Soil and waste pipes should always be the full size of the waste outlets from fixtures. The outlets should be sufficiently large to permit the fixtures being emptied quickly so as to thoroughly flush the waste pipes, and the waste outlets should be unobstructed by strainers, cross-bars or other devices that will catch fibrous materials or obstruct the flow of water. Formerly water closet outlets were made 4 inches in diameter and soil pipes were made correspondingly large; recently, however, the manufacture of closets has undergone a change both as to types and sizes, and water closets are now made with traps seldom over 3 inches, and usually only 2 1/2 inches in diameter. The consequence of this change in the size of closet traps is that soil pipes are now made 3 inches in diameter and soil stacks in ordinary cottage buildings are made the same size. A distinct advantage gained by this reduction in size of closet traps and soil pipes is that soil stacks in small buildings can more easily be concealed in partitions now than formerly when 4-inch pipes were used.

Size Of Vent Pipes

For traps 3 inches and more in diameter vent pipes 2 inches in diameter are used; for 2-inch trap, a 1 1/2-inch vent pipe is used, and for any trap smaller than 2 inches in diameter the vent pipe should be the full size of the trap, to reduce the possibility of stoppage should the waste pipe become choked and sewage back up in the vent pipes.

From experience and experiment the following sizes of soil, waste and vent pipes are derived:

Table VIII - Sizes Of Soil, Waste And Vent Pipes

Waste pipes from fixtures to the stack should have as much fall as can be conveniently given; owing to their small diameters and the proportionally large amount of frictional resistance they offer to the flow of sewage they cannot be given too much pitch.