Storm drainage and special waste systems
Storm and special waste
Storm drainage must be separate from sanitary drainage unless the code explicitly permits a combined system. Roof drains, scuppers, leaders, gutters, and area drains collect rainwater; overflow scuppers or secondary drains protect roofs when primary drains clog. Size leaders from roof area and local design rainfall intensity, not from pipe diameter alone. Special wastes include grease, oil, sand, acid, laundry, condensate, and medical or laboratory waste. They normally discharge through air gaps, indirect waste receptors, interceptors, or neutralization before entering the sanitary system.
Design and protection
Grease interceptors must be sized for flow and retention, vented, and accessible. Acid waste needs corrosion-resistant pipe and neutralization. Condensate must not create a slip hazard or discharge into a trap arm. Indirect waste receptors must be trapped and protected by an air gap or air break. Backwater valves protect low elevations. Storm and sanitary systems need separate testing and labeling.
Worked example
A roof area is 2,000 sq ft and the local one-hour design rainfall is 2 in. Rainfall volume = 2,000 x 2 x 0.623 = 2,492 gal/hr, or about 41.5 gpm. The designer uses that flow and the manufacturer's leader capacity, plus the code rainfall table, to select leader size and count. A 3 in. leader may or may not carry 41.5 gpm depending on the table and head; the table, not guesswork, controls. Secondary drainage must handle the same design event if primary drains block.
Traps
Connecting storm to sanitary; sizing leaders by roof area alone; ignoring design rainfall; no secondary overflow; improper grease interceptor venting; indirect waste without an air gap; acid waste in ordinary metal pipe.