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Component Design GRO Code · CIRIA C753 §12

Green Roof Calculator

GRO Green Roof Code of Practice and CIRIA C753 §12: extensive and intensive green roof retention and runoff sizing.

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Project & Roof
2
Rainfall & Climate
3
Build-Up
4
Performance
5
Results

Project & Roof Details

Fields marked * are required

Project Information

Unique identifier for this design

Description of roof location

Determines substrate depth, water retention, and maintenance requirements

m²

degrees (0–35°). Default 2°.

Substrate Depth Range: Select a roof type
Water Retention Capacity: —
Saturated Weight: —
Estimated Total Saturated Load: — kN

Rainfall & Climate Parameters

Outward postcode district to obtain Standard Annual Average Rainfall (SAAR)

📍 UK overview map for site context

If postcode lookup unavailable

mm/year

Frequency of rainfall event

minutes. Default 60 min.

Source: EA May 2022 Climate Change Allowances Table 3 (peak rainfall intensity, surface water). Choose by time horizon + band: 2050s central 20 % / higher 25 % / upper 35 %, 2080s central 25 % / higher 30 % / upper 40 %, for 3.3 % AEP (30-year) sizing. For 1 % AEP (100-year) use the higher column (e.g. 2080s upper = 50 %). Check EA guidance for your river-basin district.

Recommended: +40% for 2050 baseline

Annual Average Rainfall: — mm/yr
Design Rainfall Intensity: — mm/hr
Design Storm Depth: — mm
Annual Retention Volume: — m³/yr

Green Roof Build-Up Specification

Vegetation Layer

Informational only, determined by roof type

Substrate / Growing Medium

mm

fraction (0.35–0.55). Default 0.40.

Substrate Retention
Substrate depth × MWC 0
Retention capacity 0 mm
Drainage / Detention Layer

Provides temporary storage for peak flow reduction

mm

l/m² (≈ 1 l/m² = 1 mm)

Total Storage Capacity
Substrate retention 0 mm
Drainage storage 0 mm
Total capacity 0 mm
Other Layers

mm (default 1, negligible storage)

mm (default 5, negligible storage)

Essential component: prevents root penetration

Total Build-Up & Weight
Total build-up depth 0 mm
Substrate sat. weight 0 kg/m²
Drainage sat. weight 0 kg/m²
Vegetation weight 0 kg/m²
Total saturated weight 0 kg/m²
Total weight per m² (per 1000 m² roof) 0 kN
Structural Note: Total saturated load displayed above. Structural engineer must verify roof loading capacity.

Attenuation Performance Calculation

Design Storm Analysis
Design Storm Rainfall: — mm
Rainfall Volume: — m³
Retained by Substrate: — m³
To Drainage Layer: — m³
Attenuated in Drainage: — m³
Overflow (to downstream): — m³
Peak Flow Reduction
Conventional Roof (Unrestricted)
Rainfall intensity — mm/hr
Runoff coefficient 0.95
Peak flow (conventional) — l/s
Green Roof (With Attenuation)
Effective runoff coefficient 0.50
Peak flow (green roof) — l/s

Defines how water leaves the roof

Design Storm Performance:
Complete earlier steps to calculate

Green Roof Attenuation Results

Key Performance Indicators
Total Build-Up
0
mm
Retention Capacity
0
mm
Drainage Storage
0
mm
Peak Flow Reduction
0
%
Saturated Weight
0
kg/m²
Annual Retention
0
m³/yr
Design Storm Performance Summary
30-Year Storm Attenuation
Total rainfall volume — m³
Retained by substrate — m³
Attenuated in drainage — m³
Overflow to downstream — m³
Design runoff coefficient 0.50
Downstream flow rate — l/s
Annual Water Balance Estimate
Year-Round Performance
% Annual rainfall retained —
% Released as slow drainage —
% Overflow to downstream —
Total annual volume attenuated — m³
Design Checks & Compliance
Design Check Status Notes
Substrate depth minimum — Awaiting inputs
Slope limit (≤35°) — Awaiting inputs
Slope >15° (extra measures) — Awaiting inputs
Drainage layer required (slope >5°) — Awaiting inputs
Root barrier specified — Awaiting inputs
Filter fleece specified — Awaiting inputs
Structural loading verified ⚠ ACTION Refer to structural engineer
Design Notes

Key Assumptions

  • Empirical annual retention factors by roof type (extensive 0.45, semi-intensive 0.55, intensive 0.65, biodiverse 0.50).
  • Substrate retention based on MWC specification.
  • Average orifice head estimated at 75% of drainage layer depth.
  • No irrigation contribution considered.

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