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Community and Q&A

Slab against rim joist detail please

David McNeely | Posted in GBA Pro Help on

New construction: back patio will be a slab poured against the rim joist so it is nearly level with the door threshold. Rim joist will be 11 7/8″ atop a retaining wall back-filled with gravel, so there will be good drainage. The patio will be covered with a roof, and slope away from the door of course.

What’s the best way to permanently protect the rim against water, moisture, and termites?

Thanks.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    What's on the other side of the rim joist?

    Is there a capillary break between the foundation and the foundation sill?

  2. GBA Editor
    Martin Holladay | | #2

    David,
    In general, this is a bad detail. It's almost always better to create a step between the wood framing of your house and a concrete patio, so that concrete isn't poured up against wood components.

    A rubberized membrane (Ice & Water Shield) would be better than nothing, but I still wouldn't recommend this detail. I'll defer to others -- those who live in termite-infested regions -- to comment on the termite risk.

  3. David McNeely | | #3

    Dana, yes, there will be a plate gasket between foundation and sill plate. Also, there will be flashing from sheathing membrane to top of slab, with tile above that.

    Martin, that would mean two steps down.

  4. Expert Member
    Dana Dorsett | | #4

    A waterproofing layer (Ice & Water qualifies) down to at least 6" below the sill plate, as well as copper flashing for termite control would probably cut it. Copper leaching off the flashing is toxic to the gut flora of wood boring insects need to process the cellulosic fiber. Even if they tunnel up that far, they wouldn't live for long.

    There are sill gaskets and there are sill gaskets. The thin flexible foamy stuff doesn't qualify as a capillary break, but an EPDM sill gasket does. Sheet metal flashing in combination with foamy gaskets both above & below the metal or beads of can foam above & below (for air sealing) does too.

  5. KEVIN ZORSKI | | #5

    David - A slab patio connected to a wood frame house, and at the same level as the first floor is a bad idea. A slab patio connected to a slab first floor,separated by insulation and one step down is a good idea.Dana's detail may work,but I'd want an engineer to sign off on it.Plus - what is your detail for insulating the rim joist? When condensation gets in there, how will it dry out?Keep asking questions. This site, (and not necessarily me!), will help you out.

  6. Expert Member
    Malcolm Taylor | | #6
  7. GBA Editor
    Martin Holladay | | #7

    Malcolm,
    Thanks for the link! I knew that this discussion rang a bell -- and I appreciate your tracking down the old thread.

  8. Mike M | | #8

    Isn't this sometimes impossible to avoid?

    I'm planning an addition to my slab on grade home. The addition will have a basement. Where the basement side meets the slab there will have to be a "rim joist" that touches the slab which would be bolted to the foundation wall. My thoughts were to just use the treated TJI product for this rim? I guess I would leave some sort of gap here in the subfloor so the subfloor doesn't touch either?

  9. Joe Suhrada | | #9

    People in wheelchairs often need this sort of detail built in to their entry ways. The membrane, a covered stoop going out at least six feet, a copper flash and treated plate in that area might help. Keep it dry as best as possible.

  10. GBA Editor
    Martin Holladay | | #10

    Joe,
    I've been called upon to give advice for wheelchair access to a home with a wood-framed floor. Here's my usual advice: You want a wood-framed landing outside the door (at the same height as the interior finish floor) that is large enough for the door to be opened while a person in a wheelchair is on the landing. You want a wood-framed ramp up from the sidewalk or from grade -- maximum slope 1:12. And you want a roof over the landing and the ramp to keep rain and snow off of everything.

  11. Expert Member
    Malcolm Taylor | | #11

    Michael,
    I would't worry, the situations are not analogous. Your rim joist will be inside and protected from bulk water. Using treated lumber will be enough to protect it.

  12. James Morgan | | #12

    Boomers are building their retirement homes and they are looking for integrated solutions to aging in place issues, including primary accessibility. Ramps that look like add-ons are not going to cut it for many of these folks and this need is not going away so 'don't do it' is not going to be an acceptable response. We have been addressing this issue for a number of years and in our experience the core components of a flush concrete or paved entry area against a wood floor system are these:

    • Bear the inner edge of the patio slab on a brick ledge to the foundation wall so the slab can't settle away from the wall and reverse its slope. In protected locations the ledge can be the top block of the foundation wall extending 4" to the outside of the green plate. If the location is not well protected with a deep overhang the ledge should be one course down.
    • A continuous flashing over the sheathing and band which turns out over the brick ledge before the slab is poured and extends at least 6" above the patio level (we use peel and stick)
    • A second flashing over the first installed after the slab which turns out on top of the slab and is dressed up under the WRB. If a brick or stone veneer is used it would install on top of this flashing with appropriate weep holes, and bearing in turn on the slab and the ledge.
    • Door thresholds need extra protection. A porch or roof extension which is at least four feet deep and extends at least a couple of feet to each side of the threshold should be considered a minimum. Concentrated roof water flow in the vicinity such as from a valley abutment should be avoided. The patio slab can be finished to direct water away from this vulnerable area.

    We've been successful in avoiding problems with these details for a good number of years. Of course in Piedmont North Carolina while we get a lot of rain snow is a rarity, in snow country hardscape layout beyond the immediate area of the exterior wall will be critical. Covering the entire patio with a roof as David intends would seem to be a good part of such a plan.

  13. David McNeely | | #13

    James, your answer is really helpful. This was my plan for water and moisture, and I appreciate the confirmation that it will work here in Knoxville.

    What about termites in your detail? Would they be able to crawl up under the peel and stick?

    BTW, the purpose is exactly for a handicap entrance that does not look like it is a special accommodation for the handicapped, but is instead an improvement for all.

  14. Joe Suhrada | | #14

    Can you build this concrete patio over an unconditioned crawl space using the above details with the ledge? Could you use steel corrugated sheets under the ramp so that their is no backfill of soil? If you do so, can you place a poly vapor barrier down AND ALLOW SOME ACCESS so that you can get into that space and inspect and treat with pesticides if there is an issue? The other way to do this is to create a full unconditioned basenet under there with an insulated door coming from your basement. It should stay bone dry if your concrete is sealed, sloped properly away and covered. Now you have a wine cellar as well. I am not sure how much this adds in cost, but consider this: when you excavate the basement down eight feet, you may have a hard time excavating four feet or less that close to the full basement if you plan on a footing of any sort. Ask you excavator and mason.

  15. GBA Editor
    Martin Holladay | | #15

    Joe,
    I'm not in favor of crawl spaces or basements that try to use a concrete patio slab as a roof. These patios are often remodeled, and future owners may assume that the space can be used as a porch, subject to wind-blown rain and snow. It's hard to detail a concrete patio as a watertight roof, so leaks are likely. Moreover, if this is part of your basement, the concrete patio not only has to be detailed as a watertight roof -- it also needs to be insulated.

  16. Joe Suhrada | | #16

    I would definitely leave it outside the envelope. But your concerns are valid if there is no roof structure above it. Here in upstate many front porches on older homes have a small basement below which when sealed off, makes a nice root cellar or wine cellar. But maintaining the concrete porch above is essential to keeping that area dry. The ranch I grew up in and the ranch my brother built the seventies are good examples of that. No leaks, but both have roofs above.

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