The matter of venting appears in the plumbing system in several ways. In the first place there is the soil or waste vent through the roof, the main lines of vent into which the individual trap vents connect, the trap vents themselves, the fresh-air inlet, and the local vents of water closets, urinals. and slop sinks. Local vents and the fresh-air inlet have no connection with the system of trap vents, and will not be touched upon under this plate. The soil and waste vents, main vent lines, and trap vents are closely allied, however.

One of the chief steps toward the improvement of the plumbing system was taken when soil and waste stacks were carried through the roof instead of being allowed to end at the connection of the top fixture. Even without the use of trap vents the roof vent was of great benefit, as it was often the means of preventing the creation of siphonic conditions, which meant the siphonage of the unvented traps.

In addition, it proved a successful remedy for back pressure from the sewer, as the latter could not force the seals of traps, for the reason that the roof vents relieved any such pressure.

It is generally through the soil or waste vent that air is brought into the main vent lines of the plumbing system, which in turn deliver the air to the traps through their separate vents.

The trap vent should be as direct in its course from the trap to the main vent line as possible, in order that the passage of air may be secured with as great an amount of freedom as possible.

Each fixture vent or trap vent should incline upward throughout its course, in order that any condensation forming in it may be conducted back into the trap. The trap vent should in all cases enter the main line of vent above the fixture which it serves. When the vent is thus properly connected, and a stoppage occurs in the trap or the fixture waste, the waste from the fixture will back up into the fixture, thus giving warning of the trouble that exists. If the vent pipe is connected below the fixture, however, the waste in the event of such a stoppage will not back up into the fixture, but will flow off through the fixture vent into the main vent line, and thence into the drainage system, thus defeating the purpose of the vent system, and making of the trap vent and main vent a waste pipe for the fixture.

Each fixture trap should be separately vented, but vents from several fixtures may be connected into a single branch vent, provided this branch runs above the highest fixture of the group.