Toilet Rough In


The rough in for a toilet involves three different kinds of pipes. The water supply, usually formed from 1/2-inch copper pipe, terminates at a shut off valve under the tank. The waste line, which may be 3- or 4-inch PVC pipe, ties into the sewer via a vertical waste stack and connects to the toilet by means of a closet flange. In addition, a 2-inch pipe that terminates outside must vent the waste line. The configuration of these pipes depends on the location of the toilet relative to pre-existing plumbing and the construction of the house.

The first step is installing waste and vent pipes.

  1. A 4-inch vertical PVC pipe will be installed, that extends from a point at least one foot above the toilet bowl to the sewer. This is the soil stack and it will tie into the sewer with long sweep wye, or “Y”-shaped, fitting. A plastic wye is used if the sewer pipe is plastic, but if the sewer is cast iron, a cast iron wye with a threaded inlet will be installed and a PVC adapter screwed on. The soil stack will be glued to the plastic wye or the adapter with PVC cement.
  2. The drain for the toilet needs to be placed at a location in the bathroom that will provide enough clearance for the toilet when it is installed; 12 inches from the back wall is suitable for most toilets.
  3. A 3- or 4-inch diameter hole for the drain will be drilled in the subfloor with a reciprocating saw. The size of the hole depends on what size waste pipe you plan to use. A 3-inch line provides better drainage, but if the distance to the soil stack is more than 6 feet, 4-inch pipe is required.
  4. Then a closet flange of the appropriate diameter will be fitted into the hole in the bathroom and screwed to the subfloor. The flange holds the toilet to the floor and forms the connection between the toilet and the waste line. A closet elbow is glued to the flange pipe under the subfloor with PVC cement. The elbow should be pointed in the direction of the soil stack.
  5. PVC pipe is glued to the elbow and extended it to the soil stack, maintaining a downward slope toward the stack of 1/8 to 1/4 inch per foot. Then the plumber will cut into the stack and glue in a 4-inch sanitary tee with a 3- or 4-inch opening, depending on the size of the waste line. And finally glue the waste line to the tee.
  6. The size of the opening of the top of the soil stack will be reduced to 2 inches with a reducing coupling. Then he will glue 2-inch PVC pipe to the reducer and extend it through the roof. This pipe vents the toilet and should terminate 1 foot above the roofline.

Steps for Installing Water Supply Pipes

  1. Your plumber will choose an existing copper pipe that carries cold water near the toilet and shut off the water to that pipe. He then cuts into it with a pipe cutter and solder in a copper tee with a 1/2-inch outlet.
  2. Next he will assemble a supply line for the toilet with 1/2-inch copper pipe and fittings. It should run from the tee to the wall behind the toilet and extend about 2 inches beyond the wall under the tank. All the joints will be solders after he has assembled the supply line, and affixed the line to a stud in the wall behind the toilet with a pipe clamp.
  3. Finally, he needs to terminate the water supply line with a shut off valve. He can either solder the valve to the pipes, or install a valve with a compression fitting. Make sure the valve is closed before you turn the water back on.


  • If you’re connecting a new toilet to an existing stack, you only need to install the waste and supply lines. To do this, the toilet must be placed within 10 feet of the stack.
  • A configuration in which the waste line connects directly to the sewer will require venting for the waste line. You may be able to tie the vent pipe to an existing one that already runs through the roof. Vent pipes must either slope upward away from the toilet or be horizontal.