Wet basement floor - dimpled membrane OK?

wysecjDecember 27, 2009


I've got a wet basement that I'd like to finish. I've fixed all the downspouts, done some regrading away from the foundation, installed an interior french drain, but it's still damp in a couple spots. In particular, I have a 30" square spot that is permanently damp - it's been wet for almost a year now, even with the humidifier running all summer. Additionally, it's right in the middle of the floor with the french drain.

Here's some background info:

1. Basement floor installed by previous owner. He did a poor job - I don't believe that the floor is level, and it doesn't have a consistent thickness.

2. The floor was poured in sections. Doesn't appear to be any water in cracks between sections.

3. Before he had the french drain installed, he had a sump pump in that section of the basement.

4. Floor thickness appear to be very shallow in some areas, possibly as thin as 1".

Some pictures:

Joint between poured sections

View of room

Large (30") wet area 1

Large (30") wet area 2

Wet cracks near french drain

Wet cracks near french drain

Wet cracks

I was originally thinking that to finish this room, I would need to at least get the 30" wet area fixed. I was planning on chipping it out and filling it with hydraulic cement. I was also thinking I'd probably need to level the floor with self-leveling cement.

After researching on the web, it appears that I can put down a dimpled membrane instead of repairing the wet area. Is this sufficient? Am I just asking for trouble?

The pictures also show some wet cracks - these are especially troubling since they are right at the french drain. I don't understand how I'm getting water there - the french drain works, and it is covered with gravel. The wet cracks appear to be wicking - I just don't see how since there is no dirt beneath them. I could understand it if there were a river below the foundation, filling up the french drain, but thankfully, that's not happening.

Will the cracks continue to get worse? Should I deal with them now before I do the floor? How should I fix them?

What about the joints between the poured sections? Are these a problem? They're dry, but they make me nervous. My guess is that there is no bonding agent between them.

If I open up the large crack to repair it, is there potential for a major repair? A river of water running under the basement? My wife says that it's just a basement - forget about it. I'd like to make sure we do it right before we finish the room.

Regarding the walls, I'm planning on polystyrene board glued to the concrete. In front of that, I'd put the metal studs. I wouldn't use a vapor barrier, and would use waterproof (resistant?) dry wall (gypsum?). Assuming that I use the dimpled membrane, should I put the walls on top of the membrane?

Any suggestions would be appreciated. Thanks.


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A river of water running under the basement

If there were, an inch of concrete would do nothing to stop it!

Assuming no unusual soil conditions, specs for a new home would usually be 5-6 inches of crushed stone under 3-4 inches of reinforced concrete. If your basement floor were done that way, it's likely whatever moisture is seeping in would be contained in the stone and drain off to the interior drains or evaporate.

With what your pics show, you could probably get away with plywood over the Delta. But I wouldn't put down an expensive finish floor or lots of partitions in case you had a worse water problem down the line and needed to remove it. The walls go over the membrane.

I would have preferred dampproofing the foundation from the exterior.

    Bookmark   December 27, 2009 at 4:48PM
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I think there may be sand underneath the slab. I believe the concrete is between one and three inches. I don't think there is any rebar reinforcement - at least there wasn't at the edges when the french drain was put in. I'm sure it gets thin at points since there are a couple of holes where I can hit dirt (not in this part of the basement though).

I suspect that the exterior drain is clogged. I had to dig down to it for some well work, so I added a clean out, but I haven't had anyone come out to snake it. Rotorooter charged a lot when I had them snake out about a hundred foot of pipe that my basement drains connect to. I'll get them out again in the spring unless anyone has a better suggestion.

I'm also considering putting a two foot deep drain around the perimeter of the house, four feet from the foundation. I read in some other forums that this can make a big difference and is a lot easier than trying to dampproof or waterproof the foundation.

I'll wait for a few more comments, but I'm thinking that I'll try the Delta-fl as you suggested. I won't have any partitions on it, and I'll go for a less expensive floor.

I would have preferred that the foundation was dampproofed or waterproofed too, but it's too expensive to do now...

Thanks for you help.


    Bookmark   December 27, 2009 at 5:59PM
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I suspect that the exterior drain is clogged.
Could be if they didn't cover it originally with free flowing gravel. It could even be installed upside down, above the footings rather than beside them, not sloped right, not even connected properly. I've found all these things when we're demoing and on new construction.

snake out about a hundred foot of pipe that my basement drains connect to

That's a long run. If it's less than ten years since it was done last, I wouldn't bother.

two foot deep drain around the perimeter of the house, four feet from the foundation. Unless you see a tremendous amount of water sitting on the ground--and it doesn't sound like you do--I'd save the trouble. One of my recent neighbours was sure that was cure for his basement water. Ignoring my opinion (which he had asked) he had a guy with a backhoe dig a two-foot trench 70 feet long paralleling the house and drop in a footing drain surfacing at one end. A few months later--in the dead of winter--he had to hire guys to hand excavate the same foundation for about ten feet, six feet deep. They found the foundation crack and fixed it.

