What This Requirement Covers
Green roofs (vegetated roofs) provide multiple benefits including stormwater management, biodiversity, thermal insulation, urban cooling, and extended roof membrane life. While not required by Building Regulations, green roofs contribute to compliance with planning policies, SuDS requirements, and energy performance targets.
Key Requirements
Types of Green Roof
- Extensive green roofs: Shallow substrate (50-150 mm), lightweight planting (sedum, wildflowers), low maintenance, minimal structural impact. Typical weight: 60-150 kg/m²
- Intensive green roofs: Deeper substrate (200-1000 mm), wider planting palette (shrubs, trees, lawns), higher maintenance, significant structural loading. Typical weight: 200-500+ kg/m²
- Semi-intensive: Intermediate depth and planting variety
Building Regulations Considerations
- Part A (Structure): The roof structure must be designed for the additional weight of the green roof (substrate, planting, and retained water). This is typically 1-5 kN/m² depending on the type.
- Part C (Moisture): The waterproof membrane must be root-resistant and suitable for use beneath a green roof. A root barrier is typically required.
- Part H (Drainage): Green roofs retain rainwater (typically 40-80% of annual rainfall), reducing the drainage discharge rate. This can reduce the size of downstream drainage.
- Part L (Energy): The additional thermal mass and insulation of a green roof improves the building's energy performance (typically 5-10% reduction in heat loss through the roof).
Fire Safety
- Green roofs over 11 metres in height must consider the combustibility of the substrate and planting
- Gravel strips or fire breaks may be required around the perimeter and at intervals
- The substrate depth and moisture content generally provide adequate fire resistance for most extensive green roofs
Practical Compliance Tips
- Engage a structural engineer to verify the roof can support the green roof loading before specifying
- Use a root-resistant waterproof membrane; standard roofing membranes can be penetrated by plant roots
- Specify an appropriate substrate mix for the intended planting; do not use garden soil (too heavy, poor drainage)
- Include irrigation provision for the establishment period (first 1-2 years)
- Maintain the green roof by removing unwanted species and checking drainage outlets annually
- Green roofs can help meet SuDS requirements and may earn credits in BREEAM assessments
- Consider the access and safety arrangements for ongoing maintenance