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Plastic sheeting under concrete wall against existing foundation

user-5516607 | Posted in General Questions on

So when i put up my addition, the addition’s foundation is a full basement. It will therefore be somewhat below the height of my century home’s existing basement. To prevent underpinning and generally expensive problems, we are installing a step in the foundation where it butts up against the existing wall. This way the wall is still supported, the addition can be the depth i want it to be, and I dont have to worry about the expense of underpinning. And i have a handy shelf once its done. 🙂

I have gone all the way with the drainage on the addition, because I really dont want this one to be damp like the existing basement. I have drain pipe inside and outside, running to daylight, and the capillary break on the footing (which apparently no one does – even though it’s ‘best practice’ it doesn’t seem to be ‘common practice’). Dampproofing, dimple mat, gravel, geotextile wrap, sand, etc.

However, this all falls aparts where the wall is poured up against the undisturbed soil near the existing house, as i dont have drainage running along that side of the foundation and obviously i cant dampproof the outside since the step is being poured up against undisturbed soil.. And yes, water is even now leaking in on top of my footings (from under the existing house), so it is a concern. Essentially this negates my capillary break, since the water is coming in above the footing from my original basement and will hit the un-dampproofed wall in the addition. I think its too late to put in drainage tile on that side, and besides I really wouldnt feel comfortable taking away any more undisturbed soil in that location even if it werent.

Is the answer as simple as laying down 6 mil plastic against the existing undisturbed soil, so when they pour the step on top of the plastic it will be the capillary break, stopping the addition basement from getting any damp in it?

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  1. GBA Editor
    Martin Holladay | | #1

    Certainly, a layer of polyethylene plastic (or EPDM roofing, or a pond liner material) against the undisturbed soil won't hurt. There are no guarantees that you won't see a little water, though.

    I would probably have included a perforated drainpipe on the exterior side of the footing shown in your sketch, but I understand that you don't want to excavate there. The interior drain pipe will probably take care of any incidental moisture, but it wouldn't hurt to leave a gap (a slot) in the slab, exposing the top of the interior drain, until you see what happens. You can always pour more concrete to seal the slot later.

  2. Dana1 | | #2

    +1 on using EPDM instead of 6 mil polyethylene.

    If polyethylene, use something heavier than 6 mil- say 10-12 mil.

  3. user-5516607 | | #3

    OK thanks, I will put some plastic in before they pour.

  4. user-5516607 | | #4

    One or the other, that is. Depends what i can find today, as they pour tomorrow. The joys of changing things on the fly mid-build...

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