The house drainage system serves two purposes. First, it must safely carry away all waste matter from the house to some disposal point, which may be either a community sewage system or a cesspool or septic tank. Second, it must prevent the gases formed by the sewage from entering the house by flowing back through the sewer pipes and out the fixture drains.
It is very important, from the standpoint of health, that this system be properly installed and function correctly. A faulty hot water system is an inconvenience and a leaky pipe may cause damage, but sewer gases are poisonous and sewage carries many harmful germs.
Unfortunately for the home mechanic, most minor repair jobs involving the house plumbing system will deal with the sewage system. As no work of this type is considered pleasant, the home mechanic should see to it that the system is not abused.
To provide a safe means of disposing of sewer gas, an extension of the sewer pipe runs through the roof of the house. This is called a vent stack, and it allows the gases in the system to pass harmlessly into the outside air. A fresh air intake for this venting system is provided in the lower portion of the sewer pipe.
This must be done so that the gases may flow freely out of the system. If this fresh air intake becomes blocked, the ventilating system will not operate properly. In a correctly installed system, however, the intake will be so located and designed that no foreign matter can enter it, under normal conditions.
The tops of the vent stacks above the roof must not become choked with ice, leaves, or any other substance. If this should happen, the pressure generated by the sewer gas in the system will cause the water to rumble as it flows down the drain. If the pressure of the gas is great enough, it can force its way through a fixture drain, bringing sewage along with it.
Any cracks in sewer pipes should be immediately repaired, for these provide an easy escape for sewer gas.