Sanitary Drainage, Venting, and Storm Drainage
Slope, Size, and Flow
Drainage design depends on slope, pipe size, and fixture-unit load. Know the standard slopes (1/4 inch per foot for smaller horizontal branches, 1/8 inch per foot for larger pipes where permitted) and why too little slope causes solids to settle while too much slope lets liquids outrun solids. Understand that a building drain carries flow to the building sewer, while a building sewer begins outside the foundation.
Venting Fundamentals
Every trap needs a vent. Learn the difference between a vent stack, stack vent, individual vent, common vent, circuit vent, and air admittance valve, and know where each is permitted. Traps include confusing a wet vent with a dry vent, forgetting the requirement for a vent terminal height above the roof, and misapplying the rule that a vent cannot serve as a drain.
Storm Drainage
Storm drainage is separate from sanitary drainage and must not connect to it. Know sizing based on rainfall intensity and roof area, and the role of primary and secondary roof drains or scuppers for overflow protection.
Rule of Thumb
For a horizontal drain, check three things in order: fixture-unit load, slope, and developed length including fittings. Worked example: a branch with a given fixture-unit load at 1/4 inch per foot may be smaller than the same load at 1/8 inch per foot, because flatter slopes require larger pipe to maintain velocity and capacity.