5.1 Drainage Principles & Drainage Fixture Units (Tables 709.1 and 709.2)

How gravity drainage works

A sanitary drain is an open-channel system. Horizontal drains are designed to run partly full, so that air can move above the water and pressures in the system stay close to atmospheric. The vent system keeps any pressure difference at a trap within 1 inch of water column (IPC 901.2). When drainage piping is sized correctly, it carries waste at a velocity that keeps solids moving. Engineers usually aim for about 2 feet per second for self-cleansing flow.

The drainage fixture unit (dfu) is a load factor, not a flow rate. It comes from Roy B. Hunter's work at the National Bureau of Standards (Methods of Estimating Loads in Plumbing Systems, BMS 65, 1940). Fixtures are used intermittently and seldom all at once. The dfu values combine each fixture's discharge rate, duration and frequency of use, so a system can be sized for a realistic peak load instead of the sum of every fixture's maximum flow. Under IPC 709.1, the Table 709.1 values are used with Tables 710.1(1) and 710.1(2) to size soil and waste pipes.

Table 709.1: drainage fixture units and minimum trap sizes

Table notes to remember:

• Note d: trap size must match the fixture outlet size. For traps larger than 3 inches, use Table 709.2.
• A showerhead over a bathtub or a whirlpool attachment does not increase the bathtub's dfu value.
• For computing loads on building drains and sewers, water closets and urinals may not be given a lower dfu value unless testing confirms it.
• For fixtures added to a bathroom group, add their dfu values to the group's value.
• Shower dfu values depend on the total flow through showerheads and body sprays.
• See 406.2 for clothes washer standpipe sizing: the trap and fixture drain must be 2 inches or larger and must connect to a 3-inch or larger branch or stack.

The Table 709.1 lavatory value is 1 dfu. The table does not separate public and private lavatories, so a choice giving "2 dfu for a public lavatory" is wrong.

A bathroom group is a water closet, lavatory, and bathtub or shower, with or without a bidet, an emergency floor drain or both, located together on the same floor level (IPC Chapter 2).

Fixtures not in the table (IPC 709.2)

The minimum trap for an unlisted fixture is the size of its drain outlet, but not less than 1-1/4 inches.

Converting flow to dfu (IPC 709.3)

Where a discharge is known only in gpm, 1 gpm equals 2 dfu. This applies to pumps, ejectors, equipment drains and similar continuous or semicontinuous flows into the sanitary system.

Example: a mechanical room has two floor drains (2 dfu each = 4), a service sink (2), and a pumped discharge of 25 gpm (25 × 2 = 50). The total load is 56 dfu.

Indirect waste receptors (IPC 709.4 and 709.4.1)

• The dfu load of an indirect waste receptor is the sum of the fixtures discharging to it, but not less than the receptor's own Table 709.1 or 709.2 value.
• A floor drain, floor sink or hub drain that receives only clear-water waste from display cases, refrigerated cases, ice bins, coolers and freezers is rated at 1/2 dfu.

Size never decreases downstream (IPC 704.2)

The size of drainage piping shall not be reduced in the direction of flow. These are not treated as reductions:

1. a 4-by-3-inch water closet flange;
2. a water closet bend with a 4-inch inlet and a 3-inch outlet, if the 4-inch leg is upright and below the closet flange;
3. an offset closet flange.

Related rules:

• 1002.5: a trap must not be larger than the drain it discharges into.
• Table 710.1(1) note a: the minimum size of any building drain serving a water closet is 3 inches.
• 704.4: drainage piping for future fixtures must end with an approved cap or plug, and 710.2 requires future fixtures to be counted when sizing drains.

Worked example: a small apartment

A two-bedroom apartment has one full bathroom (1.6 gpf water closet, lavatory, tub), a powder room (1.6 gpf water closet and lavatory), a kitchen sink with a dishwasher, and a residential clothes washer.

You would size the horizontal branches and stack with Table 710.1(2), and the building drain with Table 710.1(1), as covered in the next two sections.