2.3 Piping Support, Trenching, Bedding, and Backfill

Hanger spacing: IPC Table 308.5

IPC 308.5 requires pipe to be supported in accordance with Table 308.5. This table is worth memorizing, not merely looking up, because it appears in both General Regulations items and in drainage and water-supply items.

Two footnotes carry real weight:

• Footnote a: cast-iron horizontal spacing increases to 10 feet where 10-foot lengths of pipe are installed. This is why a 10-foot no-hub run is supported at each joint rather than every 5 feet.
• Footnote b: for the plastics and stainless systems flagged with it, sizes 2 inches and smaller require a guide midway between required vertical supports, preventing movement perpendicular to the pipe axis.

Three traps: copper tubing and copper pipe are different rows (6 or 10 feet versus 12 feet); CPVC drops to 3 feet at 1 inch and smaller while PVC stays at 4; and lead pipe must be supported continuously.

IPC 308.3 requires hangers, anchors and supports to carry the piping and its contents, of approved material that will not promote galvanic action — the reason you do not hang copper in a bare steel strap. IPC 308.4 requires attachment to the building construction in an approved manner, and 308.2 sends seismic support, anchorage and bracing to IBC Chapter 16 where earthquake loads apply.

Colorado field note: Board Rule 1.2(E)(1)(i) and (j) add a hanger rod size table 308.5.1 that the model IPC does not have: 3/8 inch rod for 1/2-inch through 4-inch pipe, 1/2 inch for 5-inch through 8-inch, and 5/8 inch for 10-inch through 12-inch.

Sway bracing and anchorage

• IPC 308.6 Sway bracing: where horizontal pipes 4 inches and larger convey drainage or waste and a fitting changes flow direction more than 45 degrees, rigid bracing or other rigid support must resist movement of the upstream pipe in the direction of flow. A change of direction into a vertical pipe does not require the upstream pipe to be braced.
• IPC 308.7 Anchorage: anchorage must restrain drainage piping from axial movement.
• IPC 308.7.1 Location: for pipe larger than 4 inches, restraints are required at all changes in direction and at all changes in diameter greater than two pipe sizes, using braces, blocks, rodding or other methods specified by the coupling manufacturer.

Together these sections explain why large no-hub cast iron under a slab or in a chase must be blocked at every bend: shielded couplings have no inherent resistance to the axial thrust a moving column of water creates.

• IPC 308.8 limits expansion joint fittings to locations where expansion and contraction genuinely require them.
• IPC 308.9 addresses parallel water distribution systems; 308.10 addresses thermal expansion tanks and their support.

Trenching, excavation and backfill

IPC 306.1 states the governing principle in seven words: buried piping shall be supported throughout its entire length. Every rule that follows serves that principle.

306.2 Trenching and bedding

Where the trench bottom forms the bed for the pipe, solid and continuous load-bearing support must be provided between joints, with bell holes, hub holes and coupling holes provided where the pipe is joined. The reason is mechanical: a bell resting on undisturbed trench bottom becomes a fulcrum, and the barrel between bells hangs unsupported.

Pipe shall not be supported on blocks to grade. Where the manufacturer's instructions are more restrictive than the code, the manufacturer's instructions govern.

306.2.1 Overexcavation

Where the trench is dug below the pipe's installation level, backfill to the bottom of the pipe with sand or fine gravel placed in layers not greater than 6 inches deep, compacted after each placement.

306.2.2 Rock removal

Where rock is encountered, remove it to not less than 3 inches below the bottom of the pipe and backfill to the pipe's bottom with sand tamped in place for uniform load-bearing support between joints. The pipe, including its joints, shall not rest on rock at any point.

306.2.3 Soft load-bearing materials

Where soft, poor load-bearing material is found at the trench bottom, overexcavate not less than two pipe diameters and backfill to the bottom of the pipe with fine gravel, crushed stone or a concrete foundation; a concrete foundation must be bedded with tamped sand.

306.3 Backfilling

Backfill must be free from discarded construction material and debris. Loose earth free from rocks, broken concrete and frozen chunks is placed in 6-inch layers and tamped until the crown of the pipe is covered by 12 inches of tamped earth. Backfill under and beside the pipe must be compacted for pipe support, and must be brought up evenly on both sides so the pipe stays aligned.

306.4 Tunneling

Where pipe is installed by tunneling or jacking, it must be protected from damage during installation and from subsequent uneven loading. Earth tunnels require supporting structures to prevent settling or caving.

The four trench-bottom conditions, side by side

Worked example: supporting a mixed riser

A three-story building has a 4-inch cast-iron soil stack in 10-foot lengths, a 2-inch PVC vent and a 1-inch copper tube hot water riser in the same chase. Floor-to-floor is 10 feet.

• Cast iron vertical: 15 feet maximum, so a support at every floor line (10 feet) satisfies it easily.
• PVC vertical: 10 feet maximum — a support at every floor line, and because the pipe is 2 inches, footnote b requires a guide midway between supports.
• Copper tubing vertical: 10 feet maximum — a support at every floor line.
• Where the copper turns horizontal at the branch, the 1-inch tubing needs support every 6 feet; the 2-inch PVC branch needs support every 4 feet; the horizontal 4-inch cast iron in 10-foot lengths may go 10 feet under footnote a.
• The copper must not be strapped in bare steel (308.3, galvanic action), and where it crosses a steel stud it must be sheathed (305.1).