How To Connect P-Trap To Wall Drain: 6-Step Guide (Do This!)


Fixture traps are not a plumbing system optional. The code requires all fixtures and appliances that drain water into a draining system to be fitted with a P-trap. 

This trap type connects to the main drain line through a wall stub instead of a floor drain. If you’ve never installed such a fitting before, the task could sound complicated.

However, the project is generally easy enough to complete by beginners. 

To connect a P-trap to a wall drain, you must: 

  • Install the tailpiece. This is the length of the pipe connecting to the sink’s drain hole.
  • Install the horizontal arm that links the trap to the wall stub. 
  • Connect the trap to the tailpiece. 
  • Connect the trap to the horizontal arm.

Before getting started, make sure that you’re familiar with the code. Otherwise, you risk installing a trap that doesn’t comply with the current regulations, and your home might not pass an inspection.

1. Know Your Code 

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All fixture traps in the USA are regulated by the Universal Plumbing Code and International Plumbing Code. Almost all municipalities use them as a reference, and these are the codes to check before installing a trap yourself.

According to these codes, the P-traps – sometimes referred to as J-traps – are the only ones approved nationwide. There are some exceptions, such as replacing an existing S-trap with a new one, as long as you don’t upgrade your plumbing system. 

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However, most building codes now require homeowners to upgrade the systems to the use of P-traps for all fixtures and appliances, except for the toilets (which come with built-in traps).

The code doesn’t regulate the height of a P-trap, but it states that it shouldn’t be lower than 24 inches from the fixture outlet. This size includes the tailpiece, so measuring the distance correctly is paramount. 

The trap also can’t be lower than the wall drain and must have a diameter of at least 1¼ inches. Since the diameter of the trap must match the diameter of the drain line, the tailpiece and trap arm must have a diameter of at least 1¼ inches, too. 

Another dimension to keep in mind is the distance from the trap to the wall stub, which shouldn’t be greater than 30 inches. All these regulations prevent clogging and siphoning, lowering the risk of smelly P-traps, which can be dangerous.

2. Gather All Necessary Supplies 

Once you know the code, it’s time to gather all the necessary supplies. You’ll need: 

  • Tailpiece
  • P-trap (including the horizontal arm and nuts)
  • Hacksaw
  • Measuring tape
  • Pliers (optional)

3. Install The Tailpiece 

A tailpiece is the length of pipe attached to the sink drain. Not all sinks need one – depending on the sink’s height and the height of the wall stub, you might be able to connect the P-trap directly to the drain outlet. 

Following the guidelines above, measure the vertical distance between the sink’s outlet and the wall outlet.

Take the size of the trap into account, remembering that it can’t be lower than 24 inches from the outlet and that it should be installed slightly higher than the wall stub. 

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Cut the new tailpiece to size with a hacksaw. Remove the burrs, then attach the tailpiece to the sink strainer with a slip nut. 

4. Install The Trap Arm 

P-traps consist of a U-bend portion designed to trap water and create an airtight seal and a horizontal arm that connects to the wall stub. 

While you could install this arm after connecting the U-bend to the tailpiece, it is easier to make this connection first. 

Ask a helper to keep the trap in place and measure the distance from the wall stub to where the arm should connect to the short side of the trap. 

Cut the arm to size and connect it to the drain stub with a slip nut. Alternatively, you can glue it in place with plumbing cement.

5. Connect The Trap To The Tailpiece 

P-traps are fairly easy to attach to a tailpiece using the nut included in the pack.

Slip a coupling nut on the long arm of the trap and fasten it to the tailpiece. 

6. Connect The P-Trap To Wall Drain

Slip the other coupling nut over the shorter arm of the P-trap – some traps have a captive nut on this arm; skip this step if yours does. 

Align the bend to the horizontal arm extending from the wall stub and secure it with the nut. 

Common P-Trap Installation Challenges 

Installing a P-trap isn’t difficult, but there are a few mistakes most first-time installers make.

Here are the most common challenges: 

Alignment Issues 

A common mistake when installing P-traps is connecting the arm slightly offset. You can generally correct the alignment, but an awkward angle might determine DIYers to install an elbow between the arm and the wall stub.

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This could lead to clogging problems and is against the code.

Incorrect Slope

Another common mistake is installing the horizontal arm without maintaining a slope towards the wall stub. 

As a rule of thumb, all plumbing installations need a slope towards the drain. This should be angled at ¼ of an inch per linear foot, and its purpose is to let gravity aid water flow. 

The absence of a slope could lead to frequent clogging and higher probabilities of backflow.

Trap Overloading

While all fixtures and appliances that drain water into the main drain pipe need a P-trap, the code allows one trap to serve multiple outlets.

This means that one P-trap is enough for your kitchen sink and dishwasher or a utility sink and washing machine combo.

However, you should pay attention to the pipe diameter and make sure it’s large enough to ensure adequate drainage when all served outlets are in use.

Always consult the code when in doubt, or you might have to deal with backflow or a flood.

Final Thoughts 

Connecting a P-trap to a wall drain is one of the simplest plumbing jobs. Just keep in mind the code requirements above and what to avoid to prevent clogging and backflow.