How to Pipe a Bathtub to the Sewer

The “P”-Trap

• An artificial low-point, universally referred to as a “P”-trap, is installed immediately below the tub’s drain. It serves twin purposes: it stops sediment from entering your drain system, where it could cause a blockage in a hard-to-reach place, and it traps a plug of water that prevents bad smells backing up into the bathroom from the sewer.

Retrofitting an Existing Tub

• Replacing old pipework is usually necessitated by corrosion in the existing system. It is cost- and time-effective to replace as little of the original installation as is possible. First gain access to the underside of the tub, either through a removable panel in the fascia or by cutting a trap door in an adjacent cabinet, if necessary. Use a digital camera or draw a plan of how the pipes run for reference when you are installing the new components. Remove the overflow pipe that runs from the tub’s overflow to the drain.

  Remove any hardware that is attached to the drain assembly inside the tub, such as a toggle drain plug or a drain cover ring -- the latter serves to hide the more utilitarian plumbing connections, and typically subscribes to the “righty-tighty, lefty loosey” rule.

Removing Failed Pipework

• Almost all tub installations use pressure fittings to make the pipework watertight. Use either a properly-sized wrench or a set of grips with an adjustable jaw to loosen all the pressure fittings between the underside of the tub and the sewer pipe; again, turn counter-clockwise to loosen. There is normally a fitting immediately beneath the tub’s drain, another at the end of the short run of pipe that leads to the “P”-trap, another after the “P”-trap that joins to a length of pipe running to the sewer pipe, and a last one at that junction.

  When all the pressure fittings are loosened, place a bowl beneath the “P”-trap and spin the two fittings off by hand. Wiggle the “P”-trap so that its ends come away from the adjacent pipes, and -- when it has come free -- carefully empty the water out into the bowl. Remove the bowl, finish undoing the remaining pressure fittings, and pull all the pieces of pipe and the fittings out from beneath the tub. Take the old components to a hardware store, lumber’s merchants or home improvement warehouse and purchase exact replacements; install the new components in a direct reversal of the removal process.

Installing a New Bathtub

• Learn the sizes of the sewer pipe and the bathtub drain -- in most residential applications these are 1 1/2 inches, but always check with the local code enforcement office for any specific requirements in your area. Design the tub-sewer system so there is a fall -- a drop in the height of the pipe between the water source and the sewer connection -- of approximately 1 inch every 4 feet. Obtain the necessary components from a hardware store, plumber’s merchants or home improvement warehouse; install the pipes and the fittings per the manufacturer’s instructions. A regular small-toothed saw is usually adequate for cutting PVC pipe to length, and the pressure fittings should be installed so the entire system is installed and finger-tight before using the wrench or grips to finish the job.