What do you call the Japanese practice of placing gardens on top of buildings in major cities?

I saw this once on a Japanese show, they have a law requiring that at least 20% of the tops of buildings should be covered in greenery.

Answers:
Rooftop Gardens.
I think it is a FANTASTIC idea and should be required on all new large buildings.

Other answers:
Zen garden
Zen garden
This is because of lack of space in Japan. The cities are extremely crowded.
rooftop gardens?
I have also heard them called "roofscapes." Great concept, as long as the roof was built to withstand the weight.
I don't know if it's a Japanese concept but "Green Roofs" is a hot topic in Toronto right now. You could try toronto.ca as the City is promoting the idea.

This is more than a rooftop garden. It's quite complex and REALLY expensive but environmentally great.
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