Storm Drainage, Special and Indirect Waste
Storm Drainage
Systems and components
Storm water is handled by roof drains, scuppers, gutters and downspouts, leaders (conductors), and a storm building drain and sewer - kept separate from the sanitary system wherever possible. Where combined systems exist, the storm connection usually enters downstream of the sanitary. Roof drains may be trapped or untrapped depending on the code edition; trapped roof drains need venting. Secondary or overflow drainage is required where ponding could overload the structure.
Sizing
Sizing depends on roof area, design rainfall intensity, and pipe size and slope. Convert area and intensity to flow: gallons per minute = 0.0104 × area (ft²) × intensity (in./hr). Then match to the leader and roof-drain tables indexed to that intensity. Leaders are sized as vertical conductors; the horizontal storm drain is sized separately by slope.
Traps
- Using the sanitary DFU tables for storm drainage.
- Forgetting that the design rainfall intensity varies by location and that the sizing table is indexed to it.
- Sizing the roof drain but neglecting the horizontal storm drain and its slope.
- Ignoring secondary drainage - a blocked primary drain becomes a structural load.
- Connecting a downspout or sump directly to the sanitary system where prohibited.
Worked example
A 2,000 ft² roof in a 5 in./hr rainfall area: 0.0104 × 2,000 × 5 ≈ 104 gpm of design flow. That figure selects the leader and roof drain from the table; the horizontal run is then sized at the slope available. Notice how quickly the number grows with area and intensity.
Rule of thumb: 1 in. of rain on 1 ft² is about 0.62 gallon. Spread over an hour that is roughly 0.0104 gpm, which is where the conversion factor comes from.
Special and Indirect Waste
What indirect waste is
Indirect waste discharges into a receptor through an air gap or air break, never a direct connection. The purpose is to keep sewer backup and contamination away from equipment that touches food, potable water or sterile supplies. Typical sources: ice machines, steam tables, coffee urns, food-prep sinks, dishwashing machines where required, water softeners and filters, condensate drains, sterilizers, and pool or spa drains. Receptors must be trapped, vented, sized to the discharge, and accessible.
Special wastes
Acid, chemical, photographic and high-temperature wastes need corrosion-resistant piping and neutralization or dilution before entering the drainage system. Health-care and laboratory wastes may need special treatment and venting. Grease, oil and sand go to interceptors. Each has its own material, venting and access requirements - the material must match the waste, not the reverse.
Traps
- Running an indirect waste directly into a floor drain tailpiece or below the flood level rim of the receptor, which defeats the air gap.
- Forgetting that the receptor itself needs a trap and vent.
- Undersizing the receptor relative to the discharge flow.
- Sending condensate to a receptor that can back up, or burying the air gap.
- Using ordinary PVC for acid waste.
Rule of thumb
If the equipment touches food, potable water or patients, the waste is indirect and the gap must be visible and above the flood level rim of the receptor. Specify an air gap, not merely an air break, where contamination is a real risk.
Example: a walk-in cooler condensate line terminates with a visible air gap above a trapped and vented floor drain; it never connects to the drain pipe.