What This Solves
Sizes primary roof drains, overflow (secondary) drains, and scuppers for flat and low-slope roofs using IPC table values.
Best Used When
- You are designing roof drainage for a flat or low-slope commercial building
- You need to determine the number and size of primary and overflow roof drains
- You want to size scuppers as overflow protection per building code requirements
Do NOT Use When
- You are sizing edge-mounted gutters for a sloped residential roof — Use Gutter Flow Calculator
- You need to size the downspouts or leaders that carry water from the drains to grade — Use Downspout Sizing Calculator
Key Assumptions
- Drain capacity is based on IPC Table 1106.1 for primary drains
- Overflow drains are sized per IPC Table 1106.6 with hydraulic head at the overflow level
- Rainfall intensity is uniform across the entire roof area
- Drains are unobstructed and strainers are clean
- Ponding loads from primary drain backup do not exceed structural capacity
Input Quality Notes
Use local code-required rainfall intensity. For overflow sizing, the design head is the depth of water above the overflow inlet — confirm this matches the roof structural ponding capacity.
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Roof Drain Sizing Calculator
Calculate required roof drain size for flat and low-slope roofs. Uses International Plumbing Code (IPC) tables to size primary drains, overflow drains, and scuppers based on drainage area and design rainfall intensity.
Calculate Roof Drain Size
For educational purposes only. Not a substitute for professional engineering judgment.
Roof Drain Sizing Overview
Roof drains collect and convey rainwater from flat or low-slope roofs to the storm drainage system. Proper sizing prevents ponding that can cause structural damage or leaks.
- Primary Drains - Main drainage from roof surface
- Overflow Drains - Secondary drains at higher elevation for redundancy
- Scuppers - Wall openings that act as overflow relief
- Siphonic Drains - High-capacity drains using siphon action
Roof Drain Capacity (IPC Table 1106.1)
| Drain Size (in) | @ 1" Head | @ 2" Head | @ 4" Head | Max Area (sf) |
|---|---|---|---|---|
| 2" | 26 gpm | 36 gpm | 52 gpm | 5,000 |
| 3" | 63 gpm | 90 gpm | 127 gpm | 12,200 |
| 4" | 121 gpm | 172 gpm | 242 gpm | 23,300 |
| 5" | 196 gpm | 278 gpm | 392 gpm | 37,700 |
| 6" | 295 gpm | 418 gpm | 590 gpm | 56,700 |
| 8" | 563 gpm | 796 gpm | 1126 gpm | 108,300 |
Flow rates in GPM at various head depths. Max area based on 1 in/hr at 4" head. Source: IPC 2021, Table 1106.1
Dome/Strainer Reduction Factors
| Dome Type | Flow Factor | Reduction |
|---|---|---|
| Standard Dome | 0.90 | 10% |
| Flat Grate | 0.85 | 15% |
| Beehive | 0.92 | 8% |
| Vandal-Proof | 0.80 | 20% |
| None | 1.00 | 0% |
Dome/strainer reduces effective flow capacity. Select "None" for theoretical maximum capacity.
About Roof Drain Design
Roof drains are essential components of flat and low-slope roof drainage systems. Proper sizing prevents excessive ponding that can cause structural damage, leaks, and premature roof failure.
Types of Roof Drains
- Primary Drains - Main drains located at low points to collect and convey rainwater
- Overflow Drains - Secondary drains set 2" above primary drains for redundancy
- Scuppers - Wall or parapet openings that provide overflow relief
- Siphonic Drains - High-capacity systems using full-pipe siphon action
IPC Requirements
The International Plumbing Code requires:
- Primary drains sized for 100% of design flow
- Overflow drains on low-slope roofs to prevent structural overload
- Overflow capacity equal to 100% of primary drain capacity
- Overflow drains set 2" above the primary drain inlet
Design Considerations
- Head Depth - Water depth over drain inlet; more head = more flow
- Dome/Strainer - Protective covers reduce effective capacity by 10-20%
- Drain Spacing - Consider maximum tributary area per drain
- Ponding Limit - Typically 4" maximum per structural requirements
- Slope - Minimum 1/8" per foot slope toward drains
Flow Calculation
The design flow rate is calculated from drainage area and rainfall intensity:
Q = (A x i) / 96.23
Where:
- Q = Flow rate (gpm)
- A = Drainage area (sf)
- i = Rainfall intensity (in/hr)
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Last verified: February 2026