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Plumbing Basics: Sizing Traps and Drains for Household Fixtures
By the Editors of CornerHardware.com
Basic DWV systemSizing drain, waste and vent (DWV) systems can be daunting enough to make most folks not want to go there. Plumbers calculate drain and vent capacity in "fixture units," consider issues like "trap-arm loading" and, if they're really fluent in effluent, take advantage of concepts like "wet venting." See what we mean? But if all you want to do is hook up a new fixture or the DWV lines in a simple bath, most of the time you don't need to do the math at all.

To begin, keep the following rules in mind:

  • Traps must not be larger than fixture drains.
  • All fixtures must be vented in some manner to prevent septic gases from entering your home.
  • Codes vary. Your community is governed by either the Uniform Plumbing Code (UPC) or the International Plumbing Code (IPC). But the following trap sizes are consistent, except where noted.
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    Fixture Minimum trap size
    Bar sink 1 1/2 inch
    Bathtub with shower 1 1/2 inch*
    Bidet 1 1/4 inch
    Kitchen sink, disposal and dishwasher 1 1/2 inch*
    Laundry sink 1 1/2 inch
    Lavatory (bathroom sink) 1 1/4 inch
    Shower 2 inch
    Toilet 3 inch
    Washing machine 2 inch
    *The UPC requires at least a 2-inch drain immediately beyond the trap.
    Source: Code Check: Plumbing, Redwood Kardon, et al.

    When fixtures share a drain, calculating drain and vent size gets a bit more complex. Each type of fixture has a fixture unit rating, a measure of the peak rate of discharge through a drainpipe. A single-fixture unit rating of a typical bathroom sink, for example, is 7 1/2 gallons per minute. Adding together the ratings for the fixtures that will share a drain indicates the required drain or vent pipe's fixture rating. In the end, most bathrooms can be served by a 3-inch drain and a 2-inch vent. Without a toilet, a bathtub with shower and bathroom sink can share a 2-inch drain and 1 1/2-inch vent.

    Materials

    You'll need:

  • Sink fittings
  • ABS pipe fittings
  • PVC drain fittings
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