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Advice for keeping a new addition basement dry

joe_fb | Posted in General Questions on

We are adding on to a 1960s house in Eastern MA. This will include a full height basement. 

Our existing basement is dry. We are well above nearby rivers and ponds but in a slightly low spot of the surrounding houses. Only real water concern I have is when we get a layer of frozen snow followed by a long rain causing ponding on the snow in the backyard. This is rare and has never ponded up to the house, but who knows someday. 

Normal rain/thunderstorms haven’t been a problem.

What detailing should be included on the new foundation to keep the basement dry? An interior or exterior foundation drain?  Both? Neither? We cannot run a foundation drain to daylight given the lot. And bringing water from an exterior foundation drain to an interior sump pit seems counterintuitive, though that’s the advice I’ve seen here sometimes.

What’s the line for dampproofing vs waterproofing? I’ve heard waterproofing is less common here away from the coast.

Any advice is much appreciated.

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  1. Expert Member


    Everything you need to know (and then some) is in Martin's article:

  2. joe_fb | | #2

    Thanks, Malcolm. I've read the article and it's very good. A few points of confusion for me:

    1. We have the land area for a drywell. But that sounds like it means digging a very deep hole somewhere in the lot and a deep trench that is the depth of the basement. Is that right?

    2. Is it reasonable to add an interior drain and sump pit now (instead of a drywell) and not add the actual pump until we see that it's needed? Or will the sump pit hole in the floor cause a pump to be needed? Again the current basement is dry.

    3. Does an interior sump pit need to be sealed to the poly under the slab?


    1. Expert Member
      DCcontrarian | | #3

      If you're building new, exterior drains are better than interior drains. You want to keep the water on the outside of the foundation. If they can't drain to daylight code here would require a sump pump.

    2. Expert Member
      MALCOLM TAYLOR | | #4


      The three choices to deal with the water are: drain to daylight, to a municipal storm system, or a rock pit. The most potentially problematic is the rock pit because as you say, it needs to be deeper that the perimeter drains, meaning it will probably fill with water the same way they do.

      The only viable alternative then is a sump pit. That can be either inside or located in a pump chamber on the exterior. Practically you can leave the pump out until it is necessary, but it represents such a small part of the work, I'd do it now - and having a code approved path for that water to drain to will be part of the requirements of any building permit you apply for.

      Whether that pit needs to be sealed to your sub-slab vapour barrier depends on the presence of radon in your area.

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