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Information on Sizing Traps & Drains

The main features of an indoor plumbing system are:

1. To bring the desired supply of hot/cold water to the residents of a dwelling through a fixture (sink, tub, toilet, etc.).
2. To drain all waste and sewage into a public sewer or private disposal system through a drainage system.

A drainage system consists of Drainpipes, Waste Traps & Vents and is also known as a DWV system

Size of the drainpipe depends on the size of the fixture, commonly with larger fixtures having larger drainpipes in order to accommodate a greater flow of waste. Drainpipes may be installed both horizontally and vertically. Typically vertical pipes are at an angle greater than 45° and horizontal pipes at an angle of less than 45°. Drainpipes regularly use gravity to drain waste when they are above the elevation of the public sewer. Horizontal drainpipes typically run at a uniform downward slope toward the point of disposal (for specific slope grade consult your local plumbing code). If the fixture is lower than the elevation of the public sewer then the drainpipe cannot use gravity to drain waste and so a sump pump will typically be used to lift and discharge the waste.

Waste Trap
Practically every fixture drain has a trap. Some traps are built into the fixture, like a toilet, others are created with curved sections of drainpipe. The curved pipe holds a small amount of water which acts as a seal preventing septic gasses from leaking back into the home. Traps cannot be larger than the fixture drain.

To relieve pressure in fixture drains and allow water to flow out freely a separate pipe is commonly used as a vent. Vents connect to a main vent stack from each fixture and exit out the roof of the home. Vents may penetrate the roof of the home directly above the fixture being served or connected together to form a main vent stack. In some instances where roof penetration is not practical a device called an air admittance valve may be approved for this purpose.

The proper sizing of a sanitary drainage system is dependent upon the number of fixtures it serves. The more fixtures used the larger the pipes will need to be to accommodate the flow. For sizing of the DWV system consult your local plumbing code.

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