This green roof details post includes excerpts and details from our book Understanding Architectural Details – Residential.
Introduction to Green Roof Details
A green or living roof is a flat or low pitched roof with a layer of planting over the waterproofing membrane. The layers of soil and planting provide insulation to the roof along with protection to the waterproofing layer, not to mention its environmental credentials. Green roofs are largely associated with urban sites as a means to provide habitats for insects, birds and other small animals to improve biodiversity. Green roofs retain water and lower run off which in turn will reduce the drainage and water storage requirements on site. Up to 75% of rainwater can be absorbed in green roofs.
There are two main types of green roof, Extensive and Intensive.
The extensive roof has a shallow soil, typically up to 100mm, and is planted with sedum, moss and grasses. It is used where access is not required to the roof, other than for maintenance, watering and so on. The extensive roof is lightweight, relatively cost effective and requires very little maintenance. These roofs can be constructed on timber, steel or concrete decks. A low pitch or flat roof can be used with an extensive green roof design, although on a slope greater than 20 the planting layer must be strapped horizontally to the structure to prevent the material slipping when saturated.
The intensive green roof has a deeper soil layer, typically 200mm upwards, and allows for a wide variety of plant types to grow. The intensive roof can feature lawns, mature trees along with paved areas for recreational use, sometimes even water features can be seen on an intensive green roof. The intensive roof requires regular watering, usually by dedicated irrigation systems. These roofs are constructed on a flat deck, typically reinforced concrete due to the increased weight.
Typical build up
A green roof build up usually includes:
- Planting layer (grasses, sedum, etc)
- Soil, compost, growing medium
- Filter fleece(allows water through but prevents growing medium blocking the drainage system)
- Moisture retention layer and drainage system
- Protection layer (to prevent damage to waterproof membrane and root barrier)
- Waterproof membrane (with root barrier and metal foil between layers to prevent root damage)
- Insulation layer
- Vapour control layer if required
- Structural deck
Domestic and residential applications generally adopt an extensive green roof approach which will be the focus of this section.
It is possible to design the green roof with a warm roof construction or inverted construction. The build up of the green roof elements will remain the same for both types, but the position of the insulation and waterproof membrane will change.
The insulation must be capable of resisting the dead load of the green roof, along with any additional live loads such as access for maintenance if required. Although the soil does offer a little insulation to the roof it is not taken into consideration when calculating U-value.
The waterproof membrane specified must be high performance, with a good life expectancy – this is due to the difficulty in renewing or repairing green roofs. The membrane can contain its own root protection, or alternatively a root barrier must be used.
Detailing of perimeters, outlets and protrusions through the roof should be carefully detailed. A minimum 150mm upstand of waterproofing layer above the soil level should always be used, along with a 300mm to 500mm wide layer of gravel to any junctions with up stands or roof lights.
The green roof should be designed with a minimum fall of 1:60 to avoid ponding.
- Reduce stormwater runoff
- Protection of waterproof membrane
- Improved sound insulation
- Improve air quality
- Improve environmental benefits
- Create habitat and biodiversity
- Aesthetic benefits
- Initial cost, ie. increased load = increased structure
- Fire resistance