I've got a two-car garage (4-inch concrete slab on grade) that I'm turning into living space. I want to put a bathroom out there but I need to get plumbing through the floor. I didn't think it was possible. I had a contractor come over and take a look. He said no problem. We could just cut a 6-inch trench in the concrete halfway-across the garage. This did not sound like a good idea to me. Is this generally a safe thing to do while keeping the integrity of the slab intact?

The garage is on a slab, but the rest of the home is on a crawlspace. So we'd just need to cut the trench from the middle of the garage to the wall of the crawlspace to tie it in with the plumbing.

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    I'm no engineer, but as long as the cut isn't being made near a load-bearing wall, I don't see why it would compromise the foundation. I'd be more concerned with making sure you've got good drainage. If there's any water under that slab, it's going to rise. Nov 23, 2012 at 22:40

2 Answers 2


This is done all the time in slab-on-grade houses. The slab is there to give you a nice surface to park on, but is not required for the structural integrity of the garage.

So, sure, go right ahead.

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    To add - they will replace the concrete once the plumbing work is done. Nothing to worry about - as Eric said it's a standard practice. Nov 24, 2012 at 2:24
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    I also agree with Eric. Done all the time. Because of the distance, you will probably need a vent stack also. Nov 24, 2012 at 12:02

If you are not planning to accommodate a 60 tonne Abrams tank in the room, it will be OK. However, do make sure that the water-proofing under the slab where the cut is made is not punctured. If it is punctured, it must be sealed again so that you don't get a wet streak popping up in your new floor. Structurally, the trench in the concrete will not pose any problem, unless, as someone mentioned, it is next to a wall. Have fun.

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