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Community and Q&A

How to waterproof foundation walls pinned to rock

AJ__ | Posted in General Questions on

The foundation walls have been poured on my project on a rocky sloped site. I ended up having to pin the walls directly to the rock (geotech’s requirement) so there are no footings anywhere. There will be an unvented conditioned crawlspace which will be below grade on the uphill side, and above grade on the downhill side. The wall on the uphill side is about 8ft tall and the crawlspace slab will sit about 5 feet from the top of the wall, so at least a couple of feet above the sub grade. For the waterproofing strategy, I was planning on using an elastomeric fluid applied coating all the way around, with dimple mat on the uphill side. The walls were scribed to the undulating surface of the rock so I’m wondering how best to install the dimple mat? Rough scribe it or try to transition it from vertical to horizontal at the base of the wall? Do the details really matter so long as bulk water can’t sit above the slab level of the crawlspace walls? The perimeter drainage will near the base of the wall but will be above some of the low points in the rock. The crawlspace wall vapour barrier will be 4 inch EPS (Craiglist score), sub slab will be 4 inch PES and poly.

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  1. Yupster | | #1

    The dimple mat performs the function of relieving hydrostatic pressure which might otherwise drive water through your wall. As long as it goes below your slab, it will do it's job. I would carry it down to your weeping tile. No need to make it go horizontal.

  2. Expert Member


    Some small amount of water will make it's way under the wall at the high side of the slope, but it should also be able to make it's way under the wall at the bottom. All this occurring well below the slab.

    One belt and suspenders approach to this situation is to have a 3" drain pipe running through the wall just above the footings on the low side, with a small pile of drain rock on the inside, surrounded by the underslab fill.

    1. AJ__ | | #3

      Hi Malcolm

      Since stripping the forms, I have noticed small amounts of water pooling in some of the low spots on the upslope side on the outside of the wall after rain. It wicks up in to the concrete and doesn't obviously pass underneath. That's not to say I'm doubting you but just making sure your advice still stands and it's not an issue!

      In relation to you suggestion, weep holes were placed on the downslope side of the wall, and half way up the sides where the contours of the rock sloped towards the sides.

      1. Expert Member
        MALCOLM TAYLOR | | #4


        If there are low spots in the rock on the high side that would otherwise allow the surface water to drain towards the sides of the foundation, it might be worth filling them in with concrete. I wouldn't be overly concerned either way - especially as you to0k the precaution of including the weep holes.

        Still managing to keep going with your place in the midst of all the disruptions? My stuff is all on hold.

        1. AJ__ | | #5

          I'll give that some thought, I just re-read my last message and not sure I made it clear the weep holes are on the downslope wall, not the upslope one where some water is pooling. We're due for some sustained rain this week so I'll see what happens.

          All my subs have shut down and the excavation crew was in Vegas for a trade show, they're in week 2 of quarantine! All the back filling and rough ins will have to wait. Glad to live on an island at a time like this.

          I have enough to keep me busy though, while another pair of hands to help strip and move all the material would have been nice it means I have plenty to do, as well as the joy of hand digging a trench down to the existing septic system for the main house.

          1. Expert Member
            MALCOLM TAYLOR | | #6

            I understood the weep holes. That's great. The only danger from the small amount of water that might make it under the upper-wall is if it somehow built up hydrostatic pressure under the slab if it couldn't escape.

            I'm using the time to renovate my kitchen, which I've been planning to do for about twenty years. If supplies keep moving I may re-roof this spring too.

  3. AJ__ | | #7

    Haha 20 years of planning, that's thorough! Did you account for all the thermal bridging from the handle on the fridge ;) Is it a face lift or a complete start over?

    I sent you an email, I saw in an old post you prefer spray on foundation waterproofing-have you used hopper style spray guns for this?

    1. Expert Member
      MALCOLM TAYLOR | | #8

      I'm staging a complete re-do of the kitchen. Phase one is re-supporting a steel beam and turning a pass-through into an open peninsula. The biggest challenge is re-routing a lot of wiring without ending up with tons of junction boxes.

      1. Yupster | | #9

        I had a week off last year and decided to start renovating my kitchen...a week before my new baby was due. That was a poor decision. Kitchen sinks are handy to have when you have a baby.

        1. Expert Member
          MALCOLM TAYLOR | | #10

          Kitchens are like venus fly traps just baiting us into getting involved.

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