There are two ways of specifying the pipe size required for a particular class of plumbing service: (1) directly in terms of area served, as in roof-draining service (Table 14.9) and (2) in terms of fixture units (Table 14.11).
As can be seen from these tables, the capacity of a leader or drain varies with the pitch of the installed pipe. The greater the pitch per running foot of pipe, the larger the capacity allowed, in terms of either the area served or the number of fixture units. The reason for this is that the steeper the pitch the larger is the static head available and hence the larger is the amount of liquid that the pipe can handle.
The steps in determination of pipe sizes by means of fixture units (Art. 19.8) are: (1) list all fixtures served by one stack or branch; (2) alongside each fixture list its fixture unit (Table 14.4, p. 14.22); (3) add the fixture units and enter the proper table (Tables 14.4, 14.11, or 14.13) to determine the pipe size required for the stack or the branch.
Fixture branches connecting one or more fixtures with a soil or waste stack are usually sized on the basis of the maximum number of fixture units for a given size of pipe or trap (Table 14.4). Where a large volume of water or other liquid may be contained in a fixture, such as in bathtubs or slop sinks, an oversize branch drain may be provided to secure more rapid emptying.