How to install a new toilet drain

Updated November 21, 2016

A toilet's drain pipe is an essential piece of plumbing that carries the waste away from the home. Drain pipes are directly beneath the toilet, via the basement or crawl space, and they lead out through the wall and into the yard. Installing your new drain pipe includes replacing the pipes beneath the toilet that lead to the main line in your yard. Keeping the right slope angle, threading the pipes properly and renewing any connectors or seals will give quality and durability to the overall project.

Measure the pipes from where the T connector joins the drain pipe to the stack or ventilation pipe, up to the toilet itself. You will need an L pipe to join to a straight pipe, which will join to the T connector.

Turn off the water supply to the toilet and flush it until all the water has gone. Remove the toilet and the wax seal from the top of the drain pipe to free it for removal. Take the wax ring off with a wood stick, or some other disposable item that can scrape the material from the toilet base. Discard immediately without touching; the wax is very sticky and hard to remove.

Turn the bolt where the drain pipe connects to the T connector with a wrench until it is loose and ready to slip off. Do this with care, because there still may be waste water in the line. Slide the drain pipe out and set it aside. If the T connector is rusted at all, replace it as well. A wrench will loosen it free.

Wipe the T connector dry and begin wrapping the threaded grooves with thread tape. Wrap the tape around at least three times thick, making sure the thread length is covered as well. Wider pipes will require more tape to cover the threaded area.

Thread the end of the L pipe and turn it onto the straight pipe, leaving it slightly loose. Place this newly made, single pipe up to the T connector. Twist the straight pipe into the T connector and tighten with a pipe wrench. Tighten the straight pipe into the L pipe while it is in position at the hole where the toilet will sit. This way, all pipes are tight, and you avoid trying to twist the L pipe around in a small, confined area that may not work.

Place the fresh, new wax ring onto the base of the toilet. Lift the toilet at an angle, or turn it over to place the ring on straight. Set the toilet onto the drain pipe in the floor and press down firmly. The wax ring will form a seal with the pipe and the toilet, preventing water leakage and potential damage.


Using a 3-inch pipe is more beneficial when wanting to resist clogs. Since the water flow must carry the waste away, more space, as in a 4-inch drain pipe, will only result in water flowing around the waste, rather than pushing it along. For larger pipes, most home improvement stores will rent out a pipe wrench if you do not own one. Make sure your pipe slopes slightly; desirably 1/4 inch to 1/8 inch to each foot between the drain and the stack pipe. The typical length between the stack and drain is 4 feet, so the pipe should slope at least 1/2 inch from the base of the L pipe to the connection at the T joint.

Things You'll Need

  • Pipe wrench
  • L pipe
  • T connector
  • Straight pipe
  • Thread tape
  • Disposable wood stick or scraper
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