So what’s in a green roof? A lot. Green roofs include (but are not limited to) a high quality waterproofing system, root repellent system, drainage system, filter cloth, a lightweight growing medium, and of course plants. They can also include anything from irrigation systems, solar energy, and park benches. When designing a green roof, where does one start? Green roofs can be divided into three separate categories:

Extensive Category

Extensive roof designs are lightweight, cost less and have extremely low maintenance requirements. They are suitable for large areas and/or retrofit projects, and they are easy to replace. Extensive roofs are great for warehouses and large retail venues, where access to the roof isn’t needed and the costs of climate control are of greater concern.

Quick Facts:

Growing Medium Depth: 6” or less

Fully saturated weight: Low (10-35 lbs / sq. ft.)

Plant Diversity: Low

Cost: Low

Maintenance: Minimal

Semi-Intensive Category

Semi-Intensive designs can combine the best advantages of both Extensive and Intensive green roofs. Instead of covering the entire roof with vegetation, semi-intensive designs utilize the areas of the roof with greater loading capacity, can have limited access, and can have better layout flexibility than an extensive green roof.

Quick Facts:

Growing Medium Depth: 25% of the green roof area above or below 6”

Fully saturated weight: Varies (35-50 lbs / sq. ft.)

Plant Diversity: Greater than Extensive

Cost: Varies

Maintenance: Varies

Intensive Category

Intensive green roofs are often more expensive but typically yield greater benefits, especially for mixed-use developments. They have a much greater range of design, better insulation and stormwater management, and double as a green space that people can access and enjoy. They also have greater potential for biodiversity and reducing the urban heat island effect.

Quick Facts:

Growing Medium Depth: More than 6”

Fully saturated weight: High (35-300 lbs / sq. ft.)

Plant Diversity: Greatest

Cost: High

Maintenance: Varies but is generally high

Modular vs. Loose Laid/Built-up Systems

There are essentially two ways to install a green roof. One is to use a modular design that has many or all of the elements pre-built or grown for easy installation and instant visual impact. The modular systems are typically used for extensive or semi-intensive green roofs, since they can easily cover a wide surface area quickly and efficiently. They also allow for easy access and maintenance of the roof layers beneath the vegetation.

Loose-Laid or Built up systems are a lot more involved, but allow for a greater amount of customization. It is when the various elements of a green roof are installed separately. They are often used for intensive roof systems, since they allow for more flexibility.

Do you think there are opportunities to build green roofs like these in your neighborhood? Let us know in the comments!

Green Roofs for Healthy Cities:

Green roof technology: