Drainage
Good drainage is fundamental in a successful green roof design. Standing water in the soil profile has risk implications in terms of increasing the potential of moisture breaching the waterproofing membrane. If the water drains freely through the profile and is collected and drained away, the risk is reduced. In addition, most plant material requires an aerated, non water-logged soil profile for healthy growth.
Yet the capability of green roofs to retain and/or detain a certain amount of rainwater and to delay the runoff during storm events makes them valuable in managing urban storm water systems. In addition, water stored in the profile can reduce irrigation de- mands.
A balance should be found between the storage of water in the soil profile and the drainage of storm water on green roofs.
The drainage system must consider surface water, sub-surface water and transporting water from the roof to either the stormwater system or rain water tanks. Surface water may be removed by proper sloping of pervious and impervious surfaces to roof drains or gutters. Roof outlets must not be allowed to become covered with greenery or loose gravel and they must be permanently acces- sible.
Subsurface water is collected by providing sufficient void space and slope to allow excess sub-surface water to drain through the profile and be transported to roof drains or gutters where it can be removed from the roof. (Allan Wingfield, 2005)
Drainage Materials
The drainage course materials and dimensions will depend upon construction requirements and objectives for vegetation. The construction requirements relate to; the drainage function, the design loads and the protective function. The objectives for the veg- etation relate to; the prevention of water-logging, a retentive or reservoir water supply, the type of vegetation sought and increasing the depth of the course available for root penetration.
When the drainage course is being formed, a distinction is made between the following groups and types of materials.
- Mineral Aggregates:-
- Expanded slate & clay
- Lava & pumice
- Gravel & fine chippings
- Re-cycled Aggregates:-
- Brick hardcore
- Slag
- Crushed / foamed glass
- Drainage Matting
- textured non-woven matting
- studded plastic matting
- fiber type woven matting
- flock type foam matting
Drainage Materials (continued)
- Drainage Modules:-
- high strength, thermoplastic, inter-locking cells that transport and capture high water volumes and protect the w/p membrane.
- Drainage Boards:-
- boards made from foam pellets
- studded rubber boards
- shaped rigid plastic boards shaped plastic foam boards
- Drainage and substrate boards
- boards made from modified foam (FLL. 2002)
