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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. –  Carey Gregory Nov 23 '12 at 22:40
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2 Answers 2

up vote 7 down vote accepted

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. –  The Evil Greebo Nov 24 '12 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. –  shirlock homes Nov 24 '12 at 12:02
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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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