Drainage, Waste and Vent (DWV)

A value representing the probable discharge of a fixture into the drainage system, used to size drains and sewers.

Add the DFU of all fixtures on a drain, then select the pipe size from the code table for the chosen slope.

The size of a drainage pipe must not decrease in the direction of flow toward the sewer.

The building drain and building sewer are sized for the total connected DFU using the drainage sizing tables.

Horizontal drainage piping shall be run in practical alignment and at a uniform slope of not less than 1/4 inch per foot, or 2 percent, toward the point of disposal — at every pipe size, not just small pipe.

Only where 1/4 inch per foot is impractical because of the depth of the street sewer, the structural features, or the arrangement of the building may pipe 4 inches or larger be sloped at not less than 1/8 inch per foot, or 1 percent, and then only where first approved by the Authority Having Jurisdiction.

Grade should be uniform and continuous so there are no low spots that trap waste or create bellies in the line.

Slope is chosen to keep a self-scouring velocity that carries solids without leaving the liquid behind.

Each fixture trap shall have a liquid seal of not less than 2 inches and not more than 4 inches, except where the Authority Having Jurisdiction finds a deeper seal necessary.

Each fixture is separately trapped, and a fixture must not be double trapped because a second trap can lock the flow.

S-traps, bell traps, and traps with moving parts are prohibited because they can lose their seal or clog.

The trap arm is the drain between the trap weir and the vent. It may change direction up to 90 degrees without a cleanout — up to 135 degrees where the trap arm is 3 inches or larger — and for a water closet or similar fixture the developed length from the top of the closet flange to the inner edge of the vent shall not exceed 6 feet.

The developed length of the trap arm from the trap weir to the inner edge of the vent shall be not less than two times the diameter of the trap arm and not more than Table 1002.2 allows: 30 inches for a 1-1/4 inch trap arm, 42 inches for 1-1/2 inch, 60 inches for 2 inch, 72 inches for 3 inch, and 120 inches for 4 inch and larger, with 1/4 inch per foot of slope maintained.

A vent that serves a single fixture trap and connects to the vent system or extends to open air.

A single vent that serves two fixture traps connected at the same level, permitted within code limits.

A pipe that serves as both a drain for one fixture and a vent for another, allowed under specific code conditions.

A vent must extend through its flashing and terminate not less than 6 inches above the roof and not less than 1 foot from a vertical surface, and not less than 10 feet from — or not less than 3 feet above — an openable window, door, opening, air intake, or vent shaft. Where the roof is used for assembly or parking, a vent within 10 feet of that area must extend not less than 7 feet above it.

A cleanout in piping 2 inches or less must have not less than 18 inches by 18 inches of clearance in front of it, and one in piping exceeding 2 inches not less than 24 inches by 24 inches. An under-floor cleanout may be no more than 5 feet from an access door, trap door, or crawl hole, with not less than 30 inches of horizontal clearance from the means of access.

Each horizontal drainage pipe needs a cleanout at its upper terminal, and a run more than 100 feet in total developed length needs a cleanout for each 100 feet or fraction thereof. An additional cleanout is required for each aggregate horizontal change in direction exceeding 135 degrees.

Kitchens and food service that discharge grease must install an approved grease interceptor to keep grease out of the sewer.

Garages and industrial drains use sand or oil interceptors to catch heavy solids and petroleum before they enter the drainage system.