14 October 2010 – According to Sidonie Carpenter, president of Green Roofs Australia, the owners of two outstanding green roofed buildings in Toronto Canada claim they enjoy a 15 per cent premium on their rents and a 15 year waiting list for tenants.

A look at the websites of these two, www.401richmond.net and www.robertsonbuilding.com makes the claims seems plausible. Certainly Carpenter believes the financial case for green roofs stacks up in more ways than one.

First there is the looming carbon price and the opportunity for “some carbon payback and sequestration opportunities and we start putting a dollar value on that,” Carpenter says.

Next is the very little understood information that a green roof, instead of taking up space where the solar panels might go, will actually enhance the efficiency of photo voltaic panels – by a fairly sizeable margin, in fact.

According to Carpenter when it’s too hot on the roof you get “brown outs” and the solar energy system won’t produce anywhere near as much energy as promised because the system can’t cope with the heat. It works best at between 23-25 degrees Celsius.

“Solar panels work more efficiently over a green roof. You get 25 per cent more efficiency because it reduces the ambient temperature,” she says.

“Even in Hobart it would be over 25 degrees Celsius in summer so you get brown outs.

“In Brisbane even in winter the temperature rises to up to the 30s and on a high rise building in summertime the temperature gets to 70-90 degrees.

“Which is why countries like Germany are leading in photo voltaics. They don’t actually work in very high temperatures on the roof. You’re better off putting them on the ground.”

Carpenter says there is now a lot of research coming out of Germany that combines photo voltaics and green roof analysis because of the benefit of evaporative transpiration, [which is like the cool moist atmosphere that a rainforest creates.]

Green roofs can also help reduce the load on airconditioning of having to cool air of 70 degrees so it can be used inside a building.

So how much do green roofs cost?

To Carpenter that’s a bit like the piece-of-string question. It depends totally on what sort of green roof you want – and that’s anything from a lightweight solution to a full deep-soil garden that requires special structural support. But generally most jobs range between $60 a square metres to $100 a square metre.

There are times, of course, when the project is simply uneconomic. As in her own house, for instance, a “small domestic house, where the quote for the structural steel alone came in at $75,000.”

Did she go ahead?

“No, we did not go ahead with it. It would have meant no bathroom, no kitchen. So we just did a light weight retrofit on the existing roof.”

Another job currently underway at Balmoral in Brisbane may also not see the light of day, because of cost blowout, thanks to a steep site that is difficult to access and requires soil and all other elements to be craned in.

In this case the costs look like blowing out to $2000 a square metre for each of the 60-70 square metre roofs over each of the five dwellings,” Carpenter says. “And the budget is $200,000.”

A green wall, by the way, generally costs between $1200 to $2000. [This might seem pricey but at a recent conference that she spoke at – the 3rd International Urban Design Conference in Canberra – Carpenter expressed mild exasperation that while the cost of structures and maintenance in other parts of a building structure are rarely questioned, the costs and needs for the same in a green roof or green wall were so often viewed as onerous.]

Carpenter says the most expensive green roof in Australia is probably the one over the Victorian desalination plant at Wonthaggi, recently announced in a tender awarded to Fyto Green, at a cost of $4.3 million.

The biggest is undoubtedly over M Central in Sydney’s inner west at Pyrmont, she says.

In the US the Ford motor plant has a 24 acre (9.72 ha) green roof but elsewhere around the globe there are the beginnings of radical experiments – with high-rise farms in Harlem in New York and paddy fields in China.

Carpenter says that there is also great opportunity for large industrial sheds to lower their carbon footprint, especially when so many are occupied by  high profile corporates such as Coles and Woolworths under the spotlight for their high energy use.

“I think that’s where there is opportunity, for companies such as Coles and Woolworths, shipping their goods all over; their food miles are huge, and their carbon foot print is huge.

“They could start to cover their industrial buildings with very lightweight green roofs that produce carbon sequestration. We’d be looking at very shallow soil profiles – they are the ones that would really make a difference -especially with those large industrial estates, where there isn’t a blade of grass to cool the ambient temperature.”

And most, adds Carpenter, could be done for around $100 a sq m.