Plumbing Vents
One of the least understood concepts of waste plumbing systems is venting.  For water to drain freely out of the house waste system, there
must be adequate/proper venting.  Venting the drain line performs three functions:

  • Allows air in front of the water rushing through the waste pipe to be pushed out of the way
  • Allows air to be reintroduced to the waste piping after the water has gone by
  • Allows sewer gases to escape outside through a vent stack, preferably above the roof.

The second function is the most important.  There should be a drain trap at each plumbing fixture to provide a water seal and prevent sewer
gases from entering the house through the drain pipe.  After a fixture is used, there should be enough water left in the trap to provide a good
seal.  If a waste system is not properly vented, the water in the drain can create a siphon effect and suck the water out of the trap.  This is most
likely to happen if a filled sink or tub is drained.  The vent breaks the siphon effect, keeping water in the trap.

It is important to have a vent connection just downstream of the trap.  This allows air into the pipe, preventing a vacuum between the water
which runs down the drain pipe and the water which remains in the trap. With the exception of floor drains under some circumstances, all
fixtures should be vented.

As a rough rule, any fixture within five feet of the main stack does not need a separate vent. Where fixtures are more than five feet from the
main stack and do require a separate vent, the vent must extend above every other fixture in the house. At this point it may join the main stack.
It is, therefore, possible for a house to have several bathrooms, and only one vent stack going up through the roof.

Typically improper venting results in a siphoning or gurgling noise when water is drained out of a sink or tub.  A sewer odor at a fixture can
indicate a trap or venting problem, or leakage in the plumbing after the trap (e.g. around the toilet drain).  The venting system typically mostly
concealed from view, except where the pipes pass through the attic and above the roof.  Older homes quite often have drain and vent
plumbing installed at the exterior of the house.

Vents should terminate at least 3 feet above and 10 feet (12 feet in Canada) in any other direction from any door or window openings.  Vents
should extend at least 6 inches above the roof and be at least 12 inches away from a wall.  The vent pipe should extend at least six inches up
through the roof of the house.  Vents that are (improperly) terminated in the attic may allow odors to find their way into the house.  Also, in cold
weather, a vent in an attic can add moist warm air into a cold attic resulting in condensation and frost damage.

The vent should extend only about twelve inches above the roof line. Vents which are very long may be subject to frost closure in the winter.
The warm moist air passes up through the vent, and the air is cooled as it contacts the cold walls of the outdoor section of the vent pipe. The
moisture in the air condenses and freezes on the walls of the vent pipe. In a prolonged spell of cold weather, this frost can build up to a point
where it closes off the top of the vent. This, of course, negates the effectiveness of the venting system. Vents should be at least three inches in
diameter where they penetrate the roof system in order to avoid frost closure.
Vents which extend more than twelve inches above the roof should be
watched for frost closure problems. In some cases, the vents can simply be
cut shorter. In other cases, where the vent is extended to carry odors up past
a window, it may be necessary to use a larger diameter vent. A frost closure
problem can usually be solved temporarily by pouring a kettle full of just
boiled water down the vent from the top.

Because the venting piping carries mostly air, leakage is typically not an
issue, however the above roof vent pipes do collect rain water and could
leak into the attic or wall.  The horizontal sections of vent pipe need to be
sloped downward, toward the drain.  
If a horizontal section of vent pipe  in an
is improperly sloped, it can fill with rain water creating a de-facto 'trap',
defeating the vent.

Roof leakage around the vent stack flashing (where the stack penetrates the
roof) is sometimes mistaken for plumbing leakage.  A vent stack passing
through the roof membrane, creates an inherently weak spot in the roofing
system. If leakage occurs here, the water may run down the outside of the
vent stack, and appear near a plumbing fixture in the house. It is possible to
look for a long time for intermittent leaks in the waste plumbing system
which do not exist. By paying careful attention to when the leak occurs, it
may be found that the apparent plumbing leakage occurs only during or
after a rain.

When a lower level bathroom is added to a home, it is difficult to run a vent
pipe up through the house and roof. Often, a vent is run out through the wall
and up the outside of the building.  This may be acceptable (see local
codes) although not attractive, and frost closure problems are more likely
with this arrangement.

Air Admittance Valve

Where an individual fixture has been installed without appropriate venting, it
is expensive to break into walls and ceilings to add proper venting.
Mechanical devices which simulate conventional venting are available,
although not approved by many plumbing authorities. These devices, known
as automatic air vents or air admittance valves (AAV), are essentially
vacuum valves which allow air to be drawn into the waste plumbing system
when negative pressure exists, but prevent any air escaping from the
plumbing system under positive pressure. These devices provide a low
cost alternative to conventional venting for all fixtures except toilets. Again,
some plumbing authorities will not allow these.  The AAV should be
installed higher than the drain in an accessible and ventilated area.
AAV diagram
Waste System Venting
Plumbing vent diagram
Horizontal Vent Pipe in Attic