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Community and Q&A

Slab on Grade Capillary Break

cody_fischer | Posted in Plans Review on

I’m under construction on a slab on grade project with traditional frost footings at a 4′ depth.  

Below is the detailed drawing. 

We have a capillary break between the footing and foundation wall because ~12″ (it varies) of the foundation wall extends above grade and will constitute a small portion of the ground floor walls. 

We are water proofing and insulating the exterior portion of the foundation wall. 

Do we also need to water proof the interior portion of the exterior wall that is below grade? If we don’t, what is the point of the capillary break on the footing? 

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  1. Expert Member


    The capillary break is to stop moisture moving up the wall from where water may be pooling at the level of your footings. There should be very little moisture under the slab on the interior of your stem-wall. Damp-proofing the interior face of the wall w0uld do no harm, but isn't necessary.

  2. cody_fischer | | #2

    Thanks Malcom -

    Follow up question. My Architect specified a fluid applied capillary break between footing and wall. Unfortunately, the concrete sub in the field built their wall forms already and they don't have the ability to do a fluid applied solution now.

    They intend to jack up the forms temporarily and slide in the capillary break material, and would like to use something as stiff as possible.

    They suggested felt paper. Would that work as a suitable substitute in this situation?

  3. cody_fischer | | #3

    Also - we unfortunately have vertical rebar rather than a key channel for the wall/footing connection. I'm guessing that any sheet material would be compromised in this situation.

    1. woodguyatl | | #4

      I can't imagine how they will install a membrane capillary break by sliding it under forms if there is vertical rebar. It is hard enough to do that when there are no forms. This is not the sort of thing that can be fixed later. The fluid applied capillary break is specifically called out in the plans. I'd insist on that.

    2. AlexPoi | | #5

      It doesn't really matters if you punch a couple of holes through the material. Capillarity is linearly proportional to the area. The holes won't significantly alter the performance of the break.

  4. cody_fischer | | #6

    Talking to myself here...but a further update. Would love for the community to continue weighing in.

    Based on another GBA thread, it appears as though felt paper / other asphalt based damp proofing products would be inferior to poly. Further...we would burn 1 to 2 weeks in project time if we rip out the wall framing and do a latex masonry paint or other liquid applied solution at this point (drying time).

    It looks like we have to go with something like 6 or 10 mil poly to stay on schedule with the project. Should I specify overlapping or taping around the rebar to deal with the compromised poly at those locations?

    1. Expert Member
      MALCOLM TAYLOR | | #8


      Although capillary breaks between footings and the stem-walls above are good idea, they are also still exceedingly rare, even in well performing new houses. So whether its worth the gymnastics to add them now is something I'd think twice about.

      1. cody_fischer | | #9

        Really appreciate the perspective Malcom, this echoes what my architect is saying. Helpful to hear it’s an opinion shared by some in the GBA community as well.

  5. cody_fischer | | #7

    Thanks AlexPoi - helpful to know and puts my mind at ease that even with a compromise solution I'll be getting most of the benefit.

    Project timeline matters a lot here because this is a multifamily building.

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