Green Roof Construction: Extensive vs Intensive, Drainage Layers & Structural Loading

Quick Answer: Extensive green roofs (50–150mm substrate, sedums, mosses) are lightweight (60–150 kg/m² saturated) and suitable for most domestic flat roof structures with structural confirmation. Intensive green roofs (200–1000mm substrate, grass, shrubs, trees) are much heavier (200–1000+ kg/m² saturated) and require purpose-designed structures or structural engineering assessment. Both types require a root-resistant waterproofing membrane, drainage layer, filter fleece, and growing medium.

Summary

Green roofs (also called living roofs or vegetated roofs) provide multiple benefits: urban biodiversity, rainwater attenuation (reducing peak flow to drains), thermal insulation, building cooling in summer, and visual amenity. They are increasingly specified in planning conditions for new developments, particularly in London and other cities where sustainable urban drainage (SuDS) is required.

For contractors, green roof installation is a specialist skill but the principles are straightforward. The challenge is selecting the right system for the structural capacity of the supporting roof, ensuring the waterproofing is correctly detailed (root resistance is critical), and selecting the right growing medium and planting for the exposure and maintenance regime.

Domestic flat roof green roofs are typically extensive (sedum blanket) systems installed by roofers or specialist green roof contractors. Larger intensive systems on commercial or mixed-use buildings require structural engineering and specialist landscape architects.

Key Facts

• Extensive green roof — lightweight; sedums, mosses, shallow-rooting plants; substrate depth 50–150mm; saturated weight 60–150 kg/m²; low maintenance (annual inspection, occasional weeding)
• Intensive green roof — deeper substrate; grass, herbaceous, shrubs, or trees; depth 200–1000mm+; saturated weight 200–1000+ kg/m²; high maintenance; requires irrigation
• Semi-intensive — intermediate; substrate 100–250mm; drought-tolerant plants; moderate weight
• Waterproofing membrane — must be root-resistant; either inherently root-resistant (some EPDM, TPO/PVC, bitumen) or with a separate root barrier; without root resistance, plant roots will penetrate and destroy the waterproofing
• Root barrier — separate polyethylene or copper-infused membrane layer placed above waterproofing to prevent root penetration; some manufacturers integrate into membrane
• Drainage layer — egg-crate plastic drainage board, or aggregate layer; allows water to drain to outlets without saturating the growing medium continuously; typically 25–100mm
• Filter fleece — geotextile membrane between drainage layer and growing medium; prevents growing medium migrating into drainage layer and blocking it
• Growing medium (substrate) — lightweight specially formulated mix; typically 70–80% expanded shale/clay or crushed brick, 20–30% organic; much lighter than standard soil; typical density 800–1200 kg/m³ dry / 1100–1400 kg/m³ saturated
• Sedum mat — pre-grown sedum planted on a carrier mat; perishable product, must be installed quickly; provides immediate coverage
• Vegetation blanket — sedum mat variant with integrated growing medium; easiest installation
• Build-up order (top to bottom) — vegetation → growing medium → filter fleece → drainage layer → root barrier (if separate) → waterproofing → insulation (if warm roof) → deck
• Approved Document H — drainage requirements; green roofs contribute to SuDS attenuation
• Structural loading — existing flat roofs should be assessed by a structural engineer before green roof installation; most older domestic flat roofs (traditional timber joists) are not designed for green roof loading
• NHBC standards — for new build; specify minimum structural capacity for green roofs

Quick Reference Table

Detailed Guidance

Structural Assessment

Before any green roof installation, confirm the roof structure can support the additional saturated dead load. Do not assume.

For domestic flat roofs (existing buildings): Most domestic flat roofs are designed to support:

• Dead load (roofing materials): 0.5–1.5 kN/m²
• Imposed load (maintenance access): 1.5 kN/m²
• Snow load: 0.6 kN/m² typical UK

An extensive green roof adds approximately 1–1.5 kN/m² (100–150 kg/m² saturated). This is within the typical imposed load allowance for a well-built roof.

However:

• Old rafters may be undersized or in poor condition
• Birdsmouth joints and rafter to wall plate connections may be marginal
• Flat roof decks (chipboard on CLS joists) may not be rated for continuous wet loading

A structural engineer should assess any existing roof before green roof installation. Provide the engineer with the proposed build-up and its saturated load per m².

For new build: The structural engineer designs for the specific green roof system at the project specification stage.

Waterproofing System Selection

The waterproofing must be root-resistant. Failures here are catastrophic and costly to repair under an established green roof.

EPDM:

• Single-ply rubber membrane; excellent chemical resistance; long lifespan (50+ years)
• Inherently root-resistant per EN 13948
• Loose-laid with ballast, or mechanically fixed and fully adhered
• Best for domestic extensive green roofs; easy to detail around penetrations

TPO (Thermoplastic Polyolefin) / PVC:

• Single-ply; heat-welded seams; mechanically fixed
• Root-resistant per EN 13948 with appropriate grades
• Common for commercial intensive systems

Reinforced Bitumen (RBM / GRP-faced):

• Traditional roofing; some grades are root-resistant; check EN 13948 compliance
• Must use root-barrier layer if the membrane is not certified root-resistant

Root barrier: If using a waterproofing that is not inherently root-resistant, install a separate root barrier:

• Copper-infused geotextile (Permaguard or equivalent): plant roots detect copper and turn away
• HDPE membrane (minimum 1mm): physical barrier
• Lay root barrier continuous and fully lapped; join with compatible adhesive or tape

Drainage Layer Design

The drainage layer allows excess water to reach the outlets without waterlogging the growing medium. Design considerations:

Plastic drainage board:

• Dimple-profile HDPE board; dimples face down on the membrane; water flows through voids
• Available in 20–50mm profile heights
• Sized by drainage flow: calculate rainfall intensity for the area × roof area → volume per hour → select drainage board with sufficient void capacity

Outlet design:

• Roof drainage outlets must be sized for the design rainfall minus the retention benefit of the green roof
• Green roofs typically retain 30–80% of summer rainfall and 10–40% of winter rainfall
• Even with attenuation, a design storm event (1:100 year) must be able to drain
• Use root-protection outlet covers to prevent growing medium blocking the drain

Edge details:

• Green roof substrate must be retained at edges with aluminium alloy or galvanised steel edge trim
• A gravel margin (100–200mm wide) at edges and around penetrations prevents sedum growing over the membrane edge and makes inspection easier
• At parapets: extend the waterproofing up the full parapet face before returning with green roof substrate

Plant Selection and Establishment

Extensive roofs — sedum: Sedum and related succulent species are ideal for extensive roofs:

• Drought-tolerant: stores water in leaves; survives UK summer without irrigation
• Shallow roots: do not compromise drainage layers
• Self-seeding: will establish and spread without intervention
• Species commonly used: Sedum acre, S. album, S. reflexum, S. spurium, Sempervivum

Sedum mat installation:

• Pre-grown on coconut coir or similar carrier
• Perishable — install within 48 hours of delivery; keep moist if delayed
• Lay in stretcher bond (like brickwork) to prevent joint movement and gaps
• Water in immediately after installation; keep watered for 2–4 weeks until established

Seed application (lower cost):

• Native wildflower and sedum mixes broadcast over growing medium
• Slower establishment (6–12 months to full coverage)
• Higher risk of bare patches and weed colonisation initially
• Requires protective covering (jute netting) until established to prevent washout

Frequently Asked Questions

Do I need planning permission for a green roof?

In most cases, no. On a flat roof, replacing the waterproofing with a green roof system does not change the roof profile and is permitted development. However, if the green roof changes the roof height (e.g., adding an intensive system with a soil depth of 500mm+), this could trigger planning. Check with the local planning authority if the build-up adds more than 200mm to the overall roof profile.

Can I put a green roof on a pitched roof?

Yes, but it's more complex. Pitch affects drainage performance and substrate retention — steep pitches require substrate systems with root and soil anchoring (geotextile bags, batten systems). Practical for pitches up to approximately 35°; above that, specialist systems are needed. Pitched green roofs are common in Germany and Scandinavia but less common in the UK.

How long does an extensive sedum roof last before maintenance is needed?

An extensive sedum roof on a properly installed root-resistant waterproofing system requires:

• Annual inspection: check outlets are clear, no bare patches developing, no invasive weeds establishing
• Occasional weed control: bramble, ash, and buddleia can establish and must be removed before roots damage the drainage layer
• No irrigation needed after establishment

Lifespan of the substrate and vegetation: effectively indefinite with correct plant species and basic maintenance. Lifespan of the waterproofing: 30–50+ years depending on material.

Regulations & Standards

• BS 8217:2005 — Reinforced bitumen membranes for roofing; design and application guide

• BS EN 13948:2007 — Flexible sheets for waterproofing; determination of resistance to root penetration

• Approved Document H — Drainage; surface water management and SuDS

• Approved Document A — Structure; loading requirements

• NHBC Standards Chapter 7.1 — Flat roofs and balconies; loading for green roofs

• GRO Green Roof Code — UK Green Roof Organisation code of best practice (free download)

• Living Roofs — Comprehensive UK green roof resource

• SEDUM UK — Sedum mat suppliers and technical guidance

• Environment Agency SuDS — SuDS technical standards

• flat roofing — EPDM, GRP, and felt flat roofing systems

• soakaways — Combined SuDS approach with green roofs

• rainwater harvesting — Rainwater collection from green roofs

• garage conversions — Flat roof over garage conversion suitable for extensive green roof