6.2 Drainage Piping Installation and Slopes

Installation Principles and Slopes

Sanitary drainage systems must be installed to maintain proper alignment and uniform slope. The main objective of drainage installation is to keep liquid and solid wastes moving continuously toward the sewer. If a pipe is sloped too little, liquid velocity drops, causing solids to settle and clog the line. Conversely, if a pipe is sloped too steeply, the liquid may flow too fast, leaving solids behind.

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Minimum Slope Requirements (IPC Section 704.1)

To maintain the required self-cleansing velocity of 2 feet per second (fps), smaller pipes must have steeper slopes because they have a smaller hydraulic radius and higher relative friction. IPC Section 704.1 establishes the following minimum slopes based on the nominal pipe diameter:

[!WARNING] Exam Trap: Memorize the slope boundaries. A 3-inch pipe is the critical line. Anything smaller must be sloped at 1/4 in/ft. A 3-inch pipe and larger (up to 6 inches) can use 1/8 in/ft. In practice, inspectors look closely at 3-inch lines to verify they have at least 1/8 in/ft slope.

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Changes in Direction and Permitted Fittings (IPC Section 706)

Fittings used to change the direction of flow in a drainage system must be selected and installed to prevent splashing, turbulence, and air pressure fluctuations. Sharp bends restrict flow and can create positive pressures that blow out fixture traps.

IPC Table 706.3 regulates permitted fittings based on the direction of flow. Direction changes are classified into three paths:

1. Horizontal-to-Horizontal (e.g., a horizontal branch turning on a ceiling space).
2. Vertical-to-Horizontal (e.g., a vertical stack turning at its base to connect to the building drain).
3. Horizontal-to-Vertical (e.g., a horizontal branch entering a vertical stack).

Fitting Permissibility Table:

Key Fitting Rules:

• Sanitary Tees: Sanitary tees have a short-radius sweep. They are prohibited on horizontal lines because the waste will hit the back of the tee, causing splashing and solid accumulation. They are only permitted when flow transitions from horizontal to vertical (where gravity aids the transition).
• Double Sanitary Tees: Double sanitary tees are prohibited for horizontal-to-horizontal changes of direction. They are only permitted in vertical stacks to connect back-to-back fixtures (such as two back-to-back lavatories).
• Short Bends: A standard short-pattern 90-degree elbow (quarter bend) is prohibited for horizontal-to-horizontal and vertical-to-horizontal changes of direction. A long-sweep quarter bend or a wye-and-eighth bend combination must be used instead.

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Joint Types and Approved Connections (IPC Section 705)

Properly installed drainage piping must have leak-proof joints that can withstand the chemical and physical demands of the waste stream. IPC Section 705 regulates joints and connections:

• ABS and PVC Joint Connections: These plastic materials are joined using solvent cement. For PVC, a purple primer conforming to ASTM F656 must be applied before the solvent cement (ASTM D2564). For ABS, solvent cement conforming to ASTM D2235 is used, and primer is NOT required. Mixing ABS and PVC using solvent cement is prohibited; a transition fitting must be used.
• Cast Iron Hubless Connections: Joined using neoprene gaskets and stainless steel shield clamps (conforming to CISPI 310 or ASTM C1277). Clamps must be tightened to the manufacturer's specified torque (typically 60 inch-pounds).
• Cast Iron Hub-and-Spigot Connections: Joined using lead and oakum (hot poured lead at least 1 inch deep) or compression gaskets (ASTM C564).

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Stack Offsets (IPC Section 704.2)

When a vertical drainage stack has an offset of more than 45 degrees from the vertical, it creates high-turbulence zones. As falling waste hits the horizontal offset, it slows down rapidly, creating a "hydraulic jump" that fills the pipe cross-section with liquid. This creates positive air pressure upstream of the jump and negative air pressure downstream.

To protect fixture traps from being siphoned or blown out by these pressure waves, IPC Section 704.2 prohibits the connection of horizontal branches to the stack within 2 feet (24 inches) above or below the offset.

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Structural Protection and Framing (IPC Section 307)

Installing drainage piping often requires cutting through structural framing members. The inspector must ensure the building's structural integrity is maintained:

• Wood Joists: Notches in the top or bottom of wood joists must not exceed 1/6 of the joist depth and must not be in the middle third of the span. Bored holes must not exceed 1/3 of the joist depth and must be located at least 2 inches from the top and bottom edges.
• Shield Plates (Nail Plates): If a drainage pipe passes through a wood stud or plate and is located closer than 1-1/4 inches from the edge of the framing member, steel shield plates must be installed. The plates must be at least 16-gauge steel and must extend at least 2 inches above sole plates and below top plates to protect the pipe from drywall nails and screws.