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Public Health - Seattle & King County

Drainage and venting fixture unit values

Individual fixtures
Minimum size trap and trap arm b
Number of units
Bar Sink
1-1/2 inch
1.0
Bathtub or combination Bath/Shower
1-1/2 inch
2.0
Clothes Washer, domestic, 2 inch standpipe
2 inch
3.0
Dishwasher, domestic, with independent drain
1-1/2 inch a
2.0
Kitchen Sink, domestic, with or without food waste grinder and/or dishwasher
1-1/2 inch a
2.0
Laundry Sink, one or two compartments
1-1/2 inch a
2.0
Lavatory (wash basin)
1-1/4 inch
1.0
Shower Stall
2 inch
2.0
Water Closet, 1.6 GPF Gravity Tank
3 inch
3.0

a Provide a 2-inch (51-mm) minimum branch drain beyond the trap arm.
b Trap sizes shall not be increased to the point where the fixture discharge may be inadequate to maintain their self-scouring properties.

Maximum unit loading and maximum length of drainage and vent piping
Size of Pipe, Inches
1-1/4
1-1/2
2
2-1/2
3
4
Maximum Units
...
... ... ... ... ...
Drainage piping a
Vertical
1
2 b
16 c
32 c
48 d
256
Horizontal
1
1
8 c
14 c
35 d
216 c
Maximum Length ... ... ... ... ... ...
Drainage Piping Vertical, feet
45
65
85
148
212
300
Horizontal (Unlimited) ... ... ... ... ... ...
Vent Piping (See note) ... ... ... ... ... ...
Horizontal and Vertical Maximum Units
1
8 c
24
48
84
256
Maximum Length, feet
45
60
120
180
212
300
  1. Excluding trap arm.
  2. Except sink, urinals and dishwashers.
  3. Except six-unit traps or water closets.
  4. Only four water closets or six-unit traps allowed on any vertical pipe or stack; and not exceed three water closets or six-unit traps on any horizon branch or drain.

NOTE: The diameter of an individual vent shall not be less than 1-1/4 inches nor less than one half the diameter of the drain to which it is connected. Fixture unit load values for drainage and vent piping shall be computed from the above table. Not to exceed one third (1/3) of the total permitted length of any vent may be installed in a horizontal position. When vents are increased one pipe size for their entire length, the maximum length limitations specified in this table to not apply.