How to Size Your Own Plumbing Waste Lines

by Laurie Reeves
    The drain, waste and vent system goes in during rough-in construction and framing.


    Building a home requires installing systems to deliver water and remove waste. To get an idea of the size and type of pipes needed for the drain, waste and vent (DWV) system, contact your local building department for the plumbing code it uses. Most jurisdictions follow Uniform Plumbing Code requirements for pipe sizes, but each jurisdiction may have additions or deletions to the code or only use a specific year of the code. You must use the Uniform Plumbing Code reference tables to help you calculate the correct plumbing waste line sizes required for your construction project.


    Look up the requirements in the plumbing code book and compare these with any local building code requirements to ensure both agree. Make note of any differences.


    Review your home's blueprints and assign the total number of units to each fixture in the home according to the standards set in the plumbing code tables. Review the "Discharge of Fixtures by Units" table and look up each fixture for the "units" value. For instance, a bathroom tub is considered as two units, while a combination shower and tub are considered three units. A toilet is considered as six units because of how much water it discharges at once. The number of units on one drain pipe helps to calculate the size of pipe needed.


    Compare the fixture to the "Minimum Sizes of Drainage Pipe by Fixture" table. For instance, the minimum pipe diameter for a washing machine drain is pipe with a 1 1/2 inch diameter. When combined with a laundry tub, the drain pipe must be a minimum of 2 inches in diameter. And toilet requires a drain pipe of 3 inches in diameter, unless two toilets are on the same drain and then it must be a 4-inch plumbing waste pipe.


    Add the number of units together to get the total number of units for discharge into the DWV system compared with the minimum pipe diameter for specific fixtures. Systems with less than nine units on one stack can use a 2-inch pipe, unless there is a toilet discharging into the piping. If that's the case, it must be a minimum of 3 inches. The stack discharges into the main drain pipe.


    Verify that the final drain into the septic or sewer system is not smaller than the largest branch on the system. If the largest stack, or drain pipe connecting multiple fixtures, on the system is a 4-inch-diameter pipe, ensure that the main drain pipe is at least 4 inches in diameter. The main drain line cannot be smaller than the largest branch or stack on the system.


    Calculate the number of sanitary T-inlets, combination Y-inlets or 1/8-inch bend inlets that tie into a stack on the drain system to determine the maximum number of fixture units that can be on one stack. Review the "Maximum Number of Fixture Units on One Stack" table in your plumbing code book.


    Design the DWV plumbing waste system after assigning fixture units, minimum drainage requirements by fixture type and other inlets or branches coming into the drain. Draw out the design on graph paper to gain a clear understanding of how to connect the system together.

    Things You Will Need

    • Uniform Plumbing Code book
    • Local jurisdiction plumbing code requirements
    • Building blueprints


    • Make sure that each fixture that uses a trap is tied into a vent stack through the roof. A common mistake is forgetting to include the plumbing below the slab with the fixtures on the second story. Check code requirements for approved piping. Some jurisdictions only accept a certain type of pipe, such as polyvinyl chloride (PVC) or acrylonitrile butadiene styrenel (ABS). Once you have installed the DWV system during rough-in, you will be required to complete a standing water test of the DWV system for building department approval. Make sure you have installed the correct pipe protection nail plates. Also, make sure the slope of horizontal pipes complies with local building code requirements. Don't forget to include cleanouts where required by code.


    • Do not attempt to design or calculate the DWV system without building or plumbing experience. When in doubt, hire the services of a professional. Though it may cost slightly more at the beginning, it will save in the end because you won't have to do the work twice.

    About the Author

    A former journalist and newspaper managing editor, Laurie Reeves has decades of experience in accounting, marketing, museum management and small business administration. Handy with all kinds of tools, Reeves designed and helped her husband build their dream home. On a creative note, she sews, crochets, draws, paints, enjoys crafting, making homemade beer and working with clay. A native California and book author, she graduated from San Diego's Coleman College.

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