How to Choose and Install a PVC P-Trap
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Time to read 4 min
Table of contents
Under your kitchen sink, you’ll see a curved piece of pipe. Check under your bathroom sink, you’ll see the same curved piece of pipe. It’s called a P-Trap! A P-Trap is a u-shaped bend in the drainpipe that connects a sink’s drain to a home septic or municipal sewer system. How do you know which P-Trap is the correct one for you? To determine the correct size, you must distinguish between the bathroom and the kitchen sink. When deciding which material to use, look at the existing material and replicate that in your replacement P-Trap.
Choosing the Right P-Trap
You’ll need to identify the P-Trap you’re replacing. Kitchen sink P-Traps come in 1-1/2” standard size, while bathroom sinks use P-Traps of 1-1/4” standard size. The traps also come in different material types such as propylene, ABS, brass (chrome-plated or natural), and PVC. You should use the material that is currently in place when replacing a P-Trap.
How to Install a P-Trap
As we cover the steps of installing the P-Trap, keep in mind that the tailpiece should always connect with the sink drain and the shorter side past the bend should connect with the drainpipe. No matter what size or materials you’re using, the steps are the same (joining methods may be slightly different based on material.)
Step 1 - Remove the Old Drain Pipe
Remove the existing assembly from the top to bottom. Pliers may be required to remove the slip nuts. The U-shaped bend will have some water in it so it may be best to have a bucket and towel close by.
Step 2 - Attach the New Tailpiece
If you’re replacing the kitchen P-Trap, put the tailpiece washer in the flared end of the tailpiece. Attach it by screwing the slip nut onto the sink strainer.
If you’re replacing the bathroom P-Trap, know that the sink drain starts as the tailpiece and could reach the P-Trap already. If it doesn’t reach, add a tailpiece to get the correct length.
Step 3 - If Necessary, Add a T-Fitting
In a few scenarios, you may need to add a Tee fitting. A sink with two basins uses a waste T-fitting to join the tailpieces. Attach the fitting with a slip washer and nuts. Make sure the beveled sides of the washers face the threaded portion of the pipes. Apply pipe lubricant on the slip washer. It will ease installation and ensure a tight fit.
Step 4 – Attach Trap Arm
Remember, keep the beveled side of the washer facing the threaded drain stub-out and attach the trap arm to the drain stub-out.
Step 5 – Connect the Trap Bend to the Trap Arm
The beveled side of the washers should face the trap bend. Attach the trap bend to the trap arm. Tighten all of the nuts with a pair of slip-joint pliers.
*Never use Teflon tape on white plastic threads and fittings*
Using Your P-Trap
After installing your P-Trap, you can use your sink without any issues or worry. Over time you’ll want to perform maintenance on your P-Trap to ensure it’s performing its best and that no leaks have formed. Whether you’re installing a P-Trap on a bathroom or kitchen sink, it’s a plumbing fixture you need to have.
Size and Material Rules: Always match the existing pipe material. For sizing, kitchen sinks require a standard 1-1/2” P-trap, while bathroom sinks use a smaller 1-1/4” size.
Orientation and Assembly: Install the trap with the longer tailpiece connecting directly to the sink drain and the shorter side past the U-bend connecting out to the wall's drainpipe.
The Golden Rule of Tightening: Hand-tighten the slip-joint nuts and washers (ensuring the beveled sides face the threads), then finish with pliers. Never use Teflon tape on white plastic threads.
FAQs
What is the difference between kitchen and bathroom P-trap sizes, and how do I choose the right one?
The primary difference comes down to the pipe diameter. Standard kitchen sink P-traps use a 1-1/2 inch size to handle larger volumes of water and potential food waste, while bathroom sinks typically use a 1-1/4 inch size. To choose the right one, look at your existing plumbing layout or measure the diameter of the sink's tailpiece before buying your replacement kit.
Which way should the beveled (sloped) side of the slip joint washer face during installation?
The beveled, tapered side of the washer should always face toward the threaded intercept point or the fitting mouth that it is sliding into. When you tighten the slip nut, it pushes against the flat face of the washer, squeezing the sloped edge into the joint to create a completely watertight compression seal.
Should I use Teflon tape or PVC cement on the slip-joint threads of a plastic P-trap?
No, you should never use Teflon tape, pipe dope, or PVC cement on plastic slip-joint threads. These molded white or black plastic threads rely on the compression of the rubber or plastic washer to stop leaks, not the threads themselves. Adding tape or glue can interfere with the threading, prevent the nut from tightening evenly, and actually cause the joint to leak or crack.
Mark Ligon
Mark Ligon is the marketing manager at PVC Fittings Online, a supplier of commercial PVC and CPVC pipe, fittings and valves. Ligon enjoys educating businesses on the parts of piping systems so managers can make informed decisions. He can be reached at webmaster@commercial-industrial-supply.com.
