Good drainage is criticalfor green roofs and ensures that large amounts of water are not retained on the roof, compromising both the structural integrity of the building and plant health through waterlogging and oxygen-depleted substrates. Sub-surface run-off must drain efficiently from the substrate, into the drainage layer, off the roof surface, and into drains to the stormwater or rainwater collection. The growing substrate must be kept separate from the drainage layer with a filter sheet.
Older green roofs often used a permeable layer of rock aggregate (such as scoria or gravel) for drainage. The clay and silt content of materials used in a rock aggregate drainage layer should be < 10 per cent by mass. The rock aggregate should also have a suitable pH and be low in soluble salts to ensure plant growth is not adversely affected. This form of drainage is heavy and does not allow for air pruning of roots, which is now achieved with plastic drainage cells. However, in some situations, rock aggregate drainage does a better job controlling peak flow.
On modern, lightweight, green roofs, plastic drainage sheets or boards are the preferred drainage materials. Plastic drainage layers may be rigid, open mesh structures that allow unrestricted drainage of water, or they may have a cup-style, ‘egg carton’ design that enables water to be stored at the base of the profile (see image below). The advantage of the latter is that water can be stored and used later by the plant. The volume of water that can be stored varies with each product and the size and packing density of the cells.
Drainage is installed as a continuous layer over the entire surface of the green roof. Cup-style drainage sheets should be overlapped to eliminate the possibility of gaps being created between sheets. Other plastic drainage types should have adjacent sheets butted together. Very rigid drainage layers should be installed in trafficable areas of the green roof to avoid compression of the layer.