Extensive vs. Intensive Vegetative Roofs: What’s the Difference?

The primary distinction between an extensive and intensive vegetative roof is the depth of substrate—or growing medium—used to sustain the roof vegetation. Extensive vegetative roofs typically have a growing medium depth of six inches or less. Such shallow soil limits the diversity of vegetation that can grow on an extensive roof system, and accordingly, these systems often use sedum vegetation for their resilience in harsh conditions.

The thin growing medium, however, reduces the weight load the roof must bear and comes with significantly lower maintenance requirements than intensive roofing. Extensive vegetative roofs are typically inaccessible to the public and are usually installed solely for their energy- and cost-saving benefits. Extensive roofs offer the best cost-benefit balance for stormwater management.

The Vancouver Convention Centre is one such structure. It features a six-acre "living" roof, the largest in Canada. While not open to the public, the extensive vegetative roof is home to 400,000 indigenous plants, beautiful green and yellow grasses, and four beehives of European Honeybees. 

Building owners and architects interested in the practical applications of extensive vegetative roof systems often find modular vegetative units ideal, such as the SkyScape Pregrown Modular System. Modular systems are designed for fast one-person installation while remaining removable to make roof repair and maintenance easy.

• Lightweight with minimal structural support requirements
• Inexpensive to install and maintain compared to other vegetated roof systems
• Suitable for roofs with large surface areas
• Ideal for roofs with 0-30° slopes
• Low maintenance requirements
• Less technical expertise needed to install and maintain
• Often no need for a drainage or irrigation system
• Ideal for retrofit projects
• More likely to receive approval from the local building authority

When you imagine a lush rooftop garden, you're likely thinking of an intensive vegetative roof. Intensive vegetative roofs tend to be much more complex than extensive systems due to their significantly deeper soil and growing medium—which may reach several feet in depth. As a result, intensive vegetative roofs can sustain a much more diverse range of plant life, including small trees. This greater diversity in vegetation makes intensive roofs more attractive and similar to conventional gardens and parks—and as such, they are often made accessible to the public to attract visitors to that building. Moreover, the deeper growing medium and more vigorous plant growth of intensive roofs allows for greater stormwater retention compared to extensive systems.

However, the increased weight and depth of the substrate and its vegetation means that intensive vegetative roofs require significantly more structural support, investment, maintenance, and irrigation compared to extensive systems.

The School of Art Design and Media at Singapore's Nanyang Technological University is a beautiful example of a functional intensive vegetative roof. It features sloping, grassy green areas that harvest rainwater, provide insulation, and offer students a lush spot to gather.

• Greater diversity of plant life
• Better insulation properties
• Can simulate natural areas and habitats found on the ground
• More visually appealing than extensive vegetative roofs
• Can be used for a variety of purposes, such as recreation, growing food, public gathering space, and wildlife refuge

Semi-intensive vegetative roofs include properties of both roof types, meaning there may be areas above and below six inches of substrate depth within the same vegetated roof system. As a result, semi-intensive roofs have greater plant diversity than extensive roofs but are still less costly than intensive roofing. Semi-intensive roofs may be partially accessible and come with varying maintenance requirements.

The Chicago City Hall building is an excellent example of a semi-intensive vegetative roof. It is an island of green in a sea of cement, and home to a combination of 20,000 herbaceous plants, 112 shrubs, and two trees.  

Ultimately, the type of vegetative roof best suited for a building will depend on factors such as roof structure, desired vegetation, annual rainfall, and stormwater management requirements. 

Installing a vegetative roof of any kind, however, requires several layers of roofing material not found on conventional roofs. These layers include vegetation, growing medium, filter membrane, drainage layer, root repellent layer, roofing membrane, thermal insulation, vapour control layer, and structural roof support. 

Not all buildings will be suitable for intensive vegetative roofing due to certain weight limits and size constraints. Intensive vegetative roofs are best planned by an architect before a building is constructed, whereas extensive vegetative roofing can often be retrofitted onto an existing building.

Ultimately, while intensive vegetative roofs are more expensive to install and maintain and more resource intensive, they typically offer more utility and greater plant diversity than extensive roofing, plus greater stormwater volume benefits. In contrast, extensive roofs provide a low-cost, low-maintenance vegetative roof solution that can be quickly installed and produce more immediate energy savings.

Has an idea taken root in your mind? A vision for the future? Find an Elevate Sales Rep or licensed contractor near you to take the next step toward installing a vegetative roof.