Building your own living roof garden isn’t as hard as you might think, but there are some important points to consider.
Do your research
First you have to decide what kind of green roof you want, says Rosie Mohorko who is in charge of marketing and research at Green Roofs Australasia. “A lot depends on the structural capabilities. Not every green roof is possible to implement,” Mohorko says.
The main consideration is weight, and she says it’s crucial to consult a structural engineer.
Design the garden
There are two main types of green roof – extensive, which have five to 15cm of soil – and intensive, based on 15cm of soil or more. It’s got to be taken into consideration that not many lush species of plants can be grown in five centimetres of soil.
“You would have to look at plant covers or something that can survive in harsh conditions,” says Mohorko.
Choose your plants wisely
City of Sydney Green Roofs and Walls Strategic Planning and Urban Design senior project officer, Lucy Sharman, says the types of plants you choose should reflect what you want out of the garden.
“Rooftop veggie patches need around 30cm of soil to grow,” she says.
“If you want the rooftop garden to improve air quality, then look at plants that have a bigger leaf size as they clean the air better than say, succulents.”
For general aesthetics, a variety of different plants sizes or flowers that bloom at different times of year is best, she adds.
Once a green roof is established it’s hard to fix any leaking problems, so it’s important to get this right at the outset. “You can do it yourself but I wouldn’t recommend it,” says Mohorko, who suggests getting the help of a professional roofing person for this part of the process.
In the City of Sydney, and most other councils in Australia, setting up a green roof requires the lodgement of development application.
A structural engineer report and proof that the building is watertight is part of the process.
Drainage and soil
The main function is to divert excess water from the garden and there are a few different products available.
Don’t use normal potting mix soil on a rooftop garden as it stores too much water, says Mohorko.
“There is a special soil, or man made growing medium, that works for these gardens.”
Drainage is also critical as excess water standing on the roof, bears too much weight.
A filter sheet (thin sheet of fabric) should be used to separate the growing medium from drainage.
Essentially a green roof is just like a ground-level garden – only it’s raised up. Maintenance is much the same though. The main consideration, says Sharman, is how you water the garden.
Strategic use of shading and mulch can help, but for rooftop gardens, she recommends the installation of an irrigation system.
“We will be expecting more extreme weather events and on hot days if you can’t get to water the plants they will die – especially shallow green roofs,” she says.
What’s the cost?
Setting up a green roof costs between $150 and $300 a square metre. That takes in soil medium, plants and irrigation systems but not reinforcing the buildings foundations.
The biggest roadblock to having a thriving green roof is forgetting about them. If you can’t see the garden there’s more of a chance it won’t be taken care of. If no-one uses the garden, or it’s not in easy eyesight, go for something low maintenance.