How to Layout the Bathroom Plumbing in a Basement Before the Floors Are Poured

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A home’s plumbing system is a network of water supply lines, drainage pipes and plumbing vents, all of which efficiently direct water flow in and out of the home. Because breaking out a concrete floor to plumb a bathroom in an existing home is expensive and labor intensive, many communities have building codes requiring the contractor provide for a future basement bathroom by installing the drains for a toilet, a sink and a shower before pouring the basement floor.

Things You'll Need

  • 4-inch PVC pipe
  • 4-Inch PVC fittings
  • 4-inch toilet flange
  • 2-inch PVC pipe
  • 2-inch PVC fittings
  • 2-by-2-foot wood frame
  • Locate the basement bathroom as close to the main sewer line or sewage tank as possible. The drain lines must slope as they run toward the main line or the tank.

  • Run a 4-inch PVC drainpipe between the future toilet and the main sewer line or the sewage tank. The drain for the toilet requires a 4-inch toilet flange that sits flush with the concrete floor. The flange connects to a 4-inch PVC elbow fitting that connects to the 4-inch drainpipe.

  • Slope the 4-inch drainpipe that runs from the toilet to the main sewer line or to the tank at the rate of one-quarter inch for every horizontal foot. For example, if the distance between the toilet and the main sewer line is 10 feet, the pipe will descend a total of 2 1/2 inches from end to end.

  • Position a 2-inch PVC drainpipe that extends out of the floor by 1 or 2 feet. This is the vanity/sink drain. It must extend above the floor. The sink drain’s position is between two wall studs in a future wall, because the sink has a P-trap below the sink that exits through the wall behind the sink.

  • Run the sink’s 2-inch drain at the same slope as the toilet drainpipe, but use a 4-inch to 2-inch PVC reducer fitting tying the sink drain into the 4-inch toilet drain line. Install corner fittings as needed.

  • Run a 2-inch drain for the shower, but do not run its end above the floor’s level. The shower requires a P-trap, which installs when the shower is set. The shower drainpipe extends horizontally beneath the floor to the location of the future shower. Then put a PVC end cap on the line.

  • Tie the 2-inch shower drain line into the 4-inch toilet line just as you did with the vanity/sink drain line. The horizontal slope of this line is also one-quarter inch per horizontal foot.

  • Place a wood frame around the shower drain’s end cap. Fill the frame with sand. When you pour the floor, do not pour concrete in this frame. This is the shower block-out.

  • Check the drainpipes’ slopes. Filling sand around and beneath them keeps them from shifting. When pouring the floor, the only visible items are the top of the toilet flange, the 2-inch vanity/sink drain sticking up and the top of the wood frame filled with sand.

Tips & Warnings

  • While it’s always important to get the right slope on drainpipes, it’s essential that drains beneath concrete be correct because it’s difficult to repair them later.
  • Water supply lines run through walls from above, so no provisions for these lines are needed before pouring concrete.
  • When installing the bathroom fixtures, the drain line behind the vanity/sink serves as the bathroom vent. Install a T-fitting with a pipe that extends upward and ties into the home’s main vent.
  • Just because a little slope on the toilet drain is good doesn’t mean more is better. Too much slope causes flushed liquid waste to outrun solid waste, leaving it sitting the pipe.
  • Check with your local building authority before installing any plumbing. Some communities allow homeowners to run plumbing, while others do not.

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