Green Roof Layers

Figure 13. A typical green roof cross section. Image courtesy
Green Roofs of Colorado.

A typical green roof begins with a quality waterproof membrane, a root barrier, the drainage layer, the soil (referred to as media), and plants. A green roof needs to be designed to hold enough water for the plants, and to efficiently direct excess water off the roof. The media will detain some water and release any excess via the drainage layer and roofs drainage system.

1 – Base Layer

There are multiple different quality waterproofing membranes that can be used for green roofs.

2 – Root Barrier

The root barrier is simply a sheet of material that stops root growth from penetrating the membrane below. Plant selection needs to be discussed in the design phase so a proper root barrier can be specified.

3 – Drainage Layer

A layer that allows water to flow through and directs it towards the building drainage system. There are multiple options for drainage layers and can be specified based on the design intent of the green roof.

4 – Media

Green Roof media is a specialty growing layer that detains water and nutrients so plants can thrive. It is manufactured to ensure that it won’t compress over time and block the flow of water. Different medias can be produced depending on the plant selection or use of the green roof such as food production.

5- Irrigation

Green Roof designs incorporate either drip or spray irrigation, which is especially vital in the first two years for establishing plants and times of drought.

Other Green Roof Design Considerations

Pre-planning questions

  • What is the available structural capacity for the green roof area
  • What type of access is there to the green roof area?
  • Is there water available for irrigation?
  • Is maintenance part of the budget?