I'm running a 3-inch main drain and a 2-inch branch under a slab.

3-inch main drain: 1/8 inch per foot (1%) minimum. 2-inch branch: 1/4 inch per foot (2%) minimum.

Code basis (IPC 2021/2024, UPC 2021/2024):

• Table 704.1 (IPC) / Table 704.1 (UPC): Horizontal drainage piping 3″ and larger: 1/8″/ft. Piping 2″ and smaller: 1/4″/ft.
• No exceptions for “under slab” — slope applies regardless of location.

Practical notes:

• Maintain grade continuously. Use a laser/level and check every 4–6 ft. A 10-ft run of 3″ needs ≥1-1/4″ fall; 2″ needs ≥2-1/2″.
• Bedding: Compacted sand or pea gravel (per local amend.) — no voids. Pipe must not settle below grade.
• Cleanouts: Required at base of stack, every 100 ft (IPC 708.1), and at direction changes >45° (IPC 708.2).
• Inspection: Call for rough-in before backfill/cover. Photos of slope tape/level help if inspector questions it.

Watchouts:

• Don’t overslope (>1/2″/ft) — solids outrun water, causing clogs.
• 2″ branches tying into 3″ main: use a wye + 1/8 bend (not a tee) to maintain flow direction.
• If local amendment requires 1/4″/ft on 3″ (some jurisdictions), follow the stricter rule.

Bottom line: 3″ = 1/8″/ft, 2″ = 1/4″/ft. Verify with your AHJ — some adopt older codes or local amendments.

For a 3-inch main drain and a 2-inch branch under a slab, the required slope is governed by the International Plumbing Code (IPC) and Uniform Plumbing Code (UPC), which are widely adopted in the U.S.

Slope Requirements:

• 3-inch main drain: Minimum slope of 1/4 inch per foot (2% grade).
• Code Reference: IPC Table 704.1 / UPC Table 704.1 — Horizontal drainage piping 3 inches and larger requires a minimum slope of 1/4 inch per foot.

• 2-inch branch: Minimum slope of 1/4 inch per foot (2% grade).
• Code Reference: IPC Table 704.1 / UPC Table 704.1 — Horizontal drainage piping 2 inches or smaller also requires a minimum slope of 1/4 inch per foot.

Important Notes:

• Maximum slope: Do not exceed 1/2 inch per foot (4% grade) for either pipe. Excessive slope can cause liquids to outrun solids, leading to clogs.
• Under slab: Ensure the pipe is bedded in compacted granular material (e.g., sand or gravel) to prevent settling and maintain slope integrity. Avoid direct contact with soil or sharp rocks.
• Venting: Both pipes must be properly vented per IPC Chapter 9 to prevent trap siphonage and ensure flow.
• Inspection: Slope must be verified before pouring concrete — use a laser level or string line with a tape measure to confirm consistent fall.

> ✅ Bottom line: Both the 3-inch main and 2-inch branch require minimum 1/4 inch per foot slope, max 1/2 inch per foot. > ⚠️ If you’re unsure about slab penetration, structural integrity, or local amendments (e.g., some jurisdictions adopt stricter rules), consult a licensed plumber or building inspector. Slab work is permanent — get it right the first time.

Always verify with your local AHJ (Authority Having Jurisdiction), as amendments may apply.