Drainage Systems
Slope and layout
Gravity drainage depends on slope. Horizontal drainage pipe 2½ inches and smaller slopes at ¼ inch per foot; 3 inches and larger at ⅛ inch per foot. Uniform slope, no sagging, no backfall. Fall = slope × run: a 24-foot branch at ¼ inch per foot drops 6 inches.
The building drain is the lowest pipe receiving drainage inside the building; the building sewer begins where it exits the foundation. Soil stacks carry fixtures with fecal matter; waste stacks carry everything else; a stack serving more than one branch interval is sized from the drainage fixture unit (dfu) total in the tables.
Fittings and arrangement
Flow direction decides the fitting. Horizontal-to-horizontal changes use wyes and 45-degree bends - never a sanitary tee on its back. Sanitary tees are for vertical-to-horizontal connections. No fixture may be double-trapped, no drain may be reduced in the direction of flow, and every trap must be vented (see the venting section).
Cleanouts and protection
Cleanouts are required at the base of each stack, at the upper end of horizontal drains, at changes of direction greater than 45 degrees, at intervals not exceeding 100 feet, and at the junction of the building drain and building sewer. They must remain accessible. Piping under a floor slab must be protected against shear and settlement, and hangers must be spaced per the support table for the material.
Worked example
A horizontal 3-inch drain runs 32 feet to a stack. Minimum fall = 32 ft × 1/8 in/ft = 4 inches of drop end to end.