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What is a good rule of thumb for how wide to cut the trench when adding new drain/waste/vent plumbing to a foundation?

Here is a made up example rule similar to what I am hoping to find:

Start with 2x the width of the pipe and
add 1” for every 6” you need to dig down

I ask because I would like to rent a concrete saw for just a day to cut the outline for the trench to create a nice clean edge and then do the jackhammering/digging on days afterward and I’d rather not have to rent the concrete saw again (or chisel and leave a jagged edge) if I find I didn’t cut wide enough.

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That's a good rule, if a bit over-complicated. For most residential work that's within 24" of slab level, 12-16" is plenty, assuming that it's placed accurately.

You can always bell out the trench under the slab a bit if needed for a large assembly. Of course, plumbers would always want more. :)

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I always cut an inch wider than my shovel. Cutting wider than that would be a waste and cutting a narrow trench makes digging impossible especially if you run into large rocks. I also drill the edge of the cut slab and install galvanized nails every few inches this helps hold the patch from shifting in the future. I have seen a large number of patches that looked bad years later when the ground settled and the patch shifted, with nails every few inches it ties it from being able to shift in the future.

  • Thank you for the all around solid advice. Im only accepting Isherwoods answer because of more precise numbers around width (I have a 4” wide trenching shovel and adding an inch to that definitely wouldnt get me a hole wide enough for 4” pipes 2 ft deep). The intent with a normal spade shovel is clear though and gives good perspective on how to choose width along other good techniques I hadnt considered. – statueuphemism Apr 9 at 0:29

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