Step 1
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.
Step 2
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.
Step 3
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.
Step 4
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.