Step 3 · Install Underground DWV Pipes
This step involves laying the pipes that will carry waste away. Begin by locating and exposing a section of the main sewer line. Cut into the main line and install a wye fitting that will accept your new 3-inch drain line. Use a PVC pipe cutter for clean cuts. Before using any PVC primer or cement, lay out all your pipes and fittings in the trench—this is called a 'dry fit'. Assemble the 3-inch line for the toilet, connecting to a toilet flange set at the correct height. Run 2-inch pipe from the main line to the shower and sink locations, installing the appropriate shower drain assembly and a sanitary tee for the sink drain arm. Use a 4-foot level to constantly check your slope, ensuring it is at least 1/4 inch of fall for every foot of run. Once you are certain everything fits perfectly, disassemble and re-assemble the system permanently using PVC primer (usually purple) followed immediately by PVC cement. Add a layer of pea gravel under and around the pipes for support.