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I have an unfinished basement in 40 year old home. Block walls are extremely clean and show no signs of water having come down or through them from ground outside but when it rains for a couple of days, I have a small puddle, and by small, not even a half inch deep or a foot wide, on floor by wall. Since I have ruled out surface water, where is it coming from? Could it be coming in where the flor meets the wall from some underground source and if so, why only when it rains? Is there something I can use to seal the joining of wall and concrete floor or seal block? I would like to finish basement but don't dare put in insulation and drywall with this unresolved.

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We can only guess at this point, but one theory is that when it rains, you have water seeping down the outside of your foundation until it hits the footer, at which point it's seeping into your basement.

At the point where you have this puddle, what is outside? Is there a gutter downspout there? Or perhaps the ground is sloping towards your house at that spot? Check those things first.

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I had the same problem on one side of the house. I even dug down by hand on the outside to the bottom of the foundation and put a few pieces of drainage pipe into the ground, to motivate the water to drain away from the foundation. It did not solve the problem.

In the end, I was successful with a combination of measures:

  • I put a 4-foot wide concrete apron along the foundation
  • I installed a dry well for the nearest down spout
  • I re-graded the side of the yard for better surface drainage away from the house

This solved the problem for good and I have not seen any more standing water or even wet spots in the basement. It wasn't entirely cheap, though.

My takeaway: this is usually a combination of

  • soil at the bottom of the foundation being less permeable than the soil that was used to backfill along the foundation walls (from compaction during construction, and because it was undisturbed)
  • poor surface drainage along the foundation

As a result, even though there is no drainage problem visible at the surface, water will back up at the bottom of the foundation, seep underneath the foundation walls, and enter the basement through the floor or through the gap between foundation walls and the concrete floor.

One other thing to consider: If the house is built on the slope of a hill, it is also possible that you have water draining towards the house below the surface, which would require additional measures.

The picture shows the problem - water coming down the downspout on the back corner cannot drain away from the house because of the pool and poor grading - and parts of the solution - the form for the concrete apron going in. It does not show the drywell in the front of the house and the drain pipe from the downspout in the back that is already in the ground here. The concrete apron is sloped in 2 angles - away from the house and by about 6 inches from the back of the house to the front.

I agree - you have to solve this before you finish the basement. Unfortunately, it cannot be solved on the cheap from the inside.

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