the rim and let him pour up to it, then silicone the intersection?

Thanks for advice.

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  1. GBA Editor
    Martin Holladay | | #1

    I'd be interested to hear any advice from others who have faced your dilemma.

    My first reaction is that you don't want any concrete to be installed anywhere near your rim joist. The top of the concrete stoop should be at least two steps below the door threshold, and the steps between the threshold and the concrete should be built of wood.

    Here in Vermont, where we often get 12 inches or 18 inches of snow in a single night, no one would think of putting concrete up against a rim joist. If you need a handicapped access ramp, it's best to build it out of pressure-treated wood, or -- if the ramp must be made of concrete -- to design an entry roof that keeps rain and snow away from the door threshold.

    My second reaction is that, if you insist on concrete in this location, you want something more durable than 6-mil plastic to protect the rim joist: something like Grace Ice & Water Shield (protected by metal flashing). And you also want an air gap between the concrete and the metal flashing.

  2. davidmeiland | | #2

    You want a substantial air gap anywhere that the stoop comes up above the level of the mudsill. My advice is to pour the stoop one step below the level of the interior floor, and put a spacer between the rim joist and the concrete during the pour, to be removed after the pour. That spacer should extend below the mudsill, so that there is concrete-to-concrete contact between the stoop and the stemwall.

    Here are a couple of photos of a job we did a while back. In the one that shows the forms, look carefully and you can see that there are a couple of pieces of form board against the rim joist. The felt is already in place over the sheathing. The two layers allow for easier removal--if you put one piece in there, you'll never get it out. If I recall correctly, we used a piece of 1/2" plywood plus a piece of 2x6, for a total of 2" of air space. We later installed siding into that space, 5/8" thick. This is completely covered by a roof, so it never rains much on it. The space can be cleaned with a blower.

    In the second photo, you can see that we poured against the stemwall. There are rebar dowels connecting the two pours. That's not absolutely necessary, but I think it looks better. I have seen these poured with no connection to the building at all. If you don't have a roof over yours, and/or you get snow, that may be the best bet. Of course you need to decide how the siding fits in... maybe it should be installed first.

  3. user-659915 | | #3

    Nice job David. All exterior doors should be protected this well.

  4. GBA Editor
    Martin Holladay | | #4

    Thanks for the details, David.

  5. gioberg | | #5


    Do you have any additional details/drawings for this concrete touching wood scenario?

    Thank you

    Brian Gio

  6. Expert Member

    There are several ways to do this.

    The safest is to step up your foundation wall where it will meet the slab and set the floor joists back on a ledge. David's recommendations still hold: don't attach the two and leave a gap.

    If you decide to pour against the sheathing it's a good idea to replace it with cement board, especially if it's on a rain screen.

    Really though, Martins injunction is a good one. Don't pour above the top of your foundation wall.

  7. GBA Editor
    Martin Holladay | | #7

    Q. "Do you have any additional details/drawings for this concrete touching wood scenario?"

    A. No, because I advise builders to avoid the "concrete touching wood scenario" by proper design.

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