Green Roof Retention Calculator

Calculator Inputs

Formula Used

This calculator treats retention as limited by available storage depth and the storm depth. All depths are converted to millimeters internally for consistency.

• Media storage (mm): S_media = (n − θ_i) × D
• Total storage (mm): S_total = S_media + S_d + S_i
• Slope reduction: loss = min(0.30, 0.005 × slope)
• Effective storage (mm): S_eff = S_total × PF × (1 − loss)
• Retained depth (mm): R = min(P, S_eff)
• Runoff depth (mm): Q = max(0, P − R)
• Retained volume (m3): V_r = (R/1000) × A
• Runoff volume (m3): V_q = (Q/1000) × A
• Retention percent: 100 × R / P (when P > 0)

How to Use This Calculator

1. Select a unit system and enter your roof area.
2. Enter a storm depth for your design rainfall event.
3. Provide media depth, porosity, and initial moisture estimates.
4. Add drainage and interception storage if applicable.
5. Use a performance factor below 1 for conservative design.
6. Press Submit to view retained depth, runoff, and volumes.
7. Download CSV or PDF reports for submittals and checks.

Tip: If you lack measurements, start with porosity 0.55 and initial moisture 0.25.

Example Data Table

These examples illustrate how higher initial moisture reduces retention.

Professional Notes on Green Roof Retention

1) Retention purpose in site drainage

Green roofs reduce peak discharge by storing rainfall within the media and drainage layers, then releasing it gradually. For planning, retention is commonly expressed as a retained depth (mm) and an equivalent retained volume (m3) over the roof area.

2) Inputs that control storage

The main storage term is the available pore space: (porosity − initial moisture) × media depth. Higher porosity or deeper media increases retention, while wet antecedent conditions reduce it. Drainage products can add detention depth, and a conservative performance factor accounts for aging or partial clogging.

3) Example dataset and interpretation

Consider a roof area of 500 m2 with a storm depth of 35 mm. With media depth 120 mm, porosity 0.60, initial moisture 0.30, drainage storage 8 mm, interception 1.5 mm, slope 2°, and performance factor 0.90, the effective storage is about 40.5 mm. Because storage exceeds rainfall, retained depth equals 35 mm and runoff depth is 0 mm. Retained volume is approximately 17.5 m3.

4) Output metrics for reports

Use retained depth to compare storms of different magnitudes, and retained volume to coordinate with cistern sizing or downstream controls. The effective runoff coefficient provides a quick check for hydrologic models that require a single coefficient for a design event.

5) Practical checks during design

Verify that assumed media depth matches the approved assembly, and confirm whether drainage storage is manufacturer-rated or field-observed. For compliance submissions, run multiple storms and antecedent moisture cases, then export CSV or PDF outputs for traceable calculations.

FAQs

1) What does this calculator estimate?

It estimates how much rainfall a green roof can retain during a storm, expressed as retained depth, runoff depth, retained volume, and an effective runoff coefficient for the selected event.

2) How is “media storage” calculated?

Media storage is available pore space: (porosity minus initial moisture) multiplied by media depth. It represents the depth of water the media can hold before drainage begins.

3) Why include a performance factor?

Performance factor reduces storage to reflect real-world uncertainty such as compaction, aging, partial clogging, nonuniform thickness, or conservative design assumptions.

4) How does roof slope affect results?

Steeper slopes can drain faster and slightly reduce effective storage. This tool applies a modest slope-based reduction to approximate that effect for preliminary planning.

5) What should I enter for drainage storage?

Use the drainage product’s rated detention depth, if available. If not, enter 0 and rely on media storage only, then refine once product data is confirmed.

6) Can this replace a full hydrologic model?

No. It is a screening and reporting calculator for event retention. Detailed design may require continuous simulation, flow routing, and local compliance criteria.

7) Why do outputs include metric volumes even in imperial?

Many drainage reports and sustainability submittals standardize on cubic meters. The calculator keeps volumes in m3 for consistent documentation across unit systems.