Used water and wastes are carried through drain-waste pipes to public sewage lines, or to on-site sewage systems; the most common being septic systems These pipes are sloped on quarter of an inch per foot, since waste flows by gravity only. Overflowing fixtures and slow-draining wastes are signs that your fixtures or drainage system should be checked.
Decomposing waste material in the sewage system emits unhealthy fumes. To prevent sewer gases from flowing back into the house, each fixture’s drain has a U-shaped pipe called a trap. A trap should always be filled with water to create a seal against sewer gases.
Usually, water draining out of the fixture will automatically seal the trap. However, improper venting can create a siphoning action that draws the water seal out of the trap, allowing sewer gases to enter the house.
Venting is necessary to maintain equal atmospheric pressure within the drain-waste pipe system and to safely dispose of sewer gases outside the house. Vent pipes are connected to the drain-waste system at each fixture’s drain line downstream from the trap, and extend outside of the house, usually from the roof.
There shouldn’t be any cross connections in your plumbing system. A cross connection is any point where contaminated water or wastes might mix with potable water; such as the point where a sink or bathtub spout is below the flood rim of the fixture or a toilet ballcock valve that is under water.
This could present a serious health hazard, for example, if there was ever a sudden drop in water pressure resulting from a water main break. Contaminated water or wastes would be drawn into the fresh water supply system through these cross connections.