A guide by Robert Griffith BEnvSc, Hort

Planning and design, you need to understand what your roof is for. Options include extensive or intensive roofs. So you can remember the difference just call them light weight green roofs or rooftop gardens for the heavier types. Extensive green roofs are generally for plants only with thin soils and generally and low growing hardy plants. Extensive or light weight roofs provide insulation, storm water management and a place for biodiversity. Intensive green roofs are usually heavier and designed for people to enjoy the space, they can include everything from parks, intimate areas for bbq's and lush plantings of trees and shrubs. You can have both types on one roof also. Climate and aesthetic requirements also effects your design.

A competent green roof designer will take care of these and the many other requirements below.

Engineering weight loads, depending on the green roof selected weight calculations are required and an engineer is needed to calculate building load capabilities.

Water proofing, there are numerous systems of waterproofing, one with a root barrier are preferred. New concrete structures are prone to cracking so a waterproof product that allows movement is recommended. Guarantees from waterproof contractors are to be sought.

Leak detection systems, in the unlikely event that waterproofing fails leak detection systems are insurance against costs. I always specify the right system for the job as if it leaks you may have to remove the whole green roof and thats expensive.

Drainage, drainage systems vary in their ability to shed or store surplus water. Given that one benefit of green roofs is to reduce flow rates, the drainage needs to be designed for the desired Peak Discharge Rates. Drainage cells, gravels and geotextiles are available in a variety of thicknesses, flow rates and water storage capacity. Adjustments to these components, including growing media infiltration, and plant species are varied to determine accurate discharge rates. Drainage pipes and access for cleaning also need the correct design.

Growing media, there is debate on the benefits of organic matter over mineral components. Most soils, particularly with clay are unsuitable due to inevitable blockages to the drainage system. I always choose a highly mineral based media due to longevity and reduced clogging of the drainage systems. Media depth is an important determinant of plant species selection.

Evolvement Pty Ltd design roofs to last a long time, some of the ones I have been involved with in Europe are still functioning very well after 40 years.

Plant species, There are thousands of species that are suitable for green roofs. A horticulturalist with green roof experience will provide a good selection based on climate, growing media and the desired aesthetics.

Mulching, stone mulch is preferred as it can get very windy on a roof and the weight of it will keep it there. At the edges of a building there can be quite an uplift of wind, a gravel with erosion control strip can be used to avoid wind erosion.

Irrigation, although irrigation is not required in some climates, in Australia it is recommended for all but some circumstances. A well designed drip irrigation system provides a good outcome as it is not blow by wind.

Maintenance, as with all systems a maintenance manual for your green roof should explain regular tasks required.

If you need help, please don't hesitate to call Robert on +61 468 787 071 or rgriffi8@gmail.com.

I hold regular informative seminars, please book with the above contact.