What Size Should a Toilet Vent Stack Be?

Plumbing Code

• The National Standard Plumbing Code produced by the Plumbers-Heating-Cooling Contractors Association provides the guidelines by which most states determine their own codes. Any building plans submitted for approval must adhere to the code, and any construction that is deemed by the building inspector as not following the code will not be approved for use. Chapter 12 of the code lays out the various types of vents found in dwellings and how they are to be managed.

Mandate

• The code mandates a stack vent of "at least one main vent stack or vent stack, of not less than 3 inch diameter, for each building drain when connected separately to a building sewer or septic tank." Many contractors, plumbers and municipal codes still adhere to three inches as the absolute minimum. However, the code changed in 2000 to give more flexibility in the size of the vent pipe.

Three Inches Not Necessary

• The code now reads that "Every building in which plumbing is installed shall have at least one stack the size of which is not less than one-half of the required size of the building drain." According to the Georgia Plumbing Training Academy, "...It is highly unlikely ... a three inch vent would be necessary on a residential plumbing system."

Rule of Thumb

• The rule of thumb for plumbers has always been to have a three-inch diameter vent stack with one toilet, and four inches with two. Five inches is considered the maximum needed. The vent stack is directed toward the roof and is usually easily identifiable as a PVC pipe directly over the main bathroom.