Proper Fall for Drain Lines

Terminology

• Although they are technically different, the terms fall and pitch are commonly used interchangeably. Plumbing codes usually express minimum drops of drainpipes in terms of pitch, the amount of inches a pipe drops for each foot of length. The fall or slope is a separate calculation expressed as a percentage. To figure the fall for a drain line, divide the elevation change by the distance then multiply by 100. For example, if a pipe drops 1 foot for a distance of 50 feet, 1 divided by 50 multiplied by 100 equals a 2 percent fall.

Small Pipes

• The IPC sets a minimum pitch for a 2-inch pipe as 1/4-inch drop per foot. That degree of pitch means a pipe would drop 1 inch for every 4 feet of length or 1 foot for every 48 feet of length, roughly a 2 percent fall.

Large Pipes

• The IPC minimum pitch for a 3- to 6-inch pipe is 1/8-inch drop per foot. The minimum for a pipe that is 8 inches or larger is 1/6-inch per foot.

Special Considerations

• Corrugated pipe drains more slowly than a pipe with a smooth interior and should drop at a steeper pitch. Plumbing codes govern standards for interior and exterior water and sewage draining systems. You can use a pitch as low as 1/8-inch per foot for a gravel-covered outdoor French drain to redirect water from your lawn. This would be a 1 percent fall.