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When I built my garage 25 years ago, the concrete contractor nixed my idea of installing in-floor drains under each car "bay" and assured me that he would slope the floor toward the garage door so water would run out. On the morning of the pour, I watched for a few minutes, then had to run off to the office and didn't get to "supervise" or ask questions. The pour was completed in one shot - footers, foundation walls, and slab are all one pour.

After building the walls, we discovered that not only was the floor not sloped toward where the garage door would be, it was actually sloped away from the door and toward one corner!

Unfortunately (and foolishly - we were young, the internet didn't exist to ask 10,000 questions of), we rejected his offer to put one course of block on the floor, and built our stud walls directly on slab. On the bright side, though, despite water pooling in the corner, there isn't yet (visible) evidence of any serious rot in the wall, so I'm not yet worried about having to repair that.

As the spouse and I have now reembarked on our journey of house improvements and upgrades, I'm considering trying to fix the improper slope. I've read lots here about using self-leveling cement to get a nice, smooth, level floor for various purposes like covering with laminate flooring. This is not what I want since I do actually want the floor to slope, not be perfectly flat.

What are my options for changing the slope of my monolithic-poured garage floor from a south-to-north (high to low) slope to a north-to-south slope?

If it were a wooden floor, it would be fairly easy to cut some long 2x shims to adjust the level and refloor it, but I don't know how to do that out of concrete.

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    You could install a drain in the corner where the water pools. Cut a hole in the concrete, dig out some soil and replace it with gravel. Put in short piece of PVC with grill, and bevel the concrete around it. I assume the reinforcement is rebar and not post tension.
    – Jim Stewart
    Commented Jun 24, 2020 at 15:25
  • You certainly can't shift the slab, even if it's not bonded to the walls. I disagree partly with @JimStewart because I don't approve of drywells directly under the floor. It's a lot more work but I would get someone to drill a hole thru the foundation rear & run a waste pipe either to an external dry well or, if sufficient slope, just exit into the existing ground.
    – Carl Witthoft
    Commented Jun 24, 2020 at 16:31
  • Self leveling compounds don’t hold up well in garages, how much are we talking ! I usually have close to 2” in a 25’ garage floor.
    – Ed Beal
    Commented Jun 24, 2020 at 16:32
  • I wasn't expecting a self-leveling compound to be the answer @EdBeal because I don't want it level! I'll have to measure the actual amount of existing slope, I've never done that. My FIL has a transit, so I can get a pretty accurate measurement.
    – FreeMan
    Commented Jun 24, 2020 at 16:43
  • Interesting that the contractor did the footers, foundation walls, and slab with one pour. I thought the footers had to be done first, inspected; then the foundation walls, inspected; then the slab. The slab, just like a basement floor, I thought was supposed to "float" on the footers.
    – SteveSh
    Commented Jun 24, 2020 at 18:07

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Install a drain at the low spot, or rent a concrete saw and cut drainage channels (run the saw on tapered shims so the channels slope correctly.)

By cutting channels you should be able to intercept and redirect much of the water before it reaches the low spot.

It might be possible to do that to the whole floor with a terrazzo grinder, but those seem to be a lot harder to find for rental, and it would be a LOT more work. I doubt most things you'd apply on top would hold up to vehicle traffic unless you have room for a couple of inches at the thin part.

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  • I haven't taken any action on this yet, but I thank you for your answer.
    – FreeMan
    Commented Sep 22, 2020 at 13:34

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