How To Make A Plumbing Loop Vent: The Ultimate Guide

According to the plumbing code, all fixture drain P-traps must have vent pipes that connect to the main plumbing venting system. These vents are necessary to maintain atmospheric pressure in the pipes and prevent a vacuum that can cause slow or no drainage of the wastewater. Venting fixtures that are attached to the wall is easy, but if the P-trap requires the vent pipe to run under the floor, you have to install a loop vent. Making a plumbing loop vent isn’t complicated, but there are a few things to keep in mind to ensure it is code-compliant.

How To Make A Plumbing Loop Vent

The most important thing when making a plumbing loop vent is the loop vent diagram. You should draw it before buying the materials to ensure you have an adequate length of pipe to run the vent from the fixture’s P-trap under the floor and tie it into the vent stack.

Things You Will Need 

  • PVC plumbing vent pipe
  • 45° elbows 
  • 90° elbows
  • Y fitting
  • Sanitary tee
  • Vent tee
  • Plastic pipe cement
  • Tape measure 
  • Hacksaw 
  • Market 
  • Sandpaper 

1. Measure And Cut The Pipe

As mentioned above, making a loop vent should start with a diagram. Once you have determined the position of the kitchen island sink and P-trap, determine the kitchen island loop vent distance from the trap and draw a diagram to establish how to run the pipe under the floor to meet the vent network in the wall.

Use the diagram to measure and write down the distance between the loop vent and the vent network in the wall. 

You should also establish how close to the trap you want the loop vent to begin. Keep in mind that loop vents are bulky, so they should begin as close as possible to the trap. The minimum allowable distance is about four inches, but you can place it farther if you want to.

You should also measure the distance from the top of the loop vent (about six inches above the P-trap) to the floor. 

Once you’ve written down all these numbers, use the tape measure to measure the necessary lengths of pipe. Mark the cutting points with a marker and cut the vent pipe sections with a hacksaw. Remove the burrs with sandpaper before proceeding.

Note: The plumbing code requires a vent pipe that serves a fixture to have a diameter at least half as large as the drain pipe, with a minimum allowable pipe diameter of 1-1/4 inches. Remember this when selecting the vent pipe diameter.

2. Install The Loop Vent 

Once you have all pipe sections, you can start installing the loop vent using the diagram to assemble it. 

1. Tie the loop vent to the P-trap 

The first thing to do is to tie the loop vent to the P-trap with a sanitary tee. As explained above, you should install it as close as possible to the trap to maximize space under your sink cabinet. Place the sanitary tee with its sweep sloping toward the drain and glue it to the trap with plastic pipe cement. 

2. Construct the loop

Once you have installed the sanitary tee and tied it to the trap and the drain, install a vertical section of vent pipe that rises at least six inches above the trap. You can now create the loop by gluing a 45-degree elbow to the vertical section of the pipe. Continue with a 90-degree elbow followed by another 45-degree elbow.

If the loop is constructed respecting the dimensions required by the code, the highest part of the 90-degree elbow should touch the underside of the sink cabinet. 

3. Tie the loop vent to the stack vent

With the loop constructed, all you have to do is to connect the kitchen island loop vent to the stack vent in the wall. 

Start with a vertical section of pipe running from the 45-degree loop elbow of the loop to the floor. Install a vent tee at floor level, then glue a horizontal section of pipe to it – this is the pipe running under the floor to the wall. The floor section of the loop vent must be sloped upward at an inclination of ¼-inch per foot. 

At wall level, tie the loop vent pipe into the stack vent pipe with a 90-degree elbow or a Y fitting, depending on the wall plumbing configuration.

The remaining opening of the vent tee should connect to a 45-degree elbow that will then connect to a section of the drain pipe and tie into the main drain pipe running under the floor. This connection is particularly important, helping to drain any water that might accidentally end up in the dry section of the loop. 

Note: The steps above explain how to make a plumbing loop vent for a kitchen island sink. Follow the same steps if you have to build a loop vent for a washing machine or other fixtures and appliances located at a distance from the wall. In case you’re wondering how to make a loop vent for a toilet, know that toilet loop vents are only allowed for wall-hung water closets, and they should be installed by a licensed plumber. 

Frequent Questions

Do you still have questions? Below, the answers to some of the most frequent questions. 

Are loop vents legal?

Yes, loop vents are legal. Not only are they legal, but they are actually required by law for fixtures that don’t connect directly to a wall plumbing vent. That said, you should respect the loop vent plumbing code when installing them. According to the code, the horizontal section of a loop vent should raise at least six inches above the flood level rim of the highest trap or trapped fixture they serve. Also, the minimum distance between the vent opening and trap outlet is two times the pipe diameter (generally, about four inches). 

How high does a loop vent need to be?

A loop vent should be about six inches higher than the P-trap. That means that a kitchen island loop vent should be about 26 to 30 inches high. A washing machine loop vent should be about 24 inches high.

How does a loop vent work?

A loop vent works by allowing air and gas to flow up the pipe while the water flows down. By doing this, it maintains the atmospheric pressure in the drain system and prevents siphoning. Siphoning is a phenomenon caused by improper plumbing venting (or the lack of it) and could result in suctioning the water from the fixture trap. When this happens, sewer gas can leak into your house. 

To End 

Making a plumbing loop vent to connect your P-trap to the stack vent is crucial when installing a kitchen island sink. Loop vents can also serve appliances placed at a distance from the wall or wall-hung water closets. Whether you want to add a sink to your kitchen island or need a loop vent for another fixture or appliance, we hope this guide can help you create it.

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