Roof Drainage Calculator
Calculate roof runoff volume, gutter sizing, and downspout requirements based on roof area and rainfall intensity. Perfect for drainage planning and stormwater management.
Step 1: Roof Specifications
Percentage of rainfall that becomes runoff (0.95 = 95%)
Understanding Runoff Coefficient
The runoff coefficient represents the fraction of rainfall that becomes surface runoff. For asphalt shingles: 0.95, metal roofs: 0.90, green roofs: 0.30-0.50.
Step 2: Rainfall & Drainage
Typical values: Light rain 0.1-0.5″, Heavy storm 2-3″, Extreme 4″+
Drainage Formulas
Runoff Volume: Q = C × I × A
Where: Q = Runoff (in³/hr), C = Runoff Coefficient, I = Rainfall Intensity (in/hr), A = Roof Area (in²)
Downspout Capacity: Based on cross-sectional area and slope.
Roof Drainage Analysis Results
Total Roof Runoff
Downspout Capacity Comparison
| Downspout Size | Type | Capacity (GPH) | Max Roof Area | Recommended Use |
|---|
Roof Drainage Formulas
Rational Method Formula
Formula: Q = C × I × A
Where: Q = Peak runoff rate (in³/hr)
C = Runoff coefficient (0-1)
I = Rainfall intensity (in/hr)
A = Roof area (in²)
Example: C=0.95, I=2.5 in/hr, A=1,200 ft² → Q = 0.95 × 2.5 × 1,200 × 144 = 410,400 in³/hr ≈ 2,850 GPH
Downspout Capacity
Formula: Capacity = A × V × 7.48 × 3600
Where: A = Cross-sectional area (ft²)
V = Flow velocity (ft/sec, typically 1-3 ft/sec)
7.48 = Gallons per cubic foot
3600 = Seconds per hour
Example: 3″ round downspout (0.049 ft²) at 2 ft/sec → 0.049 × 2 × 7.48 × 3600 ≈ 2,640 GPH
Gutter Sizing
Rule of Thumb: 1 square inch of downspout per 100 ft² of roof area
K-style Gutters: 5″ handles ~5,500 GPH, 6″ handles ~8,000 GPH
Half-round Gutters: 5″ handles ~4,800 GPH, 6″ handles ~7,200 GPH
Always include 20-30% safety margin for heavy storms.
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