The Situation
You are designing the roof drainage system for a new 20,000-square-foot flat commercial building (a single-story retail space). The roof has a slight slope (1/8 inch per foot) toward four internal roof drains. Local building code requires the system to handle a 100-year, 1-hour design storm. The building is in a location where that storm produces 3.5 inches of rain in one hour.
You need to determine how many drains are required and what size they should be.
Which Calculator to Use
The Roof Drain Calculator determines the required drain size and count based on the roof area, rainfall rate, and drain specifications. Since this calculator does not support URL pre-fill, we will walk through the calculation methodology and point you to the calculator to run your own numbers.
Walking Through the Inputs
Roof Area (20,000 sq ft)
The total horizontal projected area of the roof. For a flat roof, this is the same as the building footprint: 20,000 square feet.
Rainfall Rate (3.5 in/hr)
The 100-year, 1-hour rainfall intensity for this location is 3.5 inches per hour. This is determined from local IDF curves (NOAA Atlas 14).
Why 100-year? Most building codes require roof drainage to handle the 100-year storm, unlike site drainage which typically uses a 10-year or 25-year storm. Roof drainage failure can cause ponding loads that exceed the structural capacity of the roof.
Number of Drain Points (4)
The architect has located four drain points in the roof plan, roughly dividing the roof into four equal quadrants of 5,000 square feet each.
Drain Type (Internal roof drain)
Internal roof drains (also called siphonic or gravity drains) are located in the interior of the roof with dome strainers. They connect to the storm piping below the roof deck. This is the standard for flat commercial roofs.
The Calculation
Step 1: Determine Total Flow
Total flow from the roof:
Q = (Roof Area x Rainfall Rate) / (96.23)
Q = (20,000 x 3.5) / 96.23 = 727 gpm (approximately 1.62 cfs)
The constant 96.23 converts square feet and inches per hour to gallons per minute.
Step 2: Flow Per Drain
With 4 drains handling equal areas:
Flow per drain = 727 / 4 = 182 gpm per drain
Step 3: Select Drain Size
Standard roof drain capacities (at 1-inch head):
| Drain Size | Flow Capacity (gpm) | Max Roof Area at 3.5 in/hr |
|---|---|---|
| 2” | 30 | 825 sq ft |
| 3” | 65 | 1,785 sq ft |
| 4” | 130 | 3,575 sq ft |
| 5” | 245 | 6,735 sq ft |
| 6” | 400 | 11,000 sq ft |
Each drain needs to handle 182 gpm. A 4-inch drain handles 130 gpm — not enough. A 5-inch drain handles 245 gpm — adequate with a safety factor of 1.35.
Select 5-inch roof drains or use 6-inch drains for additional safety margin.
Step 4: Verify Leader Pipe Sizing
The leader pipe (vertical pipe from the drain to the horizontal storm piping) must also be sized for the flow. Using Manning’s equation for vertical pipe:
- 4-inch leader: approximately 150 gpm capacity
- 5-inch leader: approximately 290 gpm capacity
- 6-inch leader: approximately 500 gpm capacity
A 5-inch leader matches the 5-inch drain.
The Results Summary
| Parameter | Value |
|---|---|
| Total roof area | 20,000 sq ft |
| Design rainfall | 3.5 in/hr (100-yr) |
| Total design flow | 727 gpm (1.62 cfs) |
| Number of drains | 4 |
| Flow per drain | 182 gpm |
| Selected drain size | 5-inch |
| Leader pipe size | 5-inch |
What This Means for Your Project
Overflow Protection (Critical)
Building codes require a secondary (emergency) overflow system in case the primary drains are blocked. Common options:
- Scupper overflows: Openings in the parapet wall at 2 inches above the primary drain level. Size for 100% of the design flow (assume all primary drains are clogged).
- Secondary roof drains: Separate drains set 2 inches above the primary drain level, piped independently to daylight.
Never rely on a single drainage path for a flat roof. Ponding water on a flat roof can exceed the structural design load and cause a collapse.
Horizontal Storm Piping
Below the roof deck, the leader pipes connect to horizontal storm piping that carries water to the building exterior. Size these pipes using Manning’s equation:
- Minimum slope: 1/8 inch per foot (1%)
- Material: Cast iron (inside building), PVC (underground)
- Size: Typically 6-inch minimum for the main horizontal run
Use the Manning’s Pipe Calculator to verify the horizontal pipe sizing.
Cost Estimate
- 5-inch roof drains (4 units): $400-800
- Leader pipes (4 vertical runs, assume 15 ft each): $300-600
- Horizontal storm piping (approximately 100 ft): $500-1,000
- Scupper overflows (4 units): $200-400
- Installation labor: $2,000-4,000
- Total: $3,400-6,800
Try It Yourself
Open the Roof Drain Calculator and enter your specific roof area, rainfall rate, and drain configuration to get a customized result.
Next Steps
- Verify horizontal pipe sizing with the Manning’s Pipe Calculator
- Read the Flat Roof Drainage guide for complete system design guidance
- Check your local building code for the specific design storm and overflow requirements
- Coordinate drain locations with the structural engineer to ensure adequate slope to drains