The plumbing code specifies minimum diameters for drains and vents in the vertical main stack, horizontal branch drains, and separate vent systems. The diameters are related to the number of fixture units.

To determine drainpipe diameter, look up the fixture or fixtures you’re considering in the code’s fixture unit chart. Add up the total fixture units. Then look up the drain diameter specified for that number of units, as shown in the chart below left.

The plumbing code also specifies the maximum allowable distance between fixtures and vents, as shown in the chart below right. This distance—from a fixture’s trap to the main stack, a secondary stack, or another vent—is called the critical distance. The code lists critical distances by size of fixture drain.

The height of the fixture drain is also regulated by code, which specifies that no fixture drain may be completely below the level of the trap’s crown weir or the drain would act as a siphon and empty the trap.

Figuring in the ideal drainpipe slope of 1/4 inch per foot, the length of that drainpipe quickly becomes limited. But if the fixture is vented properly within the critical distance, the drainpipe’s run to the stack or drain may be any length.

If your fixture is too far from its vent, you have several choices: You can increase the size of the drainpipe, you can move the fixture closer to the existing vent, or you can add a vent closer to the fixture location.

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