6.3 Storm Drainage Sizing, Roof Drains & Secondary (Emergency) Overflow Systems

6.3 Storm Drainage Sizing, Roof Drains & Secondary (Emergency) Overflow Systems

Quick Answer: Under North Carolina Plumbing Code (NCPC) Chapter 11, storm drainage systems are engineered based on horizontally projected roof area and the local 100-year, 1-hour rainfall rate, which ranges from 3.0 to 4.0 inches per hour across North Carolina. Roofs enclosed by parapet walls mandate a completely independent secondary (emergency overflow) drainage system or overflow scuppers. Overflow scuppers must have a minimum width of 4 inches (102 mm) and an invert set not less than 2 inches (51 mm) above the primary roof drain low point. Secondary piping must discharge above grade in a conspicuous location where water flow will be immediately noticed by building occupants.

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Storm Drainage Principles & Segregation Mandates

Storm drainage systems convey rainwater, surface runoff, and subsoil groundwater away from buildings to approved municipal storm sewers, drywells, or surface retention basins.

• Prohibition on Combined Systems (NCPC 1101.3): In North Carolina, storm drainage piping CANNOT discharge into sanitary drainage systems, and sanitary sewage can never enter storm drains. Combined sewers are prohibited in all new construction.
• Materials: Underground building storm sewers must comply with DWV standards (Schedule 40 PVC, ABS, Cast Iron, or SDR-26). Above-ground indoor leaders must be Schedule 40 PVC, ABS, copper (Type M, L, or DWV), or hubless cast iron.

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North Carolina Rainfall Rates & Sizing Mathematics

Storm drainage sizing is governed by the peak rainfall intensity that has a 1% probability of occurring in any given year (the 100-year, 1-hour storm event):

┌────────────────────────────────────────────────────────────────────────┐
│           NORTH CAROLINA 100-YEAR, 1-HOUR RAINFALL RATES               │
├──────────────────────────┬──────────────────────┬──────────────────────┤
│ Western Mountains        │ Central Piedmont     │ Eastern Coastal      │
│ (Asheville, Boone)       │ (Raleigh, Charlotte) │ (Wilmington, OBX)    │
│ 3.0 to 3.3 inches / hour │ 3.3 to 3.6 inches/hr │ 3.8 to 4.0+ inches/hr│
└──────────────────────────┴──────────────────────┴──────────────────────┘

Equivalent Square Footage Sizing Formula

Standard code tables (IPC/NCPC Tables 1106.2 and 1106.3) list allowable projected roof areas based on a baseline rainfall rate of 1.0 inch per hour or 4.0 inches per hour. To size piping for a specific local rainfall rate, use the conversion formula:

$\mathbf{\text{Adjusted Sizing Area (sq ft)}} = \text{Actual Projected Roof Area (sq ft)} \times \left(\frac{\mathbf{\text{Local Rainfall Rate (in/hr)}}}{\mathbf{\text{Table Base Rate (in/hr)}}}\right)$

$\text{Table Projected Area Capacity at Local Rate} = \frac{\text{Table Capacity at 1.0 in/hr}}{\text{Local Rainfall Rate (in/hr)}}$

$\text{Hydraulic Flow Rate (GPM)} = \text{Roof Area (sq ft)} \times \text{Rainfall Rate (in/hr)} \times \mathbf{0.0104\text{ GPM/sq ft/in/hr}}$

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Vertical Leaders, Conductors & Horizontal Storm Drain Sizing

1. Vertical Leaders & Conductors Sizing (NCPC Table 1106.2)

Vertical leaders (inside conductors and outside downspouts) operate under partial vacuum/gravity core flow. Their hydraulic capacity is significantly greater than horizontal piping of equal diameter.

2. Horizontal Building Storm Drains & Sewers (NCPC Table 1106.3)

Horizontal storm piping capacities vary based on pipe diameter and installed slope per foot (1/8", 1/4", or 1/2" pitch):

Worked Step-by-Step Problem: Sizing a Commercial Storm Drainage System

Scenario: A commercial retail building in Raleigh, NC (100-year, 1-hour rainfall rate = 3.5 inches per hour) has a flat roof with a projected area of 18,000 square feet. The design specifies four identical primary roof drains and vertical leaders discharging into a single horizontal main building storm drain pitched at 1/4 inch per foot.

1. Determine the required size of each vertical leader.
2. Determine the required size of the horizontal main building storm drain.

1. Calculate Area Served per Vertical Leader (4 Leaders): $\text{Area per Leader} = \frac{18,000\text{ sq ft}}{4} = \mathbf{4,500\text{ sq ft}}$
2. Size Vertical Leaders (@ 3.5 in/hr):
   • Convert 4,500 sq ft to 1.0 in/hr base rating: $4,500 \times 3.5 = 15,750\text{ sq ft capacity required at 1.0"/hr}$.
   • Check Table 1106.2:
     • A 3-inch leader handles $8,800\text{ sq ft @ 1.0"/hr}$ ($8,800 / 3.5 = 2,514\text{ sq ft}$) $\implies$ Too small.
     • A 4-inch leader handles $18,400\text{ sq ft @ 1.0"/hr}$ ($18,400 / 3.5 = 5,257\text{ sq ft}$) $\implies$ Adequate ($5,257 \ge 4,500$).
   • Result: Install four 4-inch vertical leaders.
3. Size Horizontal Main Building Storm Drain (Total Area = 18,000 sq ft @ 3.5 in/hr, 1/4"/ft slope):
   • Convert to 4.0 in/hr table base: $\text{Adjusted Area @ 4.0"/hr} = 18,000\text{ sq ft} \times \left(\frac{3.5}{4.0}\right) = \mathbf{15,750\text{ sq ft}}$
   • Check Table 1106.3 at 1/4" per foot slope:
     • A 6-inch pipe handles $7,550\text{ sq ft}$ $\implies$ Too small.
     • An 8-inch pipe handles $16,300\text{ sq ft}$ $\implies$ Adequate ($16,300 \ge 15,750$).
   • Result: Install an 8-inch horizontal building storm drain.