    Bookmark   December 27, 2009 at 7:43PM
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There was gravel above the footing drain when I dug it out to put in the cleanout (actually I was digging to fix a well problem). It was beside the footings.

The pipe was snaked from the edge of my basement to a swale in the back yard, about 100 feet away. To get from the foundation drain cleanout to that pipe (the foundation drain is connected to it), I need to go from the front of the house to the back, about 75 feet. It hasn't been cleaned out for 35 years. Is it worth doing?

I'll forget about the two foot drain. It would be a lot of work, and if I'm not going to get much benefit, then there's no sense doing it.

    Bookmark   December 27, 2009 at 10:46PM
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Hi - It's been a year and I haven't done anything in the basement until last week, when I jackhammered up the wet spot in the floor.

It turns out that there was more concrete there than I thought. Here's what I saw - wet clay soil below about a 3 mil sheet of plastic, followed by about 3 inches of concrete, topped off by a 1/2" layer of cement which wasn't bonded well with the concrete.

After a few days, I now have about a 1/2" of water in the bottom of the hole.

There are a couple of things that I don't understand. First, the basement has a working perimeter drain surrounding the hole. The water level in the hole is above level of the perimeter drain. The drain actually only covers three side, even the the hole is well inside the 'U' formed by the drain. What's going on? Is water traveling into the open 'U', or is it coming up from underneath?

Second, I have a gravity drain that is dug about a foot and a half below the floor, and drains out back. There is no water in that hole, even though it is well below the one I jackhammered last week. The two holes are about 30' apart. Does this mean that the water in th hole from last week is not due to the water table? It seems like it would be easier for the water to use the gravity drain - it's lower, and the basement floor at that point is also lower. What's going on?

Finally, how should I resolve this? Is it OK to have water directly under the floor like that? I was planning on just sealing it up with hydraulic cement, and putting down a floor (using a product designed for damp floors). Is this sufficient or should I do something else?


    Bookmark   November 10, 2010 at 9:20AM
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Pouring the floor directly onto the native clay was a big mistake. Clay does not drain well. A thick layer of non-compacting stone would have permitted any water to run off to the interior perimeter drain.

The ideal fix would be to remove the floor, excavate, fill with gravel, insulate and repour. Quite the job!

If it were my own home, I'd be inclined to use a Delta FL-type membrane or Dricore and finish with a moisture resistant flooring. And never stop dehumidifying or maintaining the existing drains

    Bookmark   November 10, 2010 at 12:00PM
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I had suggested removing the floor in the past to my wife. She was not enthused, and neither am I. It's too big an expense and/or effort.

Unfortunately, I'm going to need to settle for a partial solution and use the Delta FL or Dricore type flooring. I don't think there is much else to do except rip up all the concrete, dig out the clay, fill with stone, and repour.

Thanks for your comments.


    Bookmark   November 10, 2010 at 1:56PM
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Actually, there is one thing I could try. Before I close up the hole, I could dig it out fairly deep (maybe two feet?) and fill with stone. Since I'm seeing water at that point in the basement, it's probably a low point. If the water leaks slow enough maybe it wouldn't be continually filled. Is it worth doing this? The problem that I see is that there is no way for the water to drain from the hole. I need to rely on it eventually seeping back into the clay.


    Bookmark   November 10, 2010 at 2:02PM
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Like a mini-drywell. Presuming that the water is pooling there, rather than being sourced from below.

I'm always mindful of the home I saw where the contractors lowering the original basement floor a foot or so broke through into an underground stream that filled the basement like a swimming pool.

    Bookmark   November 10, 2010 at 6:11PM
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Are you trying to scare me? Did you make that up?

Or are you saying "Don't bother with the mini-drywell." ?


    Bookmark   November 11, 2010 at 8:13AM
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Did you make that up?

I was meeting a trade at a then 60-year old home on Rusholme Rd., Toronto. I glanced in the basement and asked the workers there about the dark line running around the walls about five feet up and got the story. By this time, there were two sump pumps running 24/7. The funny thing is that the many underground streams were well known to anybody who cared to investigate.

I once similarly nipped into an underground flow excavating for a new home. It was running maybe a gpm, so it was more easily handled. But if someday future owners decide to lower the basement in that corner, they may be mightily surprised.

    Bookmark   November 11, 2010 at 9:56AM
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I guess I'll take it slow and dig out the area. I think it's just water seeping through - I seriously doubt that I have an underwater stream. If the flow starts to pick up as I dig, I'll stop, fill what I dug with stone, and add concrete.


    Bookmark   November 11, 2010 at 10:40AM
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