PLANNING, ESTIMATING, AND SIZING PLUMBING SYSTEMS

You will provide input on the planning, estimating, and sizing of plumbing systems. This input may concern installation techniques, types of material required, quantity and size of piping or fittings, and so forth. This section provides information you must consider for planning and estimating a plumbing project. The National Standard Plumbing Code, military specifications, and job specifications provide more concise information.

SANITARY SYSTEMS

Various types of pipe and fittings are used for sanitary waste and drainage. However, the location of the installation determines the type of material you must use. Threaded pipe that is underground requires coal tar protection. Install underground sanitary waste and drainage lines in a separate trench from the water-service line. The underground water service and the building drain or sewer should not be less than 6 feet apart horizontally and placed on undisturbed or compacted earth. When separate systems of sanitary drainage and storm building drains are placed in one trench, they should be placed sideby-side. A building sewer or building drain installed in fill dirt or unstable ground should be made of cast-iron soil pipe, except that nonmetallic drains may be used when laid on an approved continuous supporting system. Table 7-1 depicts code requirements for pipe usage.

Table 7-1.-Sanitary Waste and Drain Piping

Pay special attention to the joints so roots do not grow into the piping. The depth of the piping should be below the frost line. Also, you should encase the piping with concrete or sleeve it with a metallic material when laying piping under roadways.

It may be necessary to install the building sewer and the water-service pipe in the same trench (fig. 7-1). If so, use the following precautions:

Figure 7-1.-Building sewer and the water-servicepipe in the same trench.

Ensure the bottom of the water pipe is at least 12 inches above the top of the building sewer. Place the water pipe on a solid shelf at the side of the trench. Use hot lead when joining cast-iron pipe for a building sewer; use a hot-poured compound in joining bell-and-spigot clay or concrete sewer pipe. After installation, test the building sewer with a 10-foot head of water, or equivalent test.

Grading

Install sanitary drainage piping on a uniform slope. This slope, pitch, grade, or drop per foot decides the flow velocity of liquid within the pipe. Piping with a diameter of 3 inches or less requires a slope of not less than 1/4 inch per foot. Pipe 4 inches or larger slopes no less than 1/8 inch per foot. This allows a velocity of not less than 2 feet per second, that provides the scouring action necessary to keep a pipe free from fouling. Sewer mains may have slopes of less than 1/8 inch per foot, as long as there is a cleaning velocity of 2 feet per second or greater. See table 7-1 (A) for code requirements.

Table 7-1 (A).-Discharge Rates and Velocities of Sloping Drains

Higher velocities, or greater drop per foot, will increase the carrying capacity of a drain. When designing fixture branches, keep in mind that a slope/drop of more than 1/4 inch per foot may cause siphonage of the trap seal.

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