In the UK, especially London and elsewhere in the world there is a growing interest in utilizing roof space for urban agriculture.

There are limitations to this on buildings, mainly due to the load bearing capacity of individual roofs. However some roofs can take the required substrate depths for growing food. Even on extensive green roofs with a depth of 150mm it would be possible to grow native herbs. In fact many plants associated with green roofs in continental Europe, such as Marjoram, Oregano, Basil are classic extensive green roofs plants.

There are only a few examples of food growing in the UK at present, notably RISC, the North Harringay Primary School and the Grays Inn Road Project.

Livingroofs.org in partnership with Buglife – The Invertebrates Charity have two current projects, which will include a element of food growing within the London area.

There are a number of projects in the pipeline in the UK, including a large allotment scheme on a major supermarket in North London.

Reading, UK
RISC in Reading has a very good green roof that is used to promote food growing.

Elsewhere in the world there has been more work on this, especially in North America and Australia. Some notable examples are:

Chicago
http://www.growinginchicago.com/2009/07/nations-first-rooftop-organic-farm-in.html

New York
http://www.bucolicbushwick.com/2009/07/plant-update-v-2009.html
http://newyork.seriouseats.com/2009/05/eagle-street-rooftop-farm-in-greenpoint-brooklyn-nyc-volunteering.html

Montreal
http://rooftopgardens.ca/en/about

Roots Around The World - A Pedagogical Guide on Urban Gardening
Published by Alternatives and the Rooftop Garden Project