Calculator
Formula Used
Angle to inch drop:
Drop in inches = tan(angle in degrees × π / 180) × horizontal run in inches
Pitch:
Pitch in inches per foot = tan(angle in radians) × 12
Slope percent:
Slope percent = tan(angle in radians) × 100
Estimated overflow capacity:
Capacity in cfs = 3.33 × coefficient × width in feet × head in feet1.5
Estimated runoff demand:
Demand in gpm = roof area × rainfall rate × 0.0103896
How to Use This Calculator
- Enter the scupper slope angle. Use 0.1 for a .1 degree check.
- Enter the horizontal run to the scupper.
- Select the run unit.
- Enter scupper width, height, freeboard, and water head.
- Add roof area and rainfall rate for a rough demand check.
- Press Calculate.
- Read the inch drop result above the form.
- Download the CSV or PDF report when needed.
Example Data Table
| Angle | Run | Drop | Pitch | Use Case |
|---|---|---|---|---|
| 0.1° | 12 in | 0.0209 in | 0.0209 in/ft | Small roof edge check |
| 0.1° | 120 in | 0.2094 in | 0.0209 in/ft | Short parapet run |
| 0.1° | 240 in | 0.4189 in | 0.0209 in/ft | Longer roof approach |
| 0.5° | 120 in | 1.0472 in | 0.1047 in/ft | More noticeable fall |
Overflow Scupper Calculation Guide
Why Angle Matters
An overflow scupper protects a roof edge when primary drains slow down, clog, or receive more water than expected. The small angle near the scupper matters because it creates a vertical drop. That drop helps water move toward the opening. This calculator converts a roof or curb angle, such as 0.1 degrees, into inches of fall over a chosen run.
Understanding the Inch Result
The main value is the inch drop. It shows how much lower one point is compared with another point. A very small angle can still create a useful fall across a long distance. For example, 0.1 degrees creates about 0.0209 inches of drop per foot. Over twenty feet, the same angle creates about 0.419 inches of drop.
Useful Engineering Outputs
The tool also reports pitch in inches per foot and slope percent. These values help compare drawing notes, field measurements, and drainage details. The radians result is included for engineering checks. The ratio result shows how many inches of horizontal run create one inch of rise.
Overflow Review Inputs
Extra fields support overflow scupper review. You can enter opening width, opening height, estimated water head, roof area, rainfall rate, and discharge coefficient. The page estimates weir style flow using width and head. It also estimates runoff demand from rainfall and roof area. These values are planning aids, not a replacement for local code design.
Field Checking Tips
Use conservative inputs when the roof is critical. Measure the real run to the scupper. Confirm parapet height, freeboard, flashing, and ponding limits. Check whether debris screens, strainers, or wall thickness reduce the usable opening. A scupper that looks large may perform poorly if the approach path is flat or blocked.
When to Use the Results
This calculator is useful during early design, site checks, and quick reviews. It can help you explain why a tiny angle still matters. It can also show when a scupper opening may need review. Always compare results with approved drainage standards, local rainfall data, and a licensed professional when life safety or property risk is involved.
Exporting Your Check
Keep records from each check. The CSV file stores input and output values. The PDF button creates a simple report for review notes. Use both downloads to compare several runs, then keep the final value with project sketches and inspection photos for later field reference.
FAQs
What does 0.1 degrees mean in inches?
It means the vertical drop equals tan(0.1 degrees) multiplied by the horizontal run in inches. Over 12 inches, it is about 0.0209 inches.
Can this calculator be used for roof scuppers?
Yes, it helps with quick roof scupper slope and overflow checks. Final design should still follow local code, project drawings, and professional review.
What is pitch in inches per foot?
Pitch shows the vertical change over one foot of horizontal distance. It is useful when drawings describe drainage slope using inches per foot.
Why is the result so small for 0.1 degrees?
0.1 degrees is a very shallow angle. The drop becomes more noticeable only when the horizontal run is long.
What is the discharge coefficient?
It is an adjustment factor for flow behavior. Edges, restrictions, and approach conditions affect it. Use a conservative value for planning checks.
Does the capacity result replace drainage design?
No. The capacity result is only an estimate. Use approved rainfall data, building codes, and qualified engineering judgment for final sizing.
What is freeboard in this calculator?
Freeboard is reserved height above the water flow zone. It reduces the effective scupper height used for the overflow head check.
Why download CSV or PDF results?
Downloads help store calculation records. They are useful for comparing runs, checking options, and sharing notes with team members.