the risc roof garden
We can blame Pip French for RISC's crowning glory. He came up with the unlikely idea of creating a green roof above the leaking conference hall. The garden is now so much part of RISC's identity that it is easy to forget that it was a leap into the unknown... could fruit and nut trees grow on a foot of soil, would the structure take the weight, how could we integrate it into our education work... ? We should have trusted in nature. The garden gradually revealed its secrets – the countless stories embedded in the plants and hard landscaping that attract visitors from far and wide.


Roof Garden Design

The design brief for the 200m2 site (32x6m) emerged from brain-storming sessions among the Collective: a garden which could be a tool for making connections between the local and global, including sustainable development, as well as the economic, cultural and historical importance of plants. We collaborated with Paul Barney, a local permaculture designer, who adapted the forest garden idea, championed by Robert Hart in the 1970s, to our site and needs.

Our design takes into account every aspect of the site, surrounding buildings and makes full use of the local resources. For example, the Centre produces large amounts of organic waste which are composted and help feed hungry plants, while minimising landfill waste. In exchange, herbs and cut flowers are used in the café. Water from surrounding roofs is harvested for the drip-feed irrigation system which is powered by a small wind turbine and photo-voltaic array. The hard landscaping uses a combination of reused, renewable and recycled materials – old bricks destined for landfill, paths made from woodchip and edged with cordwood (tree surgeons’ waste which would otherwise be burnt), fencing and raised beds made from locally coppiced hazel and willow.

The forest garden is a variation of the permaculture approach – using a carefully selected combination of perennial herbaceous plants, shrubs, trees and climbers in a planting scheme which mimics a multi-layered woodland ecosystem. This creates the conditions which support great diversity. Once established, forest gardens require a little pruning and lots of harvesting from early spring to late autumn. Conventional vegetable plots can also be included. The use of a 75mm layer of mulch and ground cover plants, such as herbs and strawberries, helps to conserve moisture and suppress weeds.

Our design features two winding interconnecting paths which create ecological niches for forest-edge plants, but also provides enticing access for visitors, especially children. It had to accommodate assorted skylights and ventilation ducts and shade cast by a building on the south side of the plot. Areas at either end of the site receive full sun for most of the day. One is planted with sun-loving herbs in a raised bed which surrounds a small area of decking made from local wind-blown oak.

Watch a short film about RISC’s forest garden

download an article about the garden

download information on the garden's sustainable features

download background information, including reading list and suppliers


The key technical elements in a green roof are loading and root/water proofing. Luckily for us, the roof that Blackwells used to fill the space between the original Victorian structures was made from large rolled steel joints mounted on reinforced concrete piers. A structural engineer calculated that it was strong enough to take 30cm of substrate plus hardlandscaping, plants and people. The drainage system was also easily adapted for the rainwater harvesting system.

Martin Mikhail, RISC's resident fixer, managed the various stages of the build. We employed contractors, MJ roofing, to repair the existing roof and install the elements required for a green roof. The skylights were raised to accomodate 30cm of soil and three layers of special bitumen felt, impregnated with a herbicide to prevent penetration by plant rootlets, were laid on a base of marine ply. A layer of polysterene insulation was then put down (although soil will provide good insulation when dry, it conducts heat when wet), followed by a plastic drainage/reservoir layer which allows water percolating through the substrate to flow towards the outlets and into the rainwater harvesting system. The lightweight substrate was placed on the roof by a forklift truck and spread by hand.

Our builder, Steve Hunter, took care of the hardlandscaping – laying walls and paths, and putting up the hazel fencing. Local fabricators made the staircase and railings, and Jessica Witchell, another RISC worker, coordinated a team of volunteers to implement the original planting. The wind turbine and photo-voltaic panels (inherited from Caverham Court Environment Centre) were installed by Ernest Warren of Hitech Energy and subsequently reconfigured by another volunteer, Stephen Loundes.

download a cross section of the roof


The initial plant list had about 120 different species – mainly perennial plants from around the world, most with multiple uses: food, medicine, fuel, fibre, construction, dye, scent – chosen to generate maximum interest for visitors, from school children through to garden experts. They include the full range of layers, from roots (oca Oxalis tuberosa, American groundnut Apios americana) and ground cover (strawberries Fragaria, herbs) through to climbers (hop Humulus lupulus, kiwi Actinidia arguta, grape Vitis vinifera), small shrubs (lemon verbena Aloysia triphylla, blue sausage tree Decaisnea fargesii, Chilean guava Ugni molinae) and taller trees (cherry Prunus avium, Japanese raisin tree Hovenia dulcis). Most of our fruit trees are heritage varieties dating back to Victorian breeders and beyond. Notable for their taste (though not for the uniformity required by supermarkets), they will also enrich the gene pool which will create new varieties which can cope with the weather brought about by global climate change.

The garden has matured well – looking at the dense foliage, it's difficult to remember the early years when spindly saplings swayed above little clumps of green and oceans of woodchip. Ours was no instant show garden! Most of the original plants thrived but some, eg currants, struggle and a few have died. The garden has developed its own personality and follows its own muse, occasionally shaped by