Sizing rain water piping
Local regulations on the disposal of rain or storm water are apt to vary. Some places permit this water to run off into the septic tank or public or private sewer; others insist that it be drained into a dry well. Regardless of which rule prevails, it is recommended that rain water be kept separate from sewage and that a separate pipe be used to connect the drained area with the disposal area. If this is done, the rain-water drainage piping can be a smaller, more economical size, and the danger of flooding the sewage system during a cloudburst or thunder shower is eliminated.Storm-water piping size is determined by the area drained in horizontal projection, that is, the flat area in square feet on which rain may fall. Measurements of roof and other such areas may be estimated, but in doubtful cases the larger of two possible pipe sizes should be used.
Sizing house drain
The size of pipe to be used in a house drain is determined by the number of units drained by it. Proper diameters are: for 2 units, 1 1/2"; for 9 units, 2"; for 25 units, 2 1/2".
The size of any stack, house drain, or sewer must be, however, at least the size of the largest branch connected to it. The minimum size of a house drain receiving the discharge from a water closet should be 4" in diameter, and this pipe should continue full size to the vertical stacks receiving the discharge of water closets. Even without any water closets discharging into a system, the drainage system should have at least one 3" diameter stack extended full size through the roof.
Sizing vent pipe
The proper size for the vent stack is determined by the total fixture units of the soil or waste stack it serves. A branch vent is sized by the number of units connected to it and the length of the pipe itself. No vent pipe should be less than 1 1/4" in diameter, nor should a vent stack's diameter be less than half the diameter of the soil stack it serves. This means that with a 4" soil stack, vents cannot be less than 2" pipe. In determining the length of vent piping, regard length of stack and branches as being continuous.