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Checking the Plumbing
in a Concrete Slab

The plumbing lines that are installed under a concrete slab are one of the most critical parts of your home project, and you must check every pipe! Even though the plumber believes he did it all correctly, check it anyway! Everyone can make mistakes, and mistakes in the location of these lines under a concrete slab can be disastrous.

This page covers checking the sewer and water lines in a slab. See our page, Checking Over the Concrete Slab, for a complete breakdown on the other items to review and check.

Have your plumber ready to come right away after the fill is spread and compacted under the slab. He will dig trenches as needed to run the sewer pipes and water pipes, and should smooth the dirt or sand back when finished.

Take your floor plan and measuring tape, and check the location of every pipe. Using the dimensions shown on the floor plan, measure from the outside of the foundation walls or slab! The photo above shows a sink drain, hot water line, and cold water line at a stud wall on the outer edge of the floor slab. The pipes must be at the sink location shown on the floor plan.

You will probably be checking the layout after the workers have finished and gone for the day. If anything doesn't look right, mark it with your flagging tape and talk with the plumber as soon as possible. Mistakes must be corrected before the concrete folks come back to prep and pour concrete!

The drains and vent pipes on this house fall within load-bearing stud walls, except for the toilet and tub drains. The nearest pipe in the photo is a vent pipe, the pipe to the left is the washing machine drain, and the pipe beyond in the trench is a sink drain. The pipe in the black box is for the bath tub, and the largest pipe sticking up a few inches at the outer wall, to the left, is the toilet drain. (Notice the deeper trenches that form "grade beams," or thickened slab areas, beneath the stud walls.)

The photo below shows the same area during the pouring of the concrete, viewed from a different angle.

Views of the same drain and vent lines after the concrete slab was poured, and after the wood wall framing was installed.

Plastic sand box that makes a blockout in the finished slab for the drain at a shower or bathtub. The blockout gives some space to adjust the drain piping, as shown in the picture below.