For civil engineers, planners, and municipalities, stormwater management is among the most valuable functions of green roofs. By detaining and retaining rainfall at the source, LiveRoof® reduces demand on municipal sewers, stormwater tunnels, and detention basins, enabling more sustainable and cost-effective infrastructure.
Rainwater Capture and Retention
The performance of a green roof in reducing runoff depends on many factors, including:
- Regional precipitation patterns
- Rainfall intensity (light rain is absorbed more efficiently than short, heavy storms)
- Seasonal climate variables (temperature, sun, wind)
- Roof slope (flatter roofs retain more water)
- Soil depth and engineered media composition
- Plant type, density, and transpiration rates
- Supplemental irrigation (minimal impact on retention)
In practice, annual rainfall retention varies widely:
- Hot, arid climates (e.g., Phoenix) can retain nearly 100% of precipitation.
- Temperate cities (e.g., Chicago, New York) typically see 60–85% annual retention.
Engineered Media Performance
LiveRoof’s engineered soil media is specifically designed for stormwater control:
- Maximum water-holding capacity: 48% by volume
- Total pore volume: 58.8%
- Saturated hydraulic conductivity: 0.434 in/min
These physical properties allow LiveRoof systems to act like a sponge, absorbing rainfall, holding it for plant use, and slowly releasing excess water through evapotranspiration and controlled drainage.
Roof Slope Considerations
Roof slope directly affects stormwater retention by determining how quickly water moves through the green roof system and how much can be stored within it.
- Flat or low-slope roofs (¼” per 12’) maximize water retention.
- Steeper slopes (>2:12 pitch) require buttressing, reinforced parapets, or slope stops.
- Maximum slope: 40° per international guidelines (FLL & GRHC).
RoofBlue® DETAIN – Temporary Storage
For projects with strict stormwater regulations, LiveRoof can be paired with RoofBlue® DETAIN, a subsurface accessory that provides temporary detention beneath the vegetated roof before controlled release.
- Stores up to 2 gallons/ft² (7.4 L) beneath the green roof.
- Relieves pressure on storm sewers during peak events.
- Can reduce drain sizes, utility fees, and overall project cost.
- Provides a cost-effective alternative to tanks, cisterns, or underground detention chambers.
RoofBlue® RETAIN – Passive Irrigation and Storage
Complementing DETAIN, RoofBlue® RETAIN is designed to recycle water to the plants themselves. RETAIN modules hold stormwater within the green roof profile, gradually wicking it back into the soil media.
- Functions as a self-regulating system that enhances both stormwater retention and plant performance.
- Reduces irrigation demand by leveraging stored rainfall.
- Keeps vegetation healthier during dry periods.
Optimizing Stormwater Outcomes
- Soil depth: A 4-inch LiveRoof system provides an optimal balance of storage and evapotranspiration.
- Plant selection: Dense, biodiverse plantings maximize transpiration and overall water consumption.
- Hybrid solutions: Pairing LiveRoof with RoofBlue® DETAIN or RETAIN delivers both high-capacity detention or retention plus improved plant water availability.
Conclusion
LiveRoof systems are a proven, site-specific solutions for stormwater management, reducing annual runoff by 60–100% while simultaneously improving building performance and urban resilience. With the addition of RoofBlue® DETAIN for detention and RoofBlue® RETAIN for water conservation, LiveRoof provides comprehensive options that help projects meet stormwater requirements, reduce infrastructure strain, and support healthy, sustainable landscapes.
For project-specific calculations and design assistance, LiveRoof’s regional experts can provide detailed retention estimates, RoofBlue® sizing, and plant selection guidance tailored to site conditions.
About This Series
This article is the fifth installment in The Science behind LiveRoof® System series, where we explore the technical advantages of the LiveRoof® system.
Jodi Griffin, Business Development Manager for LiveRoof® Global. Questions? Reach out anytime at jodi@liveroof.com.
