We noticed issues with our water coming through our basement cinder block wall and hoped a new interior french drain would fix them. It did not.

Short Version: We just installed a new french drain and we still have wet spots on the wall as high as six courses up. Why?

Long Version: The picture shows our basement. The black is the cinder block wall. The part highlighted in red is the unfinished part. The part in blue is finished. The front of the house is at the top. 'S' is the location of the sump pit.

The previous owner painted the walls and the paint was flaking off because of efflorescence. The efflorescence was worst in the corner (C) and along the front wall. In the corner the efflorescence went almost to the top of the wall, but it got lower farther from the corner. There were wet spots at C where the paint had been brushed away. The very bottom of the wall at W would sometimes get wet and there would be a small amount of water on the floor. I made a small hole in the finished part of the basement (at H). The drywall, sill, stud, and insulation were clean, dry, and free of mold, but the block behind them had a strong smell of mildew.

We talked to a couple of waterproofing companies and they all agreed that a new interior french drain would fix the problem, so we got one installed. A three foot section of the old drain along the left wall was totally clogged with sand but the rest was clear. They said the existing pipe going into the finished area was clean, so they connected it to the new white S&D pipe with holes in it. There were already holes in the bottom row of block, so they did not drill any new ones.

It rained really hard today and the spots at C are wet and there is water on the bottom block at W. The new pipe coming into the pit is totally dry. The wet spots at C are as high as six courses up and the pipe is dry, so I can't image there is water in the wall. Where is the water coming from in the corner? What should I do with the finished area? I really don't want to put a new drain in that area if it is not going to fix the problem.


  • Well, I think I figured out what might be part of the problem... I decided to check how they connected the old and new pipes at location W, so I dug a hole to check out out. It turns out they did not actually install a new pipe in that location. They took out the old pipe and just backfilled with sand and gravel. Sigh.
    – Stuart
    Jun 20, 2018 at 4:38

2 Answers 2


Putting a drain sump on the inside is like catching the blood from a cut on your arm into a cup. What you really need to be doing is taking a hard look at what caused the cut and remedy that problem instead. Just pasting on bandaids will not do the trick.

So getting back to the house situation. Look hard at what it takes to get water away from the outside of the basement wall. This can include any and all of:

  1. Proper working rain gutters to divert roof runoff away from the building.
  2. Soil surface preparation and grading to allow surface water to run away from the building as opposed to letting it soak into the ground by the basement walls.
  3. Exterior french drains located in areas where the soil tends to stay persistently wet to help allow water to go into the ground deeper than your basement floor and foundation.
  4. Foundation drain lines on the outside of the basement foundation that allow water from outside to collect and flow away from the building.
  5. Exterior sealing of the basement walls so water does not infuse into the concrete or blocks in the first place.
  • Thank you for the great suggestions on the next steps to take. I plan on talking with professionals regarding grading and exterior sealing, but I am trying to educate myself about what is actually happening inside the wall. The waterproofers I spoke to originally were certain that water was coming into the block and running down the hollow cores. Obviously that is not the case. Does the water wick around the cores? Is this normal or does it indicate there is a crack/hole somewhere?
    – Stuart
    May 26, 2017 at 19:39

I think you are misunderstanding how interior french drains work. Exterior french drains help divert water away from the foundation and thereby reduce the amount of water entering the basement. Interior french drains divert water that has entered the basement through the walls and help prevent mold and flooding.

Typically, in a finished basement an interior french drain is paired with a special wall covering that goes between the cement and the insulation/drywall. This allows the moisture to drip down the cement wall and special wall covering into the french drain.

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