Green roofs look better while saving money and water

In one of those curious twists of fortune, Sean Johnstone came to Hong Kong in 2011 with Japanese steel giant Kawada Industries to compete for contracts to supply heavy steel for Hong Kong infrastructure projects. That exercise was relatively unsuccessful as the Japanese currency, at 76 yen to the US dollar, was the strongest it had been for many years.

Now Johnstone spends his time marketing a sustainable roof greening system but is still working for the steel company. Johnstone is the general manger for Kawada's overseas operation and in the course of his presentations here would mention almost as an aside one of Kawada's green initiatives, Midori-Chan, which has been operating in Japan since the 1990s. It is a patented system designed to maximise vegetation growth and recycle waste, while dramatically minimising water usage.

It came about as a combination of concern over the huge amounts of formwork the company was paying to have taken away after casting steel, together with a CSR initiative. The formwork was reduced to a mulch and formed the basis of Midori-Chan.

The interest in Hong Kong was to use it to provide "greening", particularly for roofs. In an attempt to encourage green buildings the government will allow developers a bigger gross floor area if they can fulfil various requirements, one of which is a green roof. There are a number of competing systems but the standout feature of Kawada's is that it requires virtually no maintenance. So the garden is watered when it is set up and then requires no further watering. It might require weeding once or twice a year.

While the upfront costs may, though not necessarily, be higher than other systems, costs fall dramatically over the life of the garden since there are virtually no maintenance costs, nor is there a need for costly irrigation systems.

"Over a 10 year period a 100 square metre roof from a typical system will use 2.5 million litres of water, while our system will use 2,000 litres," says Johnstone. Tests show that Midori-Chan uses 0.17 litres per square metre per month, while some Hong Kong government specifications for green roofs specify 190 litres per square metre per month.

The water savings are not insignificant given that the cost of water must increase over time given the increasing demands on it. Midori-Chan roof gardens have been installed at the West Kowloon Cultural District, the Water Supplies Headquarters, and more recently on the roof of the new Kennedy Town MTR station. They will appear on more MTR projects shortly.

The garden comprises plastic trays made of recycled plastic that are then covered with a special charcoal from Japan which is made from either old formwork or wood from forests that have been thinned. This is then covered with a specially prepared soil - the "secret" ingredient - and this in turn is covered by a mulch. Rainwater is trapped in the plastic trays but is filtered as it passes through the charcoal, ensuring there is no odour or bacteria. Because water passes through the mulch and soil fairly quickly the top surface is dry so it does not encourage insects such as mosquitoes to breed.

While Kawada provides the hardware and the design, it has partnered with Birkdale International, a firm of landscape architects, that prepares the soil and installs the gardens. Indeed, with a system like this the Hong Kong government could save millions on maintenance for its various strips of garden along the middle of the roads, roundabouts and parks. A green roof can reduce rooftop temperatures by up to 30 degrees centigrade and the temperature of the room below the roof by 3-15 degrees centigrade.

They also look a lot better.