Secondary (Emergency Overflow) Roof Drainage Systems (NCPC 1108)

Flat roofs bounded by parapet walls, perimeter gravel stops, or exterior facades form catchment basins. If primary roof drains become obstructed with leaves, trash, or ice during a heavy storm, trapped rainwater rapidly accumulates, imposing immense hydrostatic loads that can cause catastrophic structural roof collapse.

The Structural Weight of Ponding Water

• Density of Water: $62.4\text{ lbs per cubic foot}$.
• Weight per Inch of Water Depth: $\text{Weight per Sq Ft per Inch Depth} = \frac{62.4\text{ lbs/cu ft}}{12\text{ in/ft}} = \mathbf{5.2\text{ lbs / sq ft per inch of water}}$
• Structural Impact:
  • $2\text{ inches of water} = 10.4\text{ lbs/sq ft}$ (exceeds many roof live load margins).
  • $6\text{ inches of water} = 31.2\text{ lbs/sq ft}$ ($31,200\text{ lbs}$ over a $1,000\text{ sq ft}$ roof bay).

Code Mandates for Secondary Drainage Systems (NCPC 1108)

Every roof with enclosed edges where water can pond must be equipped with a secondary (emergency) roof drainage system engineered to handle the full design rainfall intensity independently:

graph LR
    A["Roof Catchment with Parapet"] --> B{"Select Secondary System"}
    B --> C["Method 1: Separate Secondary Piping<br/>• Independent roof drains & leaders<br/>• Discharges above grade in conspicuous location<br/>• CANNOT connect to primary piping"]
    B --> D["Method 2: Parapet Overflow Scuppers<br/>• Openings in parapet wall<br/>• Invert ≥ 2 inches above primary drain<br/>• Min 4 inches width<br/>• Area ≥ 3× primary drain area"]

Detailed Secondary Method Specifications

1. Method 1: Independent Secondary Piping System (NCPC 1108.2)
   • Secondary (overflow) roof drains must be installed adjacent to primary drains.
   • The overflow drain weir elevation must be set at least 2 inches (51 mm) above the low-point elevation of the primary roof drain.
   • Piping must be completely separate from the primary system. It is STRICTLY ILLEGAL to tie secondary overflow drains into primary storm leaders inside the building.
   • Conspicuous Discharge Mandate: The secondary piping must discharge above grade in a conspicuous location (e.g., splashing over a main entrance canopy, loading dock, or exterior walkway) where building maintenance personnel and occupants will immediately observe active flow, signaling that primary drains are clogged.
2. Method 2: Parapet Wall Overflow Scuppers (NCPC 1108.3)
   • Open perimeter spillways cut through exterior parapet walls.
   • Minimum Width: Not less than 4 inches (102 mm).
   • Cross-Sectional Area: Must have an open area at least three times ($3\times$) the cross-sectional area of the primary roof drain pipe serving that area.
   • Invert Elevation: The bottom invert of the scupper opening must be located not less than 2 inches (51 mm) above the adjacent low-point primary roof drain elevation, and not higher than the maximum depth allowed by structural engineering calculations.

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Controlled Flow Roof Drainage Systems (NCPC 1110)

Controlled flow roof drainage systems intentionally attenuate storm runoff by metering discharge through calibrated weir rings or throttled orifices in roof drains, temporarily storing water on the roof structure during peak rainfall.

• Prerequisites for Controlled Flow Design:
  1. Structural Engineering Approval: The building roof structure must be engineered to support the full anticipated dead load of stored water ($30\text{ to }40+\text{ lbs/sq ft}$).
  2. Maximum Ponding Depth: Water depth cannot exceed the structural design depth, capped at a maximum of 6 inches (152 mm).
  3. Drain-down Time: The system must be engineered to evacuate all stored water within a maximum of 24 hours after the storm ceases.
  4. Mandatory Secondary Scuppers: Controlled flow roofs must still be provided with emergency parapet scuppers set above the maximum design controlled ponding level.

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Subsoil Foundation Drainage & Sump Pump Sizing (NCPC 1111 / 1113)

Subsoil drainage systems protect foundations and basements from hydrostatic groundwater pressure.

graph TD
    A["Perforated Subsoil Drain Pipe<br/>(Min. 4-inch PVC/Corrugated)"] --> B["Washed Gravel Bedding & Geotextile Wrap"]
    B --> C["Gravity Flow to Sump Pit"]
    C --> D["Vapor-Tight Sump Basin<br/>(Min. 18 in. Dia × 24 in. Deep)"]
    D --> E["Submersible Sump Pump with Check Valve"]
    E --> F["Discharges to Approved Storm Sewer or Surface Splashblock"]

• Subsoil Piping: Perforated pipe not less than 4 inches (102 mm) in diameter, embedded in at least 4 inches of washed gravel/stone and wrapped in geotextile filter fabric, positioned beside the footing below the basement slab elevation.
• Sump Basin: Water-tight basin not less than 18 inches (457 mm) in diameter and 24 inches (610 mm) deep, equipped with a removable gastight cover.
• Pump & Discharge Piping: Sump pump sized to evacuate peak groundwater inflow ($20\text{ to }50+\text{ GPM}$); discharge line equipped with an accessible check valve and a full-port gate or ball isolation valve.