Before you decide to add a plumbing fixture for a bathroom, laundry room, or a utility sink, you need to be sure that the drain line is properly vented. Without proper venting your sink will drain slow, gurgle or back up.
If it is an existing drain it shouldn’t be an issue. If you are installing a new waste line, venting it is very important.
The vent allows air to enter the system to prevent air locks and also allows sewer gases to exit the system outside of the living space.
Glenda at SF Gate.com gives us some great advice on sewer venting.
The old-fashioned method of venting featured a separate vent pipe for every fixture but that led to multiple vent pipes exiting the roof. Today, plumbers combine vents, and even a home with four or more bathrooms typically has only one main vent-and-soil stack that exits the roof at the top and curves to form the horizontal sewer drain at the bottom. A good drain, waste and vent layout takes bathroom, laundry room and kitchen configuration into consideration when planning where the main vent-and-soil stack is located and how each drain and vent will connect. Drainpipes all slope downward and tie into the stack, while vent pipes extend upward and meet the stack higher on the line, often in the attic. The basic rule is that a vent must not connect to the stack lower on the stack than a drain connection.
Mechanical Vents, Air Admittance Valves and Best Practices
In the case of a remodeling, it’s not always possible to install vertical vents through finished walls, although that should be the first option. When this situation arises, it might be possible to install an air admittance valve on a sink or tub drain line. An AAV attaches to an individual fixture’s drain line in the wall nearest the fixture. AAVs replace the old mechanical vents, which are not up to code in most communities. It’s essential that all plumbing plans be cleared with the local building authority. In many communities, a licensed plumber must either do the work or oversee the project.