2-Inch P-Trap Installation


It's definitely not glamorous, far from attractive and rarely the subject of conversation. Yet, without a P-trap attached to the drains leading away from sinks, showers, washers and other plumbed appliances, your and your family's health would be in danger. A P-trap's design forces a small amount of water to rest in the bottom of the horizontally curved portion of the trap. As you flush water through the pipe, a little remains after the burst flows through. This ever-present water prevents sewer gases from flowing up through the drain into your home. Installing a P-trap isn't nearly as difficult as it might appear.

Things You'll Need

  • Drywall saw, if needed
  • Pipe thread compound
  • Hacksaw
  • Bucket or pan
  • Stiff-bristled brush, scrub pad or steel wool
  • Tape measure or carpenter's level
  • Turn off the water supply to either the fixture itself, or to the entire house, if you are replacing an existing P-trap. If you're plumbing a new item, such as a washer or bath tub (items that commonly require a 2-inch trap), turning off your water is not necessary.

  • Access the P-trap location, which runs between the plumbing drain stack and the fixture's drain. Look underneath a sink basin to find the installation area. For a washing machine, cut open the wall -- if the wall framing and drain plumbing isn't already exposed -- to reveal the assembly. A shower or tub drain hides the plumbing underneath the floor, in the ceiling of a crawlspace or basement, instead.

  • Clear the area underneath and surrounding the trap installation. Set a bucket or pan beneath an existing trap to catch any escaping water.

  • Remove an existing P-trap if necessary. Unscrew the nuts on each end of the trap -- at both the fixture drain and at the wall drain pipe -- until barely attached. Supporting the trap with one hand to ensure it remains level, turn the nuts the rest of the way to loosen and remove the trap. Carefully dump the trap into the waiting bucket or pan.

  • Scrub the fixture drain piece to remove any built-up gunk or sealant. Use a stiff-bristled brush, dipped into hot, soapy water, rinsing frequently. Use steel wool or a scrub pad if preferred, especially for stubborn buildup. Wait for the pipes to air-dry completely before proceeding.

  • Join the two pieces of the P-trap -- the J-shaped portion and the straighter section that leads away from the trap to the drain pipe -- with a gasket on each side of the connecting nut. Screw on the nut loosely to allow trap adjustment later.

  • Hold the P-trap assembly in position to test the fit between the fixture and the drain pipe. Use a level or tape measure to ensure the horizontally running tail piece section drops 1/4 inch per foot (about half a bubble from level) and fits precisely between the drain and the drain stack pipe.

  • Mark the tail piece with a marker, if it's too long, leaving enough length to ensure it inserts into the drain pipe properly. Cut to length with a hacksaw. Alternatively, lengthen a too-short P-trap assembly with a 2-inch slip-joint extension piece. Slide a gasket onto the end of the tail piece and attach the slip-joint extension with a nut.

  • Disassemble the P-trap, separating the curved J-bend portion from the straighter tail portion, once you know it fits properly. Swab pipe thread compound around the pipes and gaskets where they seal against each other, as specified by the product manufacturer, using a thread compound suited to the pipe material. Reassemble the P-trap, tightening the nut snugly but not so it's overtight.

  • Spread additional thread compound on the end of the trap that attaches to the fixture as well as at the end of the piece to which it connects. This ensures a watertight seal between the pieces.

  • Lift the trap into position and turn the nut that connects the trap to the fixture drain piece. Slide a slip joint tail piece into the wall drain pipe. Alternatively, connect with a nut and thread compound if appropriate -- this will be obvious when needed, as the wall drain will feature a threaded pipe. Firmly tighten all nuts.

  • Run water through the P-trap to test your installation. Watch for drips and leaks -- sometimes holding a dry paper towel over the fittings helps pinpoint any joint failures.

Tips & Warnings

  • Consult your local building code authority to ensure your P-trap and drain meets the building code. Never assume that it does -- certain trap sizes, shapes, material or even professional installation, not to mention a building permit, may be required in your area.

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