How Much to Slope the Drain Piping?

Installing sloped drainpipes isn't as simple as making your waste run downhill. If the slope is too steep, the liquid waste won't sufficiently push the solid waste through the pipe. If the slope is too gradual, neither liquid or solid will drain. Local plumbing codes prescribe the standards for building residential drain, waste and vent piping systems. Along with knowledge of local regulations, an understanding of conventional drain system construction allows you to determine which drainpipes you must slope and how steep to pitch them.

Drain piping exists within a larger system called the drain waste and vent system. Drain and waste pipes deliver liquid and solid waste from plumbing fixtures to septic or sewer systems. Vent piping allows sewer gases to escape the system and admits air to regulate the system's pressure and prevent siphonage from drain traps. While vent piping generally runs straight to the building's roof in a vertical path, drain and waste branch pipes must slope toward main drainpipes, which slope toward main drain outlets.

Plumbers and building authorities express slope in either percent grade or inches per 1-foot unit. Expressing slope in percentage form allows builders to quickly apply the information to U.S. customary units, such as inches, or metric units, such as centimeters. For example, multiplying 12 inches by 2 percent, or 0.02, equals 0.24, which represents a slope of roughly 1/4 inch per 12 inches. Alternatively, builders and codes might simply express the code as "1/4 inch per foot." This is the case for drain pipe diameters of 2 1/2 inch or less, according to the International Plumbing Code.

Drainpipe sloping requirements vary according to the pipe's diameter. Smaller pipes are more likely to experience blockages than larger pipes and therefore require a steeper slope. In general, plumbing codes require pipes less than 3 inches in diameter to slope a 1/4 inch per foot of run. Codes often allow pipes 3 to 6 inches in diameter to slope an eighth of an inch per foot, and pipes 8 inches or larger are allowed to slope 1/16 inch per foot. But some codes advise steeper slopes for long runs of pipe, such as 50 feet or longer. Additionally, many codes advise against slopes between 1/2 inch per foot and a 45-degree angle; slopes within this range often allow liquid waste to drain while solids remain stuck. Check with a local contractor or your building authority for definitive slope specifications.

Spirit levels are the most common tools for gauging slope. Spirit levels consist of several liquid-filled vials set within a bar-shaped body. Plumbers place the level on top of a pipe and note the position of bubbles in the vials to determine slope. Alternatively, if you have a dependable frame of reference, such as string line that spans the pipe's horizontal distance, or "run," you can use a tape measure to gauge the difference in height between the opposite ends of the pipe. If you divide both the run and the decrease in height by 12 inches, you determine the ratio that represents the pipe's downward slope in inches per foot of run.