How to Rough-In Plumbing for a Basement Bathroom

The first and most critical step is to create a detailed plan. Begin by measuring the space and deciding on the exact layout of the toilet, sink, and shower. Use a tape measure and marker to draw the outlines of the fixtures and the walls directly onto the concrete floor. Consult your local plumbing code to understand requirements for pipe sizes, distances from fixtures to vents, and clearances around each fixture. For example, a toilet typically requires at least 15 inches of clearance from its centerline to any side wall or fixture. Create a clear plumbing diagram showing the drain, waste, and vent (DWV) layout, as well as the hot and cold supply lines. This plan will be submitted to your local building department to obtain a plumbing permit. Do not begin any physical work until you have the approved permit in hand.

This is the most physically demanding step. Wearing full PPE (safety glasses, hearing protection, dust mask, gloves), use a chalk line to mark the paths for your drain trenches. Begin breaking the concrete along these lines using a sledgehammer for smaller adjustments or an electric jackhammer for the main trenching. Break the concrete into manageable pieces and remove them from the basement using a wheelbarrow. Once the concrete is removed, use a shovel and pry bar to dig the trenches in the underlying dirt and gravel. The trench must be deep enough to accommodate the pipe and maintain a consistent downward slope of 1/4 inch per foot towards the main drain line. Check the slope frequently with a level as you dig.

This step involves assembling the Drain, Waste, and Vent (DWV) system. Start by laying a bed of gravel in the trench to support the pipes. Using a PVC pipe cutter, cut your Schedule 40 PVC pipes (typically 3-inch for the main line and toilet, 2-inch for sink and shower) to the required lengths. Use a deburring tool to clean the inside and outside of each cut. Dry-fit all pipes and fittings (wyes, elbows, etc.) together, from the main sewer line connection back to each fixture. Use a level to ensure a constant slope of 1/4 inch per foot. Once you have confirmed the fit, disassemble the pieces. Apply purple PVC primer to the outside of the pipe end and the inside of the fitting socket, then immediately apply PVC cement to the same surfaces. Push the pipe firmly into the fitting with a slight twist and hold for 30 seconds. Install the toilet flange so its top will be flush with the finished floor height. Connect the shower drain assembly and its P-trap. Run the vent pipe (typically 1.5-inch or 2-inch) up inside the new wall framing, ensuring it connects to the drain line correctly and terminates by tying into an existing vent or running a new one through the roof, per code.

With the DWV system in place, you can now run the hot and cold water supply lines. First, shut off the main water supply to the house and drain the lines. Identify the nearest hot and cold supply lines to tap into, typically 3/4-inch main lines. Cut into these lines and install tee fittings. From the tees, run new 1/2-inch supply lines (PEX is popular for DIY due to its flexibility and ease of use) to the new bathroom. Route the lines through the wall studs to the locations for the sink, toilet, and shower valve. Use a PEX crimping tool to make secure connections if using PEX, or a soldering torch for copper. Secure the pipes to the framing using pipe hangers or clamps every 3-4 feet and at every change of direction to prevent movement and water hammer. Install stub-outs at the correct height for the toilet and sink, and install the shower valve body in the wall framing according to the manufacturer's instructions.

Before you can cover any of your work, it must be inspected by your local building official. Schedule your rough-in inspection. To prepare, you must conduct a pressure test. For the DWV system, use inflatable or mechanical test plugs to seal all the openings, including the lowest point connecting to the main sewer line. Leave the highest vent stack opening accessible. Fill the entire DWV system with water up to the point of overflowing the highest vent on the roof or up to a 10-foot head of pressure (a 10-foot vertical pipe attached to the system). The system must hold this water without any drop in level for a specified time (usually 15-30 minutes) to prove it is leak-free. The inspector will verify this test, along with checking all pipe sizes, slopes, materials, connections, and support. They will also inspect the water supply lines for proper installation and support.

Once you have passed the inspection, you can close up the floor. First, carefully backfill the trenches with the gravel you previously set aside or new gravel. Surround the pipes completely and compact the gravel gently to ensure the pipes are fully supported and will not shift. Be careful not to damage the pipes with sharp rocks or excessive force. Lay down a vapor barrier (a sheet of plastic) over the gravel to prevent moisture from wicking up through the new concrete. Mix your ready-mix concrete according to the package directions in a wheelbarrow or a rented concrete mixer. Pour the concrete into the trenches, slightly overfilling them. Use a shovel or trowel to work the concrete around the pipes. Finally, use a straight piece of lumber (a screed board) in a sawing motion to level the new concrete patch flush with the existing basement floor. Let the concrete cure fully, which can take several days to a week before it's ready to be walked on or framed over.