Adding a bathroom, a floor drain or a sump line to a basement usually means the same thing: a trench through the concrete floor. The pipe has to get under the slab, and the slab is in the way.
It is a small job that goes wrong in predictable ways. This is how it is done properly, and what you can do to make the day go quickly.
What is under a basement slab
More than most people expect. A basement floor can hide drain lines, water lines, electrical conduit, radiant heat tubing, reinforcing steel and, in some buildings, post-tension cables. Almost none of it is on a drawing you can find.
Calling 811 does not solve this. The 811 service has the utility companies mark their own buried lines outdoors. Lines that belong to the property are the owner's to find, and nobody comes out to mark what is inside your floor. We wrote that up separately: 811 marks the yard, not your slab.
Why the scan comes first
We scan every slab with ground penetrating radar before a blade touches it. The scanner rolls across the floor and shows what is embedded in the concrete and roughly how deep. We mark what we find on the floor, then lay out the trench with you and your plumber.
Sometimes the scan moves the trench over. That is the point. Shifting a line on the floor costs nothing. Cutting through a live conduit or a drain you did not know about is a repair, a delay and a hard conversation.
Radar has limits, and it is fair to know them. Very new or wet concrete is harder to read, tightly packed reinforcing can hide what sits below it, and plastic pipe is harder to pick out than metal. A scan lowers the risk a long way. It does not replace care with the saw.
How the trench is cut
A trench is two parallel saw cuts with the strip between them removed. That is flat sawing: a walk-behind saw where there is room and a hand saw where there is not. The blade is diamond, and the depth is set so the cut goes through the slab and stops.
Sawing matters because of the edge it leaves. A jackhammer alone breaks the slab wherever the concrete is weakest, which leaves a ragged trench and cracks running out into the floor you wanted to keep. Two clean saw cuts give a straight edge, and the patch can sit tight against it.
One thing homeowners often find out the hard way: basement slabs are not always the thickness you expect, and a rented saw may not reach the bottom. If there is a floor drain or a crack where you can see the edge of the slab, measure it and tell us.
Dust, water and noise
Cutting concrete dry fills a house with fine silica dust. Breathing it is a known health hazard, and it settles on everything you own. The federal safety rules for construction treat water fed to the blade as the standard way to control it.
So we cut wet. Water at the blade turns the dust into slurry on the floor, where it can be collected. We manage the water and leave the site clean.
It is still loud, and there will be water on the floor while we work. Move what you can out of the room, cover what you cannot, and plan for the basement to be a work area while the cut is made.
Closing it back up
Once the plumber has set the pipe and any inspection is done, the trench is filled and the floor is poured back. A good pour back is flush with the floor around it, matched to the same grade and finish, so the next trade can frame or lay flooring over it.
We do pour backs as well as cuts, so one crew can open the floor and close it again. Ask when you call who is removing the cut concrete and who is backfilling the trench, so nothing is left between trades.
General information, not engineering advice — every slab is different.
