The purpose of a plumbing vent is to break the suction and allow water to drain quickly while preserving the trap seal that keep sewer gas from wafting into the house.
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Locating Vents
Figure: Max. Distance Between Fixture Traps and Vents
Max. Distance Between Fixture Traps and Vents
Size of Trap (in.)
Distance from Trap (ft.)
1 1/4
5
1 1/2
6
2
8
3
12
4
16
Based on International Residential Code; local codes may vary. Min. slope of trap arm: 1/4 inch per foot for traps sized 1 1/4 to 2 inches; 1/8 per foot for traps sized 3 to 4 inches.
By code, vents must be placed at least two pipe diameters from a fixture trap. In addition, the length of the trap arm is limited to the distances defined in the figure below.
Offset Vent Runs
A vent must rise at least 6 in. above the fixture flood rim before running horizontally. If necessary, a pair of 45- degree offsets may be used, as shown in the figure below.
Figure: Horizontal Offset
A vent must rise above a fixture’s flood rim before turning horizontally. If an offset is needed below the rim, use sixteenth or eighth bends. Do not use quarter bends.
