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Community and Q&A

Should I overlap wall sheathing on slab foundation or stop at the mudsill?

GREENGEEK | Posted in General Questions on

Hello,

Looking for some input on whether or not I should overlap wall sheathing 3/8″-1/2″ with slab on grade foundation? I have found tons of information available on this subject however, I have not been able to come to a personal decision on this. I’m hoping that you all can help with this.  

My build details:

Foundation is a slab on grade that will easily keep the mudsill 6″ above grade. House envelope will be 7/16″ zip over 2x6x116-5/8″ framing 16″ OC finished off with a rainscreen siding assembly.

I am considering the extra cost and reducing time by using 10′ zip sheathing and installing vertically as is permissible per Huber and my building codes / lack thereof. This would give me the flexibility to either overlap the sheathing and slab by 3/8″ with the sheathing lining up to the first top plate or flush the sheathing and mudsill running the sheathing 3/8″ up into the 2nd top plate. Either way, I am using Conservation Technologies EPDM sill gasket at the mudsill and then Huber Zip liquid flashing to flash the sheathing / mudsill / foundation joint. The rainscreen and flashing details will overhang the foundation by at least 2″. 

At this time, water / moisture are lesser concerns due to foundation and grade levels, however I am concerned about occasional condensation wicking over a prolonged period of time and thermal bridging between the back side of the sheathing and slab. If I were to lap the sheathing over the slab, I would treat / tape the first couple of inches of the zip board to help mitigate this. I feel that this could all be avoided if I simply start the sheathing at the mudsill. 

What are your thoughts / recommendations?

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    greengeek,

    Your sheathing acts as a shear diaphragm, and helps distribute loads, but also ties the framing behind together against uplift. I would make sure I could fasten the sheathing to the top-plate by overlapping it at least 1".

    1. GREENGEEK | | #2

      Thanks, for watching out Malcom.

      Rest assured that the sheathing will be fastened into the first top-plate and mudsill with a full 1 1/2" of contact. For me the decision to be had is whether or not to raise the sheathing 3/8" into the second top plate or drop the sheathing 3/8" to lap over / outside the slab.

      1. Expert Member
        MALCOLM TAYLOR | | #3

        Greengeek,

        Sorry, I wasn't clear: The sheathing should attach to the upper top-plate. The trusses or rafters will be attached by clips that go into that plate. If it is just held by nails into the lower one there isn't much stopping uplift. One way around that problem would be to use truss screws, which tie the two plates together.

  2. GREENGEEK | | #4

    Thanks!

    Yes, I will be using the Simpson Strong Tie Rafter / Hurricane Screws via a QuikStik installation tool.

    1. Expert Member
      MALCOLM TAYLOR | | #5

      Greengeek,

      Great. So back to your original question: I share your unease at having the plywood in contact with the slab. I always keep mine up 1/4".

      1. GREENGEEK | | #6

        Thanks for the weigh in.

        I read somewhere a simple trick would be to temporarily tack a couple of nails between the mud-plate and slab helping to keep the sheathing even while working out a level plane and tacking it in place.

        1. Expert Member
          MALCOLM TAYLOR | | #7

          Greengeek,

          Or sheath the wall before you stand it.

          1. Expert Member
            Akos | | #8

            +1 on sheathing before standing.

            I do all the taping, install rigid, house wrap and rain screen strapping before standing it up. This way once up, the wall is weather tight.

            You do have to do a bit of planning on how to connect the air barrier around corners/foundation/roof once all the walls are up.

          2. GREENGEEK | | #11

            Thanks.

            I have been considering that.

  3. jollygreenshortguy | | #9

    I don't know your termite situation. But if others should come across this thread with similar questions, one reason for keeping the sheathing flush with (or a fraction of an inch above) the foundation concrete is that it makes detailing a termite shield simpler.

    Keep in mind also that termites are gradually moving northward with climate change and if we build our homes for the long term, future generations, God only knows how far north termites will have gotten in 50-100 years.

    I'd also aim for 8"-12" clear from ground surface to top of concrete. I also typically specify a 2' zone around the foundation, with an impervious clay surface sloped away from the house. There are to be no plantings or irrigation lines in this zone. Under the clay is drain rock to a foundation drain. This effectively creates a dry moat around the house that further discourages termites and generally reduces moisture issues.

    1. GREENGEEK | | #12

      Thanks!

      I'm in Texas so termites here are more prevalent than people and the size of your average cow. All joking aside, termites are terrible and are a big concern. I am considering pre-treating with Bora-Care, although I am also concerned about health and wellbeing. Bora Care seems to be the least chemically volatile and government friendly treatment I can find. I will be using a solid EPDM mud / sill gasket which will help protect the bottom of the mud plate, although laying down a piece of copper flashing would be best.

      Because of my climate I will have to have drip irrigation within the first 6" of the perimeter of my foundation to keep a small but constant amount of moisture in the soil to prevent shifts in PVR / PVD. I did however build the pad well above grade and will be finishing off the landscape grade so I have 10+" of clearance between the foundation ground below. I have worked on far too many homes with flower beds and seen the damage first hand from plants and bugs. I don't plant on having any plantings within the first 12' of the house except a small solid concrete trough (more of a retaining wall) out front.

      I really appreciate all this input and cannot stress the value it's providing. Thank you all!

  4. maine_tyler | | #10

    "I'd also aim for 8"-12" clear from ground surface to top of concrete."

    I agree, 6" doesn't actually seem that much.

    +1 on keeping sheathing off the concrete. What's the reason to have it overlap really? Plenty of reasons not to.

    1. GREENGEEK | | #13

      Thanks!

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