Some green roofs are installed in one large integrated section, while modular green roofs use small portable planting beds placed together to create a larger green roof. Image credit: Chicago City Hall, Sika Group.

The increased focus on green building has led to a renewed interest in green or eco-roofs, from commercial buildings to residential homes.  

A green roof is a layer of vegetation planted over a waterproofing system that is installed on top of a flat or slightly sloped roof. These fall into three main categories – extensive, intensive and semi-intensive. 

Definitions

Roofing material layers 

All three types of roofs require specific layers of roofing materials that are not found on regular roofs. Every layer performs a specific function, keeping the plants alive and protecting the structure beneath.  

The basic anatomy of a green roof.

The basic anatomy of a green roof consists of: 

• Vegetation. 
• Growing medium. 
• Filter membrane. 
• Drainage layer. 
• Waterproof/root repellent layer. 
• Roofing membrane support for plantings above. 
• Thermal insulation. 
• Vapour control layer. 
• Structural roof support. 

Growing medium 

The growing medium is different from a house plant or garden, as traditional soil is heavy. 

A green roof growing medium is composed of mineral aggregates, organic material, good consistent drainage and aeration. It is lightweight, resistant to decomposition and compression, and physically and chemically stable. 

Green roofs allow for more diverse utilisation of the roof, for recreation, growing food, or as an open space. Image credit: Vox Humana Architects.

Modular planting beds 

Some green roofs are installed in one large integrated section, while modular green roofs use small portable planting beds placed together to create a larger green roof. Modular units are often plastic or metal trays, filled with a growing medium. 

Advantages: 

• Can be installed incrementally. 
• Easy removal for maintenance and inspection of the roof layers underneath.  
• Can often be cultivated in a greenhouse and be “ready to plant” on the green roof. Since the plants are already established, there are fewer issues with replacement plants that do not take root or thrive.  

Disadvantage: 

• Moisture cannot flow between units, increasing the impact of wet and dry spells on the plants.  

Comparison of extensive and intensive roofing systems 

Explore the difference between extensive and intensive green roofs, and the pros and cons of large or modular planting beds. 

Full acknowledgement and thanks go to https://www.nps.gov for the information in this editorial.