Layers of a Green Roof Assembly for Vancouver

Green roofs, weaving vibrant ecosystems into Vancouver’s urban fabric, reduce stormwater runoff by 50–70%, boost biodiversity, and cut cooling costs by 15–25%, aligning with the city’s Greenest City goals. In Climate Zone 5’s wet, temperate climate (1200 mm rainfall, 0–10°C), their complex assemblies must withstand saturation and support ecological benefits. Simon Green Works champions green roofs, using building science to ensure durability and performance. This article details the eight layers of a green roof—vegetation, growing medium, drainage and water retention, root barrier, rigid insulation, drainage mat, roof membrane, and structural deck—exploring their engineering roles, material choices, installation techniques, and Vancouver-specific considerations for sustainable, resilient roofs.

Vegetation: Ecological and Aesthetic Core

The vegetation layer, from sedums to grasses, attracts pollinators and manages stormwater, sequestering 20–40% of runoff. Extensive roofs (100–150 mm soil) support shallow-rooted sedums, like those from Sempergreen (Sempergreen), while intensive roofs (300–700 mm) accommodate trees. In Vancouver, drought-resistant plants thrive with minimal irrigation, reducing water use by 50%. Vegetation stabilizes soil, mitigates erosion, and cools urban heat islands, saving $300–$500 annually on cooling for a 200 m² roof.

Growing Medium: Sustaining Plant Life

The growing medium, a blend of perlite, shale, and organic matter, nourishes vegetation while facilitating drainage. Extensive roofs use 100–150 mm depths, intensive 300–700 mm, retaining 30–50% moisture, per ASTM standards. In Vancouver, porous aggregates from Lafarge (Lafarge Aggregates) prevent ponding, critical for 25 mm stormwater retention per city bylaws. Structural loads (100–300 kg/m² wet) require robust deck design, verified by engineers to handle 1.5 kPa snow loads.

Drainage and Water Retention Layer

A composite of filter fabric and a perforated dimple mat, like Delta-Drain from Dörken (Delta-Drain), manages water flow, retaining 10–20 liters/m² for plants while draining excess to prevent hydrostatic pressure. The fabric filters sediment, reducing clogs by 80%. Dual drainage planes (above and below insulation) ensure longevity in Vancouver’s saturated conditions.

Root Barrier: Protecting the Assembly

A 15 mil polyethylene root barrier, like Firestone EcoWhite EPDM (Firestone EcoWhite), prevents root penetration, minimizing seams to reduce leak risks. In intensive roofs, reinforced HDPE withstands maintenance traffic, ensuring a 20–30-year membrane lifespan.

Rigid Insulation: Thermal Efficiency

XPS insulation, like Owens Corning FOAMULAR (FOAMULAR XPS), in a PMR assembly (100–150 mm, R-20–30) maintains thermal performance with 0.3% water absorption. Staggered joints minimize energy loss, meeting BC Step Code requirements and saving 15–25% on energy costs.

Drainage Mat: Preventing Hydrostatic Pressure

An entangled mesh or dimple mat, like Delta-MS (Delta-MS), prevents water pooling, extending membrane life by 30%. A 2% slope ensures drainage, critical for Vancouver’s rainfall.

Roof Membrane: Waterproofing Foundation

Fully adhered TPO or PVC membranes, like GAF EverGuard TPO (GAF EverGuard TPO), provide watertight protection with 30-year warranties. Heat-welded seams ensure durability under maintenance traffic.

Structural Deck: Load-Bearing Base

Concrete or steel decks support 200–500 kg/m² loads, with a 2–4% slope to drains, flashed with Sika sealants (Sika Sealants). Seismic reinforcement (0.46g) ensures stability.

Vancouver-Specific Considerations

Vancouver’s climate demands robust drainage and insulation. Extensive roofs suit high-rises, intensive roofs enhance biodiversity, per Green Roof Bylaw (30% of buildings >20,000 ft²). Seismic design and dual drainage planes prevent saturation. Density bonuses ($500,000–$1M) offset costs ($20–$50/ft²). Modular systems, like Sempergreen mats, address urban access constraints.

Why Choose Simon Green Works?

Simon Green Works crafts green roofs that enhance sustainability and resilience in Vancouver’s climate, using products like GAF TPO and FOAMULAR XPS. Our building science expertise ensures durable assemblies. Ready to grow your roof? Contact us for a consultation.

Sources

This article draws on insights from Sharif Asiri’s article, “Layers of a Green Roof Assembly,” published by ASIRI Designs (Layers of a Green Roof Assembly). Additional perspectives are informed by “Choosing a Durable Roof Assembly with High R-Value” from Green Building Society (Choosing a Durable Roof Assembly).