How to finish a basement bathroom: Mark the floor layout for the shower stall, toilet and sink. Cut the concrete slab floor to relocate and install the shower drain. This project is continued from How to Finish a Basement Bathroom – Part 1. See the series introduction for the project index.

Basement Bathroom Layout: Toilet, Sink and Shower Stall

The next task is to decide where the bathroom fixtures – sink, toilet and shower stall will be installed. The builder had already roughed-in the plumbing so I didn’t have a lot of choices but it’s necessary to measure and mark the locations to check clearances and guide the work.

  • I verified the toilet plumbing rough-in dimension (distance from the wall to the soil stack) because this is an essential measurement for ordering the right toilet. In my case, it was a standard 15 inch rough-in distance from the wall.
  • I discovered that the builder placed the shower drain plumbing rough-in too close to the toilet soil stack, meaning the toilet would be too close to the shower. The shower drain will have to be relocated.
  • The width of the shower stall is limited to 3 feet due to the placement of the soil stack and clearance for the toilet.

I measured and marked the dimensions on the cement floor as shown in blue:

Basement Bathroom Plumbing Rough-in: Floor Blueprint

Basement Bathroom Plumbing Rough-in: Floor Blueprint

Basement Bathroom Shower Drain Plumbing Rough-In

The basement bathroom shower drain is off center and had to moved by several inches beyond the rim of the existing opening made by the home builder in the cement floor. A hammer drill and 1/2 inch masonry bit is used to drill weakening holes to chisel a knockout in the concrete slab floor to relocate the shower drain.

Basement Bathroom - Drill Holes in Concrete to Relocate Shower Drain

Basement Bathroom – Drill Holes in Concrete to Relocate Shower Drain

After drilling several holes to weaken the concrete slab floor, a brick chisel and sledge hammer are used to make the remaining cuts and shape the sides. Be sure to wear your safety glasses because cement chips fly everywhere.

Basement Bathroom Shower Drain: Cut the Concrete with a Brick Chisel

Basement Bathroom Shower Drain: Cut the Concrete with a Brick Chisel

The finished cut in the cement floor to relocate the shower drain the center of what will be the tiled shower pan:

Basement Bathroom - Chisel Concrete Slab for Shower Drain

Basement Bathroom – Chisel Concrete Slab for Shower Drain

Basement Bathroom Shower Drain P-Trap

I need to determine if the home builder has already installed a P-trap for the shower drain; if not then I must install a P-trap to prevent sewer gases from wafting up from the drain. I dug out the dirt around plumbing rough-in, sawed off several inches of vertical pipe (see above photo and compare to the following picture), then shined a flashlight into the PVC drain pipe. I saw what appeared to be trap (bend) in the drain and some dirt and PVC cuttings in the bottom, but to be certain I filled it with water which rose to a certain level and remained steady as I poured more water in the drain. I was now confident the builder installed a P-trap below grade because there’s hardly any room here for a P-trap. There is room for an S-trap but those are illegal per the Building Code and will siphon water allowing sewer gases to rise out of the drain. (Update: After finishing the bathroom, everything has worked perfectly with no sewer gas odors – I got down on my hands and knees to sniff around! I noticed the water level in the toilet moves slightly up and down when the sewage pump runs indicating the plumbing traps are sealed and working.)

The PVC drain pipe 45 degree elbows are dry fitted before gluing to ensure the shower drain is correctly aligned:

Basement Bathroom - 2 inch PVC Shower Drain Pipe in Concrete Slab

Basement Bathroom – 2 inch PVC Shower Drain Pipe in Concrete Slab

The PVC pipe and shower drain base for a tile floor are glued together. The shower drain clamp ring, adjustable barrel and strainer are shown here. The shower drain flange (base unit glued to the PVC drain pipe) must be high enough above the concrete slab floor to be even with the primary mortar bed for the tile shower. I didn’t want to too shallow mortar bed so I installed the drain base about 1 to 1-1/2 inches above the concrete floor. The shower mortar bed will be installed and sloped to the match the drain, which allows for a good deal of height adjustment.

Basement Bathroom - Tile Shower Drain on Concrete Slab

Basement Bathroom – Tile Shower Drain on Concrete Slab

I’m using an Oatey 130 Series drain for tile showers, model # 42213. It’s nearly identical to the Sioux Chief 821 Series which has excellent online instructions. Both brands are available at the home improvement stores or Amazon.com.

Basement Bathroom Shower Drain Installation in Concrete Slab

Basement Bathroom Shower Drain Installation in Concrete Slab

Foundation Rock Fill

The hole in the concrete slab floor is filled with “fine foundation rock” instead of dirt or even concrete. (If I filled the hole with concrete I’d have to jackhammer it out if I ever needed to access the drain pipe!) Concrete screenings, slag sand or paver base are equivalent substitutes for foundation rock. It packs very well for a solid fill. Pour about 3 or 4 inches at a time, tamp and repeat. Misting the foundation rock with water helps it pack more firmly. Note the rag stuffed in the shower drain to keep it clean:

Basement Bathroom Shower Drain - Foundation Rock Fill

Basement Bathroom Shower Drain – Foundation Rock Fill

The bathroom walls are painted a light blue color while there’s no worry about paint drips on the bare floor. The Wagner Paintmate roller is my favorite for painting walls. I didn’t paint the shower stall walls which were primed with white paint by the home builder because the shower walls will be covered with cement backer board and tile.

Finish a Basement Bathroom: Paint the Walls

Finish a Basement Bathroom: Paint the Walls

The shower pan and mortar bed is built in How to Finish a Basement Bathroom – Part 3.

Thanks for reading,

Bob Jackson

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