Green Roofs Are Both Performance Systems and Architectural Elements
A green roof (also called a vegetated roof or living roof) is a roof system that incorporates growing media and plants on top of a waterproofing and drainage assembly. Green roofs have been used in Europe for decades and have become increasingly common in North America as architects, owners, and municipalities recognize their benefits: reduced urban heat island effect, stormwater management, improved building thermal performance, extended membrane life (the growing media protects the waterproofing from UV and thermal cycling), and amenity space for building occupants. The ARE PPD exam tests green roofs because they are a building systems choice that integrates structural, waterproofing, drainage, and landscape design - a true multi-discipline coordination challenge.
Extensive vs. Intensive Green Roofs
Green roof systems fall into two broad categories based on growing media depth and program:
Extensive green roofs use a thin growing media layer (2–6 inches typical) planted with low-growing, drought-tolerant plants - typically sedums, mosses, and grasses. Extensive systems are lightweight (10–35 psf when saturated), require minimal maintenance, and are not intended for human occupancy. They are primarily performance-oriented systems: stormwater management, thermal insulation, membrane protection. Because of their light weight and low maintenance, extensive systems can be applied to roofs that are not designed to support intensive green roofs.
Intensive green roofs use deep growing media (8 inches to several feet) that can support trees, shrubs, and even lawn areas. Intensive systems are designed for human occupancy and may include paths, seating, and programming as roof gardens. They are significantly heavier (80–150+ psf saturated) and require a structural system designed for the additional load, regular irrigation (since the deep growing media can retain more water but also supports plants with higher water demands), and ongoing maintenance. Intensive green roofs represent a major structural design commitment and a significant capital investment.
Green Roof Assembly Components
From the structural deck upward, a typical green roof assembly includes:
- Root barrier: Prevents plant roots from penetrating and damaging the waterproofing membrane.
- Waterproofing membrane: The critical component - must be a robust, root-resistant membrane (typically a mechanically fastened or adhered single-ply or modified bitumen membrane). The waterproofing is under the growing media and essentially permanent - repairing leaks requires removing the entire growing media, which is extremely expensive. The membrane must be thoroughly tested before the growing media is installed.
- Drainage layer: Allows excess water to drain off the roof while retaining a water reservoir for plants. Typically a dimple mat or modular drainage panel.
- Filter fabric: Prevents fine growing media particles from migrating into the drainage layer and clogging it.
- Growing media: Engineered lightweight growing media (not typical garden soil, which is too dense and poorly draining); depth varies by system type.
- Vegetation: Selected for climate, depth of growing media, and maintenance requirements.
Structural Loading Considerations
The additional dead load from a green roof assembly must be accounted for in the structural design. Saturated weights: extensive systems 10–35 psf; intensive systems 80–150+ psf. For existing buildings being retrofitted with green roofs, structural analysis is essential - many existing roof structures cannot support even an extensive green roof without reinforcement. Green roof loading should be coordinated between the architect and structural engineer early in design to ensure the structural system is adequate.
Key Exam Points
- Extensive: 2–6" growing media; 10–35 psf; low maintenance; no occupancy; primarily performance-oriented.
- Intensive: 8"+ growing media; 80–150+ psf; occupiable; trees and shrubs possible; major structural commitment.
- Assembly layers: waterproofing → root barrier → drainage layer → filter fabric → growing media → vegetation.
- Waterproofing is permanent - must be fully tested before growing media installation; leak repair requires removing growing media.
- Stormwater benefit: extensive roofs retain 50–80% of rainfall in the 90th percentile storm event (varies by system depth and design).
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