Slopes and wind protection
Landscapes on pitched roofs are subject to wind forces and gravity affecting the stability and retention of the growing substrate and plants. In many cases slippage can lead to poor plant performance and ultimately green roof failure. For roof pitches of up to 15 degrees, no additional protection is needed, unless there are strong wind issues on the site. Waterproofing must be root-resistant, and covered with a protection mat. A drainage layer is not always required as the roof can drain effectively through gravity. The stability of steeply pitched green roofs is increased by maintaining vegetation cover: provision of irrigation is essential.
For green roofs constructed on pitches greater than 15 degrees, basic protection can be provided through anti-erosion jute netting installed just below the substrate surface to provide some anchorage to plants (see image). This netting breaks down over time, and is used simply to help keep the substrate in place whilst the plants establish.
Greater protection can be provided on steeper sites by using a drainage layer with large cells, or cups. The growing substrate fills the cells of the drainage layer, reducing slippage and providing spaces for plant roots to grow, ensuring further stabilisation (see Figure 20). A filter sheet must be placed beneath the drainage layer to reduce wash-through of fine particles from the substrate.
For green roofs with slopes between 20 degrees and 45 degrees, ‘honeycomb’ webbing, comprising multiple, enlarged drainage cells, can be installed above the drainage and filter sheet layers (see image). This holds the substrate in place, increases stability and reduces slippage. Other specialised structural elements can be incorporated to transfer shear forces into stable, reinforced abutments that form part of the roof structure, or wind barriers can be installed.