Belly

A belly is a depression in a horizontal or near-horizontal drain pipe where the slope is insufficient or reversed, allowing wastewater to pool instead of flow continuously to the main stack or sewer. Gravity alone can't move water through the low spot efficiently, leaving behind grease, hair, paper, and other debris that accumulates and hardens over time.

Bellies form from settling of the house structure, poor original installation, or pipes laid without proper slope—typically when a run doesn't maintain the minimum 1/4-inch-per-foot fall. Under a sink, under a slab, or in crawl spaces, they're killers because you can't see them building up until the line is completely blocked or backing up into fixtures.

The fix is to replace or abandon the sagging section and re-run the line with correct slope, or sometimes to tie into an existing properly-sloped line above the belly. Camera inspection confirms the diagnosis. In older homes, bellies in cast iron or clay lines are common and often the reason for recurring clogs that can't be cleared by snaking.