Extensive Green Roof
Key Benefits
- Water Management: High Runoff Volume Reduction via storage & evapotranspiration (Interception); Peak Flow Attenuation (moderate); Water Quality Improvement (filtration). (C753 S12.4, S12.5)
- Building Performance: Thermal Insulation (reduced energy use - C753 S12.1); Increased roof lifespan (protection from UV/thermal stress); Noise Reduction/acoustic insulation (11 db). (GI Guide)
- Environmental: Urban Cooling/Mitigates Urban Heat Island Effect (GI Guide); Air Quality Improvement (particulate capture); Carbon Sequestration/Storage (C753 Ch 5).
- Biodiversity: Creates habitat, esp. for invertebrates; Can support specific target habitats (e.g., simulating brownfield sites); Contributes to ecological networks (stepping stone habitat). (C753 S12.7; GI Guide)
- Amenity: Improves visual appearance of roofs (esp. if overlooked). (C753 S12.6)
- Metrics: Rainwater Runoff Retained: 62%; Stormwater Runoff Delay: 73%; Surface Temperature Reduction: 6.5°C; Air Temperature Reduction (Above): 0.8°C; Carbon Sequestered: 1.28 kg/m²/yr; Carbon Stored (Veg): 0.375 kg/m²; Air Pollutant Removal (NO2): 24%; Air Pollutant Removal (PM10): 14%; Water Quality (NO3 Reduction): 80%; Water Quality (TSS Reduction): 70%; Water Quality (P Reduction): 67%; Property Value Uplift (Non-Accessible): 2.9%.
Technical Guidance
Structural Assessment (C753 S12.2)
CRITICAL FIRST STEP. Existing roof structure MUST be assessed by a qualified structural engineer for load-bearing capacity. Extensive green roofs add saturated weight (typically 60-150 kg/m²). Assessment must confirm capacity for dead load (system + vegetation) + live load (snow, maintenance access).
Waterproofing & Root Barrier (C753 S12.8)
Existing roof waterproofing must be in excellent condition, appropriately detailed, and ideally root-resistant (FLL certified). If not, a separate root barrier layer (e.g., HDPE) MUST be installed over the waterproofing. Protection fleece often recommended above root barrier/waterproofing.
Roof Slope & Stability (C753 S12.2)
Suitable for flat roofs or slopes up to 10-15 degrees standard build-up. Steeper slopes (>9 degrees) require specific retention measures (e.g., battens, shear barriers, geocells) to prevent substrate slippage, designed by a specialist.
Drainage Layer (C753 S12.8.3)
Essential layer to allow excess water drainage and aeration. Typically cuspated (dimpled) sheets (e.g., HDPE), drainage/reservoir boards, or lightweight aggregate. Ensure sufficient flow capacity.
Filter Layer (C753 S12.8.4)
Geotextile filter fleece placed above the drainage layer to prevent substrate fines washing out and clogging the drainage void/outlets. Must be permeable and filter-stable.
Growing Medium/Substrate (C753 S12.9; GI Guide - Soils)
Use lightweight, engineered extensive green roof substrate (typically 60-150mm deep). MUST meet standards (e.g., FLL guidelines/GRO Code or equivalent UK specs - C753 S30.4) for particle size, bulk density, water retention, air capacity, organic content, pH. Do NOT use standard topsoil. Consider substrate variation for biodiversity (GI Guide).
Vegetation (C753 S12.10; GI Guide - Planting)
Primarily drought-tolerant, low-growing, low-maintenance species adapted to shallow, exposed conditions. Sedums common (often pre-grown mats/blankets for ease). Can include drought-tolerant herbs, grasses, and UK native wildflowers/plugs suitable for extensive roofs (e.g., Kidney Vetch, Biting Stonecrop, Sheep's Fescue, Wild Marjoram, Thyme). Match species to substrate depth and biodiversity goals (GRO Code).
Edge Detail & Fire Breaks (C753 S12.8.1)
Install vegetation-free margins (min 300-500mm) of rounded gravel/pebbles or paving slabs around perimeters, rooflights, penetrations, and potentially compartmenting large roofs (>40m - C753 S12.8.1). Essential for drainage access and fire safety (compliance with Building Regs Part B & GRO Code). Use appropriate edge profiles/retainer trims.
Installation (C753 S12.11)
Install layers sequentially: waterproofing, root barrier (if needed), protection fleece, drainage layer, filter fleece, substrate, vegetation. Follow GRO Code and manufacturer/system supplier guidelines precisely. Protect layers during installation. Ensure safe working practices (working at height).
Maintenance (C753 S12.12; GRO Code)
Low but essential (esp. establishment). Typically 1-2 visits/year after establishment. Includes: removing unwanted vigorous weeds (esp. tree seedlings), checking/clearing outlets and margins, applying slow-release fertiliser ONLY if deficiency observed (often unnecessary for sedum - check GRO Code), watering ONLY during establishment phase or prolonged extreme drought.