How to Rough-In Plumbing for a Bathroom Vanity

Obtain the specification sheet for your new vanity to confirm its dimensions. Mark the vanity's total width on the wall. Determine the centerline of where the sink will be located within that width. Using this centerline, measure and mark the final locations for the drain and supply lines. A standard rough-in height for a vanity drain opening is 18-20 inches above the finished floor. The hot and cold water supply lines are typically located 2-3 inches above the drain and are spaced about 4 inches to the left and right of the drain's centerline (8 inches apart total). Use a tape measure, a 4-foot level, and a pencil to mark these precise locations on the wall surface or directly on the studs if the wall is open.

First, use a stud finder to locate the studs that frame the bay where your plumbing will be installed. Mark the stud locations. To create the access opening, use a sharp utility knife to score the perimeter of the area you intend to remove. Then, using a drywall saw or a reciprocating saw with a suitable blade, carefully cut along your scored lines. The opening should be large enough to provide ample room to work with pipes, typically from the floor to about 48 inches high and spanning the full width between two studs. Remove the cut piece of drywall and set it aside if you plan to reuse it.

This step requires precision. Identify the main vertical drain stack (typically 3" or 4" PVC pipe). At the correct height, use a reciprocating saw to cut out a section of the stack just large enough to fit a new 1-1/2" sanitary tee fitting (using reducer couplings if needed). The tee's sweep must point downwards. Next, assemble the drain arm. Use a PVC cutter or hacksaw to cut sections of 1-1/2" PVC pipe. Deburr the cut ends with a deburring tool or file. Dry-fit all components first. To glue, apply purple PVC primer to the outside of the pipe end and the inside of the fitting socket. While the primer is wet, apply a liberal coat of PVC cement to both surfaces. Firmly push the pipe into the fitting with a quarter-turn twist and hold for 30 seconds to ensure a strong bond. Build the drain arm from the sanitary tee to the vanity location, ensuring a constant downward slope of 1/4 inch per foot. Use a level to check the slope. Terminate the arm with a 90-degree sanitary stub-out elbow that will emerge from the wall. Secure the entire drain arm assembly to the studs using plastic pipe straps and a drill/driver.

Ensure the main water supply is off. Locate the nearest hot and cold water lines. Use a tubing cutter to make clean cuts in the existing pipes. Install PEX-compatible tee fittings into the lines. From these tees, run new 1/2" PEX tubing (red for hot, blue for cold) towards the vanity location. You may need to drill 3/4" holes through studs to route the tubing; use your drill/driver for this. Install a horizontal 2x4 wood block between the studs at your marked height for the supply lines. Securely mount two 1/2" drop-ear elbows to this block. These will serve as your stub-outs. Cut the PEX tubing to the final length with a dedicated PEX cutter. To make the connection, slide a PEX crimp ring over the end of the tubing, push the tubing onto the barbed fitting of the drop-ear elbow, position the ring 1/8" to 1/4" from the end of the pipe, and compress it using a PEX crimp tool until it clicks or fully closes.

To protect your new plumbing, place steel nail plates on the face of any stud where water or drain pipes pass through. Hammer them into place. These prevent accidental puncture from future drywall screws. Now, test the system. For the water lines, wrap the threads of two 1/2" NPT threaded caps with Teflon tape and screw them tightly onto the drop-ear elbow stub-outs using an adjustable wrench. Go to the main water shutoff and turn it on SLOWLY. Listen for running water. Return to the bathroom and meticulously inspect every new fitting and connection for any signs of dripping or moisture. Leave the system under pressure for at least 30 minutes. For the drain, you can cap the stub-out with a test plug and fill the new pipework with water from a higher cleanout (if available) to check the solvent-welded joints.