How to Calculate a Plumbing Vent Requirement

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If you look at the top of a roof, you may notice a pipe sticking out of it. The pipe is called a vent stack and is used in all plumbing systems. Vent stacks allow air to enter the drains of your fixtures, including the toilets, showers and sinks. Without the air, negative air pressure inside the pipes would prevent liquid from moving downward through the drain. Figuring out the proper size of a vent stack requires you to know how many fixtures will be connected to the line.

Things You'll Need

  • Pen and paper
  • Walk through every room in the house and count the number of devices that are attached to plumbing drains. Include toilets, showers, faucets, washing machines, dishwashers and anything else that connects to the drain. An accurate count is essential.

  • Look at the plumbing schematics for your home, if possible, to determine whether there is a single vent stack or multiple vent stacks. Most homes will have only a single vent stack.

  • Consult the following formula to determine the size of your vent requirements: one fixture requires 1 1/4-inch vent pipe; two to eight fixtures requires 1 1/2-inch vent pipe; eight to 16 fixtures requires 2-inch pipe; 16 to 48 fixtures requires 3-inch vent pipe; and 48 to 256 fixtures requires 4-inch vent pipe.

  • Consult your local plumbing codes to see whether the formula changes based on the types of fittings used to connect the drain pipes to the main discharge line. Some areas allow fewer fixtures per vent stack.

Tips & Warnings

  • The minimum size of a vent pipe is 1 1/4 inches.
  • No vent stack should be less than half the diameter of the soil stack that it connects with.

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  • Photo Credit Comstock/Comstock/Getty Images
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