Many bathroom remodeling jobs involve moving or replacing the shower stall. This often requires moving a drain connection that penetrates a concrete floor. During the construction process plumbers lay out the drain system in dirt-covered trenches before the cement workers pour the concrete slab. This leaves the shower drain under the concrete slab. The drain system must slope from the shower, or other plumbing fixture, down to the home's septic tank or sewer system. Local building codes determine the rate of slope, but often the drain runs with a slope of 1/4 inch per foot.
Draw a 6- to 8-inch wide path on the concrete floor from the old drain location to the new drain location with a red crayon. Choose the shortest route. Red crayon remains visible while cutting the concrete.
Open the bathroom window and turn on the exhaust fan. Close all doors to eliminate drafts and keep the dust as localized as possible. Put on safety glasses and a dust mask.
Cut through the surface of the concrete with a side grinder equipped with a 4-inch concrete-cutting blade. Follow the crayon lines. A concrete cutting blade has a diamond studded blade with grooves that extend to the blade's center.
Chip away the concrete slab between the cut lines with an electric chipping hammer. Start in one corner and work through the slab until the chipping hammer's blade reaches the dirt underneath the slab. Remove all of the broken concrete from the path. Discard the debris.
Dig a trench in the dirt underneath the removed concrete with a narrow shovel. Start at the old drain location and work to the new spot. The depth of the trench must equal the depth of the bottom of the old drain line. Save this dirt.
Check the label printed on the side of the old drain line for its size and material type, usually "SCH 40" and "1 1/2." The drain line extension must use the same size and material as the old line.
Cut the old drain's horizontal pipe with a reciprocating saw. Make the cut at least 3 inches from the 90-degree fitting that turns the drain upwards.
Place the old drain line's cut-off piece at the new drain location. Aim the 3-inch cut-off section towards the old drain line.
Measure the distance between the horizontal pipe and the end of the cut-off piece with a tape measure. If the new drain location requires a turn in the extension pipe, place a 45-degree fitting in the trench where needed, and measure from the fitting to each cut end. Never use a 90-degree fitting when extending a drain line. Transfer the measurements to a piece of drain pipe and mark the pipe with a pencil.
Cut the drain pipe with a reciprocating saw at the pencil marks.
Glue a coupling onto the old horizontal pipe, and the 3-inch side of the pipe that was cut off of the horizontal pipe, using PVC glue.
Glue the extension pipe to the couplings with PVC glue. If the extension pipe requires 45-degree fittings, glue them in place. Allow the glue to dry.
Place a bubble level on the extension pipe. If the extension pipe does not slope down to the old drain pipe, lift the end of the pipe and pack dirt underneath. Continue to pack dirt under the extension pipe until the slope reaches 1/4 inch per foot and all hips and valleys disappear.
Pack the remaining dirt on top of the drain pipe.
Mix concrete and water in a bucket, with a trowel. Use the concrete manufacturer's instructions to determine the correct amounts of concrete mix and water. Stir the mix until it has the consistency of toothpaste.
Fill in the cut-out section of floor with the wet concrete. Use the trowel to smooth the concrete.