Cast Iron Soil Pipe & Fittings Handbook

Chapter 2 - Installation of Cast Iron Soil Pipe and Fittings

The Sizing of Soil, Waste and Vent Lines

             The sizing of soil, waste and vent lines should be based on "fixture load." The most accurate method of calculating fixture load is by using the "fixture unit basis." One fixture unit is defined as 7.5 gallons of water per minute. A lavatory in a private home is considered to use approximately 7.5 gallons of water per minute under maximum conditions, and other fixtures are governed by this yard stick. For example, a water closet requires more water than a lavatory, and thus it has a higher number of fixture units assigned to it. A pedestal type urinal will use more water than a wall hung urinal, and hence there are different values of fixture units for these fixtures. Another variable to be considered is that a lavatory in a residence will likely use less water than a lavatory in a public building. For this reason, different fixture unit values are assigned for the type of building in which the plumbing fixture is to be used. Table 2 lists the fixture units that have been assigned to the various types of plumbing fixtures and takes into account the type of building (private or public) in which the fixtures are installed. Table 4 lists data for the sizing of vents, and of the building drains. The information in this table has been used with satisfactory results. Code requirements for a given vicinity may vary. 

             The procedures used to size soil, waste and vent lines are: 

  1. Familiarize oneself with the plumbing code as to the minimum requirements, fixture unit tables, and pipe size tables. 
  2. Add up the fixture units on each branch. 
  3. Add up the total fixture units for the stack. 
  4. If there is more than one stack in the system, add the fixture unit totals for the various stacks. 
  5. From these totals look up the sizes of the pipes in the correct table. 
  6. Compare this size with the minimum allowed by the code; and if it is equal to or greater than the minimum, it is the correct size. 

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