8.3 Fittings for Change of Direction and Drainage Flow Patterns

The question Table 706.3 answers

Drainage flows by gravity, and the enemy is hydraulic jump — the sudden deceleration where falling water turns horizontal. A fitting that turns flow too abruptly produces a standing wave that fills the pipe bore, collapses the air core above the flow, and pulls a vacuum on nearby trap seals.

Table 706.3 therefore governs, for each fitting, which of three patterns it may create:

• Horizontal to vertical — a branch dropping into a stack. The gentlest case, because gravity is already pulling the flow the way it is going.
• Vertical to horizontal — the base of a stack, or a fixture drain turning out of a wall. The most demanding case.
• Horizontal to horizontal — an offset in a horizontal run.

Table 706.3 — Fittings for change in direction

Footnotes:

• a. The fittings shall only be permitted for a 2-inch or smaller fixture drain.
• b. Three inches or larger.
• c. For a limitation on double sanitary tees, see Section 706.3 (the back-to-back water closet rule).

Colorado field note (not exam-scored): Board Rule 1.2(E)(1)(ll) deletes footnotes a and b from Table 706.3, and Rule 1.2(E)(1)(kk) adds that fixture crosses are not required to meet the exception of IPC 706.3.

Reading the table correctly

The sanitary tee is the fitting to watch. It has exactly one X, in the horizontal-to-vertical column, with a footnote. A sanitary tee laid on its back to turn a stack into a building drain is a defect, because its short-radius throat produces a violent hydraulic jump precisely where the vertical flow is fastest. A long sweep or a combination wye and eighth bend belongs at the base of a stack.

Quarter bends and short sweeps are conditional. Both are permitted horizontal to vertical without restriction. Turning vertical to horizontal or horizontal to horizontal, both are limited to a 2-inch or smaller fixture drain — and a short sweep gets an additional path through footnote b at 3 inches or larger.

Wyes and combination wye and eighth bends are unconditional, which is why they are the workhorse fittings of any well-built DWV system.

Long sweeps are unconditional too, and short sweeps are not. The difference is centerline radius: a long-sweep quarter bend has a radius roughly equal to its diameter or more, a short sweep considerably less.

The base of a stack and horizontal branches — IPC 704.3

Horizontal branches shall connect to the bases of stacks at a point located not less than 10 times the diameter of the drainage stack downstream from the stack. Horizontal branches shall connect to horizontal stack offsets at a point located not less than 10 times the diameter of the drainage stack downstream from the upper stack.

Water reaching the base of a stack has reached terminal velocity and fills a large fraction of the pipe bore for some distance downstream. A branch tied in inside that zone is subject to backflow and to severe pressure fluctuation.

Worked example

A 4-inch soil stack lands on a horizontal building drain. How far downstream may a lavatory branch connect?

• 10 x 4 inches = 40 inches, so not less than 40 inches (3 feet 4 inches) downstream from the base of the stack.

For a 6-inch stack, the distance becomes 10 x 6 = 60 inches (5 feet).

Compare the parallel rule for sump discharge in IPC 712.3.5: a pump discharge connecting into horizontal drainage piping must be made through a wye fitting into the top of the piping, located not less than 10 pipe diameters from the base of any soil stack, waste stack or fixture drain.

Heel- and side-inlet quarter bends — IPC 706.4

The distinction is where the inlet sits relative to the flow line. A low heel inlet enters near the bottom of the bend, where it can be flooded by the main flow; a side inlet enters at the springline, above the flow.

Fitting selection in practice

A three-story building's 3-inch waste stack must land on a 4-inch building drain, and a 2-inch lavatory branch must turn from horizontal in a wall to vertical, then back to horizontal beneath a slab.

Two further Chapter 3 rules apply to that offset: IPC 308.6 requires rigid bracing where horizontal drainage 4 inches and larger changes direction more than 45 degrees (except into a vertical pipe), and IPC 308.7.1 requires restraints at all direction changes for pipe larger than 4 inches.