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Our 100-year-old home's basement floor is very thin (1/2"?) concrete. A friend once called it a "rabbit floor", only intended to keep critters from burrowing up into the basement :) It is uneven, cracked in places, and it appears to have been poured directly onto the dirt.

The basement walls are also concrete, about 7' tall (5.5' under grade, 1.5' above). They aren't perfectly smooth and flat; it looks like they may have been poured into trenches dug into the ground, without forms, or perhaps someone simply did a very rough job of skim-coating them in the past...

The basement walls seem strong: the house doesn't show any signs of settling problems over the years.

I want to smooth out the floor and make it nicer. I'm envisioning covering it up with more concrete and/or leveling overlay. I don't intend to finish the basement because it sometimes gets wet in the winter and taking care of the water intrusion is far down on my list of home improvements :)

The water issue is seepage from the joints between the basement walls and the floor. It usually is just dampness, but some years I'll get small puddles (a foot or two long) that last a day before drying. As far as I can tell, these puddles are formed by the joint seepage and not from water coming up through the floor. There is a drain along one wall, but never enough standing water to make use of it.

So, my questions:

  • Is it OK to pour new concrete over old? Do I need to prep the existing stuff first?
  • What thickness of concrete would be appropriate as an overlayer? I don't want the new concrete to break away from the old...
  • Should I put some sort of water barrier between them?
  • How do I interface the new concrete to the walls? (Expansion joint, etc)
  • Is this just a bad idea??

Oh, I forgot to add:

There's a washer/dryer in the basement. Other than that, I'll use it as a shop and for storage.

  • You should get a masonry contractor to come and take a look. There are number of factors that you haven't mentioned, such as substrate (soil) type and any movement issues. There's really no right way except pouring a proper 3-4" slab, as you would if you had nothing now. – isherwood Aug 7 '17 at 17:43
  • Are you located in an area where there is significant seismic risk? For example, within 500 miles of Saint Louis, the Pacific, or the Rockies? – Jasper Aug 8 '17 at 6:38
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    @Jasper I'm in Portland, Oregon. There's no regular earthquake activity here, although they say we are overdue for a devastating quake. If the predictions are correct, my basement floor will be the least of my worries :) One of the FEMA directors said, "Our operating assumption is that everything west of Interstate 5 will be toast.” – bitsmack Aug 8 '17 at 16:29
  • @bitsmack -- Given that risk, my first concern would be to improve the earthquake safety of the house. That's why I own stock in a company that sells earthquake-safety equipment. I suggest consulting a structural engineer before making any significant changes (like altering your basement floor). I have had a good experience with the Galli Group's geotechnical engineering services for the Pacific Northwest. – Jasper Aug 8 '17 at 18:10
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Get a contractor to take a look at it for sure. But I suggest to you that you will really need to rip out all the old concrete, re-prep the substrate and level it before putting in any new concrete. This will be the only good way to get a decent long lasting floor. Make sure to include heavy gauge wire mesh (5x5 or 6x6 inch grid) and re-bar if called for. When the slab is the proper thickness (3.5 to 4.5 inches) this can really help keep it together in the case cracks develop.

While you have it all opened up you should strongly consider adding drain lines along the bottom of the foundation over to a sump in one corner to help divert water from coming in on your new floor.

The type of patch up fix you asked about is really a waste of money because it is unlikely to last. Especially if you intend to use it for a shop floor.

  • There might also be an opportunity to increase the headroom in most of the basement (but not the outermost 18 inches of the basement), by making the new floor be lower than the old floor. This would require making sure that the water seepage does not erode the soil that supports the foundation walls. Depending on the soil, that soil extends outward at least 1 foot in each direction for each foot downward you go from the base of the foundation walls. – Jasper Aug 8 '17 at 6:45

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