Downloads
Inputs
Example Data Table
| Scenario | Area (m²) | Depth (mm) | Retention (%) | Intensity (mm/h) | Green C | Runoff Volume (m³) | Peak Flow (m³/s) |
|---|---|---|---|---|---|---|---|
| Extensive roof, moderate storm | 250 | 25 | 55 | 60 | 0.50 | 2.813 | 0.002083 |
| Deeper system, higher retention | 400 | 35 | 70 | 80 | 0.40 | 4.200 | 0.003556 |
| Shallow system, low retention | 180 | 20 | 35 | 50 | 0.60 | 2.340 | 0.001500 |
Formula Used
Rainfall volume (m³) = Area (m²) × Rainfall depth (m)
Retained depth (m) = Rainfall depth (m) × Retention fraction
Runoff depth (m) = max[0, Rainfall depth − Retained depth]
Runoff volume (m³) = Area × Runoff depth
Peak flow (m³/s) = C × Drainage factor × i × A
Where i is rainfall intensity in m/s and A is area in m².
How to Use This Calculator
- Enter the roof area contributing to the drain or scupper.
- Choose a storm event depth and a matching peak intensity value.
- Set retention based on system depth, media, vegetation, and season.
- Adjust coefficients to reflect expected runoff behavior in your region.
- Click Calculate to view volumes, peak flows, and reduction percentages.
- Download CSV or PDF for design notes and project documentation.
Professional Notes
Why Green Roof Runoff Matters
Green roofs help manage stormwater by retaining rainfall in vegetation and growing media, then releasing the remainder more slowly. This reduces nuisance ponding at drains, lowers short-term discharge to leaders, and can support compliance targets for onsite control and water-quality objectives.
Key Inputs and Data Sources
Enter plan area in square meters and select an event rainfall depth in millimeters from local design criteria or approved intensity–duration–frequency datasets. Use a matching peak intensity (mm/h) near the time of concentration. Choose coefficients representing expected surface behavior: conventional roofs often approach 0.9–1.0, while vegetated roofs commonly fall around 0.3–0.7.
Interpreting Volume Results
The calculator converts rainfall depth to meters and computes total rainfall volume, retained volume, and runoff volume. The effective runoff depth is the portion of rainfall leaving the roof after retention is applied. Comparing green-roof runoff to a conventional baseline provides a simple percent reduction metric for drainage notes and stormwater narratives.
Using Peak Flow for Drain Sizing
Peak discharge is estimated using the rational method, Q = C × i × A, with intensity converted to meters per second. The drainage factor can represent controlled outlets or throttling that reduces peak release. A safety factor increases the recommended design peak to provide conservative sizing for scuppers, drains, and conveyance.
Quality Checks and Reporting
Validate inputs with a quick sense check: larger area or storm depth must increase volume, and higher retention must reduce runoff depth. Run sensitivity checks across plausible retention and coefficient ranges, then export CSV for calculation logs and PDF for submittals. Record storm return period, duration assumptions, and any roof zoning for review. If the roof includes overflow scuppers, confirm their invert elevations and routes so extreme events bypass sensitive areas without causing facade staining or interior leaks later.
FAQs
1) What retention percentage should I use?
Start with project guidance or monitoring data. Extensive systems often retain most small events, while larger storms reduce percentage retention. Adjust for season, media depth, irrigation, and antecedent moisture.
2) Why are there both retention and a runoff coefficient?
Retention estimates event volume stored on the roof. The runoff coefficient is used for peak-flow estimation in the rational method. Using both helps separate “how much” runoff occurs from “how fast” it may discharge.
3) What is the drainage factor?
It is a 0–1 multiplier that represents detention controls, restricted outlets, or throttling devices that reduce peak discharge. Use 1.0 if there is no control and reduce it when an outlet limit is specified.
4) Can I model multiple roof zones?
Yes. Run the calculator separately for each zone with its own area, retention, and coefficients, then sum volumes. For peak flow, consider the controlling zone or combine flows based on your drainage layout.
5) How do I choose rainfall intensity?
Select an intensity matching the design storm duration near the time of concentration for the drainage path. Use local IDF curves or project criteria. Keep intensity consistent with the return period used for depth.
6) Are results suitable for final construction drawings?
They are a planning estimate. Final design should follow local standards, confirm drainage geometry, verify drain capacities, and include overflow provisions. Coordinate assumptions with the project civil engineer and roofing team.
7) What if my runoff volume is higher than expected?
Check units, retention, and coefficients first. Lower retention may reflect saturated media or shallow build-ups. Consider increasing substrate depth, adding detention layers, or using outlet controls to reduce peak discharge.