The sewer pipe connecting a house to the municipal branch or main sewer line under the street (in a typical installation) is called a lateral. This is usually a 4-inch pipe that is buried below the frost line in a trench that is properly prepared and sloped. The slope is the most critical aspect of the job, as it determines how well the pipe drains by the force of gravity. The basic installation process of a new sewer lateral can be broken down into five major steps. All installation details are governed by the local code authority and must be approved through one or more inspections.
Determine the Elevations
The first main step of a sewer pipe installation is to determine how far the pipe needs to go and the elevation (depth) of each end of the pipe. For a residential installation, the sewer pipe typically starts where the home's main drain exits the house foundation. The line then slopes downhill to where it connects to the city branch or main, at a connection called a tap. Elevations can be measured and laid out with a laser level, GPS, or traditional surveying equipment.
Calculate the Slope
The elevations and the length of the pipe run are used to make the initial slope calculations. Subtracting the ending elevation from the starting elevation yields the total drop in elevation. Dividing the drop by the total run (length) of the pipe yields the slope.
For example, if the pipe meets the city main at a depth of 6 feet and it starts at the house at a depth of 4 feet, the total drop is 2 feet. If the pipe run is 80 feet, the slope is:
2/80 = 0.025 or 2.5 percent
The standard minimum slope for laterals (in most areas) is 2 percent, or 2 feet of drop per 100 feet of run. The actual slope can be slightly steeper than this target but must meet the requirements of the local code. A slope that is too steep will cause the liquids to run faster than the solids, leading to clogs. A slope that is too shallow does not create enough velocity for proper drainage.
Dig the Trench and Lay the Bedding
Trenching for a sewer lateral must be done carefully to prevent unnecessary disturbance of the soil. Any disturbed soil must be thoroughly compacted to minimize settling. In most cases, it's best to make the trench as narrow as possible (to minimize soil disturbance), but this requires precise knowledge of where the pipe will terminate. Sometimes a wider trench is necessary.
The bottom of the trench must be smooth and compacted and must follow the desired slope. Often, a bedding material, typically sand, is laid in the trench to provide continuous support for the pipe. Sand is also much easier to slope than soil, allowing for fine-tuning of the slope before the pipe is installed.
Install the Pipe
Sewer pipe is installed one section at a time, typically starting at the lower end of the pipe run and working up to the higher elevation. Modern sewer pipe is made of PVC and is joined either with gasketed fittings or with solvent glue. Standard pipe size for residential lines is 4 inches. The local code specifies the required pipe material and size.
The bell (female) end of each pipe section should face uphill to minimize the changes of leaks at the pipe joints. The pipe connects to the city main per the city's specifications. At the house end, most codes require one or two cleanouts to provide easy access to the pipe for scoping or for cleaning with a sewer auger.
Backfill the Trench
After the sewer lateral is completely installed and passes final inspection, the trench is backfilled to complete the project. The backfilling process often begins with a layer of sand to protect the pipe and eliminate any voids around the pipe. Then, the trench is filled with soil added in layers. Each layer of soil is carefully compacted to prevent settling in the future.
It is always a good idea—and often required—to install a warning tape over the first layer of compacted soil. This alerts crews to the presence of the pipe to prevent accidental damage when doing any excavating in the area.