Can I Use a 1 1/4 Inch Drain?

Importance of Plumbing Codes

• The source to consult for the correct size drain you need for a particular fixture is your local plumbing code. Code requirements vary from city to city, due to such variables as piping materials and the local water pressure. If you are unable to locate your local plumbing code online, visit your local building department and obtain a code book or a code fixture unit chart, which is used to determine the minimum drain size that you can use for different plumbing fixtures.

Plumbing Fixture Units

• The required minimum diameter for a drain depends on the fixture unit rating for that particular type of fixture. This rating measures the flow of water that is expected for the fixture drain. Typically, the fixture unit rating needs to be 1 for a drain with a 1-1/4 inch diameter. If you ignore the fixture's rating in the chart, and install a 1-1/4 inch drain on a fixture whose rating is 4, such as a toilet, you not only set the stage for clogs within that drain, you affect the reliability of the overall drainage system, which includes all connected drain pipes and vents.

Possible 1-1/4 Inch Drains

• There are certain fixtures within the home, and also in other places, for which a 1-1/4 inch drain is suitable. These fixtures share a fixture unit rating of 1, allowing 1-1/4 inches as the minimum drain size. The most common of these is probably a bathroom sink. A bidet also can have a minimum 1-1/4 inch drain. Outside of the home, fixtures such as a drinking fountain and a dental cuspidor can have drains of the same size.

Additional Consideration

• The size of the drain is only one factor to consider when installing or replacing fixtures. Also look at the critical distance for the drain, which is the distance from the fixture's curved trap to the vent. The vent pipe equalizes pressure within the drainpipe system and also allows an escape route to outside air for sewer gases. The drain size determines this critical distance for the fixture. Consult the local code for the requirements to make sure the drain is not too far from the vent.