Formula Used
Roof flow is estimated from rainfall volume over the drainage area.
The formula is Q = I × A × C ÷ 43200.
Here Q is flow in cubic feet per second.
I is rainfall intensity in inches per hour.
A is roof area in square feet.
C is the runoff coefficient.
The design flow is Qd = Q × safety factor.
Flow per scupper is Qs = Qd ÷ number of scuppers.
For weir flow, Qs = Cw × L × H^1.5.
L is the required opening width in feet.
H is the head depth in feet.
For orifice flow, Qs = Cd × A × √(2gH).
A is opening area in square feet.
Cd is the discharge coefficient.
g is 32.174 ft/s².
The calculator compares methods when requested.
How to Use This Calculator
- Enter the roof length and roof width in feet.
- Add the design rainfall intensity for your location.
- Set the runoff coefficient for the roof surface.
- Enter the number of scuppers sharing the drainage load.
- Enter the allowed water head above the scupper sill.
- Enter the planned opening height and safety factor.
- Choose a sizing method or compare both methods.
- Press calculate and review the result above the form.
Example Data Table
| Roof Size |
Rainfall |
Runoff |
Scuppers |
Head |
Suggested Width |
| 120 ft × 80 ft |
4 in/hr |
0.95 |
4 |
3 in |
8 in each |
| 90 ft × 60 ft |
5 in/hr |
0.90 |
3 |
4 in |
7.5 in each |
| 150 ft × 100 ft |
3.5 in/hr |
0.95 |
6 |
3 in |
7 in each |
Understanding Roof Scupper Sizing
A roof scupper is an opening through a parapet or wall. It lets storm water leave a flat roof. Good sizing prevents ponding, overload, leaks, and interior damage. The opening must pass the design storm flow before water rises too high. This calculator uses roof area, rainfall intensity, runoff coefficient, head depth, and scupper count. It then estimates the flow each scupper must handle.
Why Flow Matters
Rainfall over a roof becomes drainage flow. The basic physics is volume per time. A large roof or intense storm creates more cubic feet per second. Smooth roofs often have high runoff values. Gravel, green roofs, and rough surfaces may reduce quick flow. Safety factors are added because storms vary. Debris, leaf screens, poor slope, and construction tolerances also reduce real capacity.
Scupper Geometry
Most parapet scuppers behave like rectangular weirs when water spills through the lower edge. Flow increases strongly as head depth rises. A small rise in water depth can move much more water. When an opening runs full, it may behave closer to an orifice. The orifice method uses opening area and pressure head. This page can compare both ideas and select a conservative size.
Practical Design Notes
Sizing is not only a math task. The roof should slope toward the drain point. The scupper bottom should be set at the correct elevation. Emergency overflow scuppers should sit above primary drains. They should discharge where water is visible and safe. Designers must also check local plumbing code, rainfall maps, structural limits, and manufacturer details.
Using the Result
The recommended width is rounded upward. This helps provide buildable values. The result shows total roof flow, flow per scupper, required width, and required area. Use the value as an early design guide. Final drawings should include flashing, strainers, leader heads, downspouts, wall thickness, and overflow routing. Never reduce a code minimum because a calculator gives a smaller number.
Common Checks
Always inspect the roof edge after major storms. Look for stains, sediment lines, and standing water. These marks show real water behavior. Keep screens and outlets clean. Recheck sizes when roof equipment, additions, or surface coverings change the effective drainage area and confirm overflow paths before final approval.
FAQs
What is a roof scupper?
A roof scupper is an opening through a parapet or wall. It allows storm water to leave a roof and discharge safely.
What units does this calculator use?
It uses feet for roof dimensions, inches per hour for rainfall, inches for opening dimensions, and cubic feet per second for flow.
What is rainfall intensity?
Rainfall intensity is the design storm rate. It tells how many inches of rain may fall in one hour for the selected storm event.
What does available head mean?
Available head is the water depth above the scupper sill. More head usually increases discharge capacity through the opening.
Why use a safety factor?
A safety factor allows for storm variation, debris, construction tolerance, surface roughness, and partial blockage at the scupper opening.
Should I use the weir or orifice method?
Use the weir method when water spills over the lower edge. Use the orifice method when the opening runs full under pressure.
Can this replace local code sizing?
No. It is an estimating tool. Always confirm final sizes with local code, rainfall tables, and a qualified design professional.
What can I export?
You can export the calculated result as a CSV file or a simple PDF report using the buttons shown after calculation.