The venting of drain and soil pipes in your home is essential. Between every fixture and the drain pipe is a trap that holds water. The water prevents dangerous sewer gas from escaping into your home. Like a finger held over a straw dipped in water, the water in the trap prevents the drain from flowing easily unless it is properly vented, similar to taking your finger off the straw.
Install a vent line for every plumbing fixture such as a toilet, vanity, shower or bath within 3 m (10 feet) of a fixture trap. If the trap empties directly into the main soil pipe, a vent may not be required. Vents typically are made from 3.7 cm or 5 cm (1 1/2 inch or 2 inch PVC or ABS plastic pipe. In multistory homes, vent pipes for fixtures on the first or middle floor may connect to a secondary vent pipe that travels up to the attic to the main vent, or simply exit through the roof.
Connect a vanity or sink p-trap outflow to the drain in the wall with a waste tee fitting. Install a vent pipe going up and a drain line going down to join with the main soil stack or secondary soil pipes. Run the vent pipe up to a convenient location where it can be joined to another vent pipe or the main soil vent with a Wye fitting.
Install a toilet waste line vent as close to the toilet as possible. The trap for a toilet is located within the toilet itself in most cases. The vent for a toilet or a shower must be made from 5 cm (2 inch) pipe. Vanities and kitchen sinks can use 3.7 cm (1 1/2 inch) pipe if it complies with local building codes. If the toilet connects to a soil pipe located directly behind the toilet in the wall, a vent is not required.
Kitchen islands present difficult venting situations. The island sink is sometimes located more than 3 m (10 feet) from a wall, and if not vented correctly, it will won't drain properly. Run a vent line from the wall over to the island drain pipe, above the level of the drain pipe, and connect it to the drain pipe with a Wye fitting along the top surface of the drain pipe. Use 5 cm (2 inch) pipe for both the drain and the vent. Connect the vent pipe in the wall to the main or secondary vent closest to it.
Vent a drain pipe before it enters another drain pipe or it will be "wet vented." Wet venting is allowed in some cases, but it is not a best practice installation. A small, wet vented drain can have the water sucked from its trap, allowing sewer gas to escape into the home.
Dry fit all your PVC or ABS plastic vent pipe together before gluing. It is good practice to prime all the fittings and pipes as you work. Glue all the pipe and fittings after you have fit everything together. It is helpful when using fittings that turn the pipe to mark the pipe and fitting with a permanent marker so their orientation is maintained while you glue them.