Drainage and vent piping

An important, but often neglected and misunderstood, part of the plumbing system is the vent piping. In some localities where there is no health authority to exercise jurisdiction, it is sometimes left out of an installation entirely. To see why it is required, we must understand a little of the physics of the waste system.

The principles involved are those of the siphon and aspiration. Sometimes, as in flushing the water closet, a positive siphoning action is needed; but more often it is guarded against. According to the siphon principle, when there are two columns of varying length, both containing water, when the weight of water in the longer one exceeds that in the shorter, greater weight draws the water out of the shorter length if the longer one extends below it. If we regard the soil stack, as the longer column, and the branch with its trap as the shorter column, we can see that when a large water discharge, as from a water closet, passes the connection of a fixture branch, the tendency will be to draw out the water in the trap.

Called aspiration, it breaks the trap seal, letting foul gases and air enter the building. To prevent this from happening, a ventilating pipe should be provided for each fixture so that back pressure cannot develop and siphonage cannot occur. A loud sucking noise in the piping of a fixture indicates that there is a siphonage action which needs correction.

When vent pipes are provided, air pressure on both sides of a trap seal are equalized. When it is so balanced, water does not tend to be drawn out in either direction. In addition, sewer gases which find their way back into the system can be drawn off before they build up enough pressure to force trap seals. Pressure generated by sudden large discharges of hot water is also relieved in this manner.

During those times when the drainage system is not carrying any discharges, properly installed vent lines permit free travel of air throughout the system, reducing the formation of slime and retarding corrosion. Foul air which might be injurious to health passes off through roof openings instead of remaining and becoming worse in the pipes. All vent piping should be run so that all horizontal lines pitch toward the fixtures they serve. This will keep condensed moisture from accumulating.

Kinds of vent piping

Main vent: This is the line of pipe running vertically beside the soil stack. Branch and fixture vents axe connected with it, and through it each trap is supplied with air from the outside atmosphere. The main vent is connected to the outside either by being extended through the roof separately or by being run back into the soil or waste stack at a point above the highest fixture connection.

Branch vent: Connected to the main vent, this pipe serves two or more fixture vents. It and the fixture vent should be run to a height above the overflow level of the fixture being served. This prevents sewage from draining through the vent line if the fixture waste branch gets clogged.

When properly connected, vent lines will remain free and any stoppages in the drain piping will be indicated by the backing up of waste into the fixture.

Fixture vent: Each fixture should be vented and trapped separately so that trouble with any one fixture will not affect any other part of the system. Fixture vents are used to prevent siphonage, to relieve pressure on trap seals, and to draw off back pressure from gases generated by decomposing sewage. It is usually run as a continuous vent, that is, only one fitting is used to connect vent pipe, waste pipe, and fixture trap or branch. This is most often accomplished by the use of a T-Y fitting.

Individual trap vent: A trap may be supplied with air near its crown by the installation of a vent at the trap itself. In this way, the possibility of a vacuum forming at this point is eliminated.

Dual connection: Though separate venting and trapping of fixtures is advised, there are certain exceptions to this rule. In the case of a combination unit, such as a laundry tub and sink, or when two such separate units are set back to back, a single trap and vent will suffice if the dual connection is made above the level of the trap crown. The water closet has its own trap built into the bowl and is vented separately to prevent back pressure, but not siphonage.

Materials used in vent piping

All types of pipe, including wrought, cast, and galvanized iron, lead, brass, plastic, and copper, have been used for vents, depending on the pipe used in other parts of the system, the cost involved, and the type of sewage handled by the drainage system. As certain kinds of sewage emit different gases, they may corrode certain metals more readily than others.

Of all the materials mentioned, lead pipe is the hardest to work with, copper the easiest, and either copper or brass the most durable. At no time should plain or black iron pipe be used for vent lines.

Installing vent piping

Liquids of all kinds should be kept out of vent piping if it is to function properly. To assure that condensation drains into the stack, vertical vents should be as perpendicular as possible, and horizontal sections should pitch downward toward the fixture in straight runs and without dips or bends so that full advantage of gravity can be taken. If turns occur in horizontal runs, there is a constant danger of water collecting from condensation.

Vent piping should never be less than half the diameter of the fixture drain it serves, and usually should be closer in size. For example, waste pipes of 3" and 4" diameters should be serviced by a minimum 2" vent pipe. And regardless of piping size, no vent should be smaller than 1 1/4" pipe. Dual vents which are to service more than one fixture should be slightly larger than required for a single fixture.

In all installations of horizontal vent piping to a waste pipe, vent connections must be made at a point which would be above an imaginary line drawn from the center of the connection with the soil stack to the center of the fixture. This line represents the theoretical fall of the waste, and unless the vent connection keeps above it, water and solid waste will find their way into it and eventually the vent will become clogged.

Like all other piping that is run for a considerable length, vent piping requires support on both horizontal and vertical runs. The principles are the the same as those described in section on the soil stack.

Plumbing basics

Bathroom fixtures
Bathroom floor plans
Bathtub installation
Installing lavatory
Water closet install

Cesspool / septic tanks

Fixture pipes
House drain
House sewer
Soil stack
Vent piping

Finishing touch

Heating a home


Dishwasher install
Garbage disposer install
Kitchen plans
Kitchen sink installation
Laundry install
Room plans
Work areas

Cast Iron

Plumbing layout
Pipe plan
Water distributing

Faucet repair
Pipe problems
Repair other
Toilet repair


Plumbing materials
Specialty tools

Water Supply

Hot water heaters
Running pipes
Service connection
Water meter
Water savings

Wells / pumps