Advanced Roof Runoff Inputs
Formula used
Plan area: A = length x width, or direct area input.
Slope factor: SF = sqrt(1 + (rise / run)2).
Sloped roof surface: As = A x SF.
Net rainfall: Pn = max(P - first flush loss, 0).
Runoff volume: V = A x Pn x C x efficiency.
Peak flow: Q = C x efficiency x i x A. The calculator applies the safety factor to peak flow.
Storage size: S = V x safety factor.
How to use this calculator
- Choose whether to enter roof dimensions or direct roof area.
- Add roof pitch to estimate the sloped roof surface.
- Enter storm rainfall depth for event runoff volume.
- Enter rainfall intensity for peak flow and gutter sizing.
- Set runoff coefficient, first flush loss, and collection efficiency.
- Enter gutter sections and each section capacity.
- Press calculate, then review volume, flow, storage, and chart results.
- Use CSV or PDF buttons to save the calculation.
Example data table
| Example roof | Area | Rain depth | Runoff coefficient | Approximate volume | Peak flow example |
|---|---|---|---|---|---|
| Small porch roof | 30 m2 | 25 mm | 0.90 | 675 L | 0.38 L/s at 50 mm/hr |
| Average house roof | 120 m2 | 50 mm | 0.95 | 5,700 L | 2.53 L/s at 80 mm/hr |
| Large workshop roof | 300 m2 | 60 mm | 0.90 | 16,200 L | 9.00 L/s at 120 mm/hr |
| Warehouse roof | 900 m2 | 65 mm | 0.90 | 52,650 L | 24.75 L/s at 110 mm/hr |
Roof Stormwater Runoff Planning Guide
Why roof runoff matters
Roof runoff is simple to picture, yet it is easy to underestimate. A short, intense storm can move thousands of liters from a clean roof. That water must leave the building safely, or it can overload gutters, soak foundations, and erode nearby soil.
Inputs that shape the answer
This calculator uses roof area, rainfall, runoff coefficient, losses, and duration. It estimates event volume, average runoff, peak flow, storage size, and gutter demand. It also separates horizontal catchment area from sloped roof surface. The horizontal plan area normally controls rainfall capture. The sloped area helps when checking panels, coating, and roof material coverage.
A high runoff coefficient means more rain becomes flow. Metal roofs and smooth tiles usually shed water quickly. Green roofs, rough surfaces, and roof gardens retain more water. First flush loss represents wetting, small depressions, debris, and diversion water. Collection efficiency accounts for splash, leaks, overflow, or imperfect downspout routing.
Peak flow and storage
Peak flow is vital for gutters, downpipes, and storm drains. It is based on rainfall intensity, not only total depth. Two storms can have the same depth, yet very different peak demands. A long gentle storm may fill a tank slowly. A cloudburst may flood a gutter within minutes.
Storage planning needs a safety margin. Tanks, soakaways, rain gardens, and detention basins rarely work at perfect capacity. Sediment, outlet restrictions, blockages, and future roof additions can reduce performance. The safety factor raises the recommended size and peak design flow.
Design cautions
Use local rainfall data whenever possible. Municipal drainage manuals often provide design intensities for different return periods. Choose a higher return period for critical buildings, steep sites, or properties with past flooding. Also check legal discharge rules, overflow routes, and minimum setbacks from foundations.
For advanced checks, review each roof plane separately. Valleys, parapets, scuppers, and internal drains can concentrate flow. Split the roof into drainage zones when downspouts serve different areas. Then size each zone independently. This approach prevents one undersized outlet from controlling the entire drainage system during severe rain and hail bursts.
Results are planning estimates. They do not replace a licensed drainage design. Still, they help compare roof shapes, gutter sections, tank sizes, and rain scenarios before detailed work begins.
FAQs
What is roof stormwater runoff?
It is rainwater that lands on a roof and flows toward gutters, downpipes, drains, storage tanks, or the ground around a building.
Should I use sloped area or plan area?
Use horizontal plan area for rainfall capture. Sloped area is useful for roof covering, coating, panels, and surface checks.
What runoff coefficient should I enter?
Smooth metal, tile, and sealed roofs often use high values. Rough, vegetated, or absorbent roofs need lower values because they retain more water.
Why does rainfall intensity matter?
Intensity controls peak flow. Gutters can fail during a short cloudburst, even when total storm depth looks moderate.
What is first flush loss?
It is the initial rainfall depth deducted for wetting, dust, debris, and diverted dirty water before useful runoff is collected.
How is tank storage estimated?
The calculator multiplies event runoff volume by the safety factor. This gives a practical storage allowance for uncertainty.
Can this size my final drainage system?
It supports early planning and comparison. Final drainage design should follow local codes, rainfall data, soil limits, and professional review.
Why add a safety factor?
A safety factor allows for blocked gutters, larger storms, rough construction, future roof changes, and imperfect collection efficiency.