Drain slope by pipe size — smaller pipe needs steeper fall

Smaller pipe needs steeper slope: 1/4" per foot for 2½" and under, 1/8" per foot for 3–6" (IPC 704.1). Too flat strands solids; too steep outruns them.

Side-view comparison of two drain pipes sloping toward the sewer. A 2-inch pipe requires 1/4 inch of fall per foot; a 4-inch pipe requires only 1/8 inch per foot (IPC 704.1). Each pipe shows a right-triangle of the fall over a one-foot run, with a water-flow arrow. Smaller pipes need steeper slope to keep solids moving; too flat strands solids, too steep lets liquid outrun them. — Source: IPC 704.1

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Side-view comparison of two drain pipes sloping toward the sewer. A 2-inch pipe requires 1/4 inch of fall per foot; a 4-inch pipe requires only 1/8 inch per foot (IPC 704.1). Each pipe shows a right-triangle of the fall over a one-foot run, with a water-flow arrow. Smaller pipes need steeper slope to keep solids moving; too flat strands solids, too steep lets liquid outrun them.

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