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There was a recent flood in my neighborhood that caused several wet basements (directly from the river, or indirectly through rising water levels). My house didn't suffer water damage but a week or two after the river receded, I noticed dark wet spots on the concrete floor of the basement. There are also some efflorescence visible.

A few contractors who have come to fix some foundation cracks (this was noticed before the flood) have said the dark patches is due to hydrostatic pressure but they don't recommend putting in a sump pump. One said he would do it but also said it won't solve the hydrostatic pressure problem.

My question is why won't a sump pump help? If water is trying to get into the basement wouldn't pumping it out alleviate the problem? or will installing a sump pump cause the water to overflow the hole and flood the basement anyway?

  • Once the water is close enough to the foundation to be pumped out it's already making things damp. A pump probably won't solve the minor patches you mentioned. Really, it's mostly for preventing standing water. Your foundation isn't perfectly waterproof, so if there was a serious problem you'd know it. – isherwood Jun 6 '17 at 17:58
  • For what it's worth I have exactly this situation right now. I have a functioning sump pump and a dehumidifier in my basement and I still see moisture in a few areas after extended rain. – isherwood Jun 6 '17 at 18:33
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why won't a sump pump help? If water is trying to get into the basement wouldn't pumping it out alleviate the problem?

A sump pump is designed to be located in a low area, usually a basin or pit, and is for removing water that is actually in the room.

The hydrostatic pressure that you are talking about is the pressure of the entire water table in the earth surrounding your home. There is no way you could effectively lower the entire water table (and subsequently reduce the hydrostatic pressure on your basement walls and floor) with a single sump pump.

...will installing a sump pump cause the water to overflow the hole and flood the basement...?

Yes, if I am understanding what you are proposing. If you cut a hole through your basement floor and dig a pit, the surrounding water that is exerting hydrostatic pressure on your basement walls and floor would come inside, it would be a bad day for you.

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Where I live, we have a very high water table (about -24"). So, we deal with hydrostatic water pressure on our basement walls year-round.

We use a 4" - 6" perf pipe encapsulated in drainrock with the perf pipe located 6" or more below the basement slab. We cover the basement wall with a mastic membrane covered with a protection board (to keep the membrane from getting damaged when drainrock is backfilled against the wall) and then backfill with drainrock up to within 6" of finish grade.

This does two things: 1) it allows all SURFACE water to run down the wall and flow into the perf pipe, and 2) it keeps the SUBSURFACE water off the wall and eliminates the hydrostatic pressure on the wall.

However, the perf pipe needs some place to discharge, (I.e.: embankment, creek, etc.) We don't use sump pumps around here, but if we didn't have a place to discharge the water, I guess a sump pump would work, as long as it drained away from the site.

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