Storm Drainage and Special Plumbing Systems
Storm Drainage and Special Plumbing Systems. This domain tests roof drainage, storm leaders, overflow systems, and special systems such as grease interceptors, indirect waste, sump pumps, and acid neutralization. Expect questions on rainfall-based sizing, separate storm and sanitary systems, secondary drainage, and special waste protection.
Core rules
- Storm drainage must be separate from sanitary drainage unless an approved combined system exists. Roof drains, leaders, and storm sewers carry rainwater only.
- Size roof drains and leaders using the design rainfall rate, roof area, and code tables. The steeper or more intense the rainfall, the greater the flow.
- Provide primary and secondary roof drainage where required. Secondary drainage may use overflow scuppers or overflow drains, and it must discharge safely.
- Special systems need approved pretreatment: grease interceptors for kitchen waste, acid neutralization for corrosive waste, and indirect waste with an air gap or air break for fixtures that must not connect directly.
Worked example
A roof area is 2,000 sq ft and the design rainfall is 3 in per hour. Use the conversion factor 0.0104 gpm per sq ft per in per hour: 2,000 × 3 × 0.0104 = 62.4 gpm. The reasoning: rainfall rate is a depth per hour, so roof area converts it to volume per time. Select the roof drain and leader from an approved table for at least 62.4 gpm, not by roof area alone.
Common traps
Do not connect a downspout or roof drain to a sanitary sewer where separate systems are required. Do not ignore secondary drainage. Do not size a grease interceptor by pipe size alone. Do not omit an air gap on indirect waste.