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Drainage and/or waterproofing for stem walls

maine_tyler | Posted in General Questions on

Hi all.

I am planning to dig a trench around a garage (stem walls with interior slab on grade) to install some EPS foam.

The soils are clay heavy.

Would it be worth:

1) Also adding drain pipe next to the footing with crush stone, geo-fabric and sand back fill thereafter? NOTE: the garage is an attached and is slightly uphill of the house, which itself does not have exterior drains. So my thought is that, at the least, this may help divert some water from the house which has a basement.

2) Also waterproofing the concrete.

My inclination is yes on number 1, and no on number 2 (since they’re just stem walls, no basement), but what do you all say?

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  1. user-2310254 | | #1


    Where are you located?

  2. maine_tyler | | #2

    Maine. Zone 6a

  3. GBA Editor
    Martin Holladay | | #3

    1. How high are the stem walls? In other words, is the top of the stem wall even with the interior slab, or higher than the interior slab?

    2. Is the exterior grade lower than the slab on all sides of the garage? Or is the exterior grade higher than the slab on the uphill side?

    3. Is there any polyethylene under your garage slab?

  4. maine_tyler | | #4


    1.The stem walls come above the interior slab by 3 or 4 inches. The slab is poured inside the stem walls (save for the doors, which appear to have the slab poured over the wall).

    2. the exterior grade is about 3-4 inches below the top of the stem walls, which is to say, about even with the top of the slab. It really isn't much of a hill. It is rather flat really with a gentle trend from high on the garage side of the house to lower on the opposite side (where the sewer exits).

    3. I do not know if there is polyethylene. Built, I believe, in the mid 1990's the garage was. I assume there is no easy way I can test for the presence of poly? There is a small chance I will be cutting into the concrete where it connects to the house foundation, and could perhaps tell then.


  5. GBA Editor
    Martin Holladay | | #5

    Since the slab is even with grade -- that's low -- and there is uncertainty about the polyethylene, both suggested measures to limit water entry make sense. It's good to keep your slab dry.

  6. maine_tyler | | #6

    Thanks Martin.

    My 'plan,' if it can be called that, involved tying into an existing pvc pipe, already below ground, that exits the basement for the sump. This pipe goes to a storm drain (through neighbors property). I realized that this pipe exits the foundation perhaps not even quite 2 ft below grade, which means I will not be able to put the drain title 4ft down (by footing) where I had planned.

    The way I see it I have (at least) 3 options.

    1) Put the pipe only as low as the existing pipe to storm drain allows, thereby creating a passive system, but one that does not address water down to the footing. In this scenario, I assume I would only waterproof/damp-proof the foundation to this point. Additionally I would only insulate to this point, if it is a true 2 ft.

    2) Put the pipe next to the footing and route it into the basement to the sump pump, which will then pump it up to existing pipe to storm drain

    3) Put the pipe next to footing and route it to some sort of drywell. The effectiveness of a drywell seems hard to predict from what I have gathered, but this option would allow a passive system with the pipe at full foundation depth.

    I am unsure exactly how much high ground water is a problem (vs surface water). Our neighbors mentioned they have some ledge on their yard (only hundred or so yard away) which is perhaps indication that there will be high water.

  7. GBA Editor
    Martin Holladay | | #7

    You don't have to install a drain at the footing. All you really want is for the drain to be below the level of the slab.

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