What Is the Minimum Slope for an Interior Sewer Pipe?

Sewer Systems

• Sewage systems in a house or residence drain out to either a municipal sewer supply or a septic tank system. In most setups, there is typically one or two major pipes, called stacks, that collect used water from various fixtures such as toilets, sinks and bathtubs. In the United States, these fixtures, except for toilets, utilize 1-inch to 1.5-inch pipes. The main stacks, on the other hand, can be 3, 4 or 5 inches wide, depending on the location.

Installing Systems Yourself

• Before doing any type of sewage installation or remodeling, check with your municipal or county region for the plumbing codes in your area. Plumbing codes give strict guidelines for plumbing installation so that standards are maintained and safe procedures are followed. These codes can tell you the minimum amount of vent stack you need to have above your roof, for example, as well as the maximum amount of bend that can take place over the course of a pipe run.

Water Flow and Sloping

• The chief thing to remember when plumbing is that water always flows downhill. This is the principle behind just about every plumbing device, including the need to keep pipes sloped at a certain level. The proper slope on a pipe also keeps the water from backing up and creating a siphon that can draw dirty water in from the sewer system.

Maximum Slopes

• The maximum slope for an interior sewer pipe depends on the pipe width. For smaller 1.5 inch pipes, a maximum slope of 1/4 inch per foot should be kept along the pipe. A 5-foot length of pipe running from a sink to a stack, then, would need to drop 1.5 inches from where it connects to the P-trap underneath the sink to the vent stack. The same maximum slope holds true for pipes larger than 1.5 inches, such as 3-inch toilet pipes, although these pipes can have a minimum slope of 1/8 inch per foot.