Sanitary Drainage, Vents, and Storm Drainage
Drainage fundamentals
Slope is the first check: ¼ inch per foot for pipe 2½ inches and smaller, ⅛ inch per foot for 3 inches and larger. Drainage fixture units come from the tables and are accumulated from fixture to branch to building drain. Changes of direction use long-sweep or long-turn fittings where required; two 45-degree bends can substitute where the code allows. Cleanouts belong at the base of each stack, at horizontal intervals not exceeding 100 feet, at changes of direction greater than 45 degrees, and at the building sewer.
Venting
- Every trap is vented. The vent rises at least 6 inches above the fixture's flood level rim before any offset.
- Trap arm length is limited by diameter and by fall: the drop along the arm cannot exceed one pipe diameter. Commonly cited limits are 3 ft 6 in for 1½ inch and 5 feet for 2 inch.
- Vent terminals extend above the roof per code and are kept away from operable windows and air intakes.
- Wet venting and circuit venting carry strict size and fixture-count limits - verify before using them.
Storm drainage
Roof drains and leaders are sized from roof area and the design rainfall rate. Storm drainage is a separate system; it never connects to the sanitary drain. Large roofs need secondary drainage.
Worked example
Is a 30-inch trap arm of 1½-inch pipe at ¼ inch per foot legal? Length: 30 inches is under the 42-inch limit. Fall: 2.5 ft × 0.25 = 0.625 inch, well under one pipe diameter (1.5 inch). Legal on both counts - show both calculations, since examiners grade each independently.
Traps
- Using ⅛ inch per foot on 2-inch pipe.
- Measuring trap arm length past the vent connection.
- Connecting a downspout or sump to the sanitary sewer.
- Venting a trap through a Studor-type device where the code prohibits it.