1.4 Hangers, Supports, Rod Sizes, Dissimilar Metals, & Expansion (UPC 313, Table 313.3)

Why Support Spacing Is Tested

Hangers keep drainage piping on grade, carry the weight of pipe and contents, and restrain movement. A sagging drain loses its slope and creates a belly that collects solids. Under UPC 313.1, piping must be supported as the code, the manufacturer's installation instructions, and the AHJ require. Section 313.2 requires hangers strong enough for the pipe and its contents, and requires that piping be isolated from incompatible materials. Section 313.4 requires support that maintains alignment and prevents sagging. Section 313.5 requires underground piping to lie on a firm bed for its entire length.

Hanger spacing is a favorite closed-book question because it is pure recall. Table 313.3 is not one of the tables PSI provides, so memorize the core rows below.

UPC Table 313.3: Hangers and Supports

Notes to the cast-iron rows:

1. A support must be adjacent to the joint, not over 18 inches from it.
2. Brace at intervals not exceeding 40 feet to prevent horizontal movement.
3. Provide support at each horizontal branch connection.
4. For hubless pipe, hangers shall not be placed on the coupling.

Memory hook: "5-6-10-12-4-3-32." Cast iron 5 ft, copper 6 ft (small) or 10 ft (2 inches and up), steel 10 or 12 ft, PVC and ABS 4 ft, CPVC 3 ft (small), PEX 32 inches.

Hanger Rod Sizes (UPC Table 313.6)

Worked example

A 3-inch hubless cast-iron building drain runs 60 feet horizontally in a crawl space using 10-foot pipe lengths, so joints fall every 10 feet. The pipe is over 4 feet between joints, so support each joint, with each hanger within 18 inches of the joint and never on the coupling. Add a support at each branch connection. Because of the 40-foot bracing note, the run also needs bracing at intervals not exceeding 40 feet. Each hanger rod must be at least 3/8 inch.

Dissimilar Metals and Transitions

Galvanic corrosion occurs when dissimilar metals such as copper and galvanized steel touch in the presence of water. The UPC addresses it several ways:

• 310.6: Except for necessary valves, dissimilar-metal connections must be in exposed or accessible locations.
• 313.2: Piping must be isolated from incompatible materials. In practice, that means copper-plated or plastic-coated hangers, or isolation pads, on copper tube.
• 605.15: Dielectric unions installed where metals differ must comply with ASSE 1079.
• 605.16.1: Copper to threaded pipe in water systems must use a copper alloy adapter, a copper alloy nipple at least 6 inches long, a dielectric fitting, or an ASSE 1079 dielectric union.
• 507.1: The AHJ may require an approved dielectric insulator on water heater connections.

Expansion and Contraction

Section 312.1 requires "approved provisions … for expansion of hot water piping." Section 312.2 requires provision for expansion, contraction, and structural settlement. Table 313.3 requires PVC and ABS DWV to allow for expansion every 30 feet. Plastics move far more than metals:

Formula: growth (inches) = length (inches) × coefficient × temperature change (°F).

Example: 80 feet of PVC DWV carrying 140°F discharge that was installed at 60°F: 960 in × 0.00003 × 80°F ≈ 2.3 inches. That movement is why the table requires expansion provisions and why plastic stacks get mid-story guides rather than rigid clamps at every point. Coefficients vary by manufacturer; use the manufacturer's data for real designs.

Unions (UPC 315.1)

Unions are allowed in drainage piping only where accessible in the trap seal or between a fixture and its trap. In the vent system they are allowed except underground or in wet vents. They are allowed anywhere in the water supply system.