Vents and Traps

Vents

Venting is necessary to provide air to displace the water that is moving down and through the drainage system.

Plumbing vents at the exterior should be at least 6 inches above the roofline, on the high side of the penetration. Vents must be at least 10 feet from windows. “In the wall” or “under-the-counter” air admittance vents (AAV) may not be permitted and must be authorized by local code. A cooking island in a kitchen, with a sink, often will have a vent of this type under the counter. Vents should be within 42 inches of the fixture being vented.

Traps

Traps prevent odorous sewage gases from entering the house through the sink and tub drains. There are two basic types of traps: (one acceptable, one not acceptable)

• “P” traps discharge is horizontal rather than vertical. The liquid seal is dependable and acceptable.
• The full “S” trap or 3/4 “S” trap should not be used in plumbing installations. These traps are almost impossible to vent properly. The 3/4 “S’ trap forms a perfect siphon. The problem with “S” traps is that the waste water may siphon out, causing the trap to lose its liquid seal. Every plumbing fixture, such as sinks, tubs and showers, should have a proper trap. Toilet traps are built into the bowl of the toilet. If a sink, etc. does not have a trap, trace the drain for other inequities that may be present (i.e. draining into a sump pump, a dry well or to the exterior surface). In older construction, there are thousands of “S” traps in use. Older building codes did not address this issue. In many cases it may be very difficult to change an “S” trap to a “P” trap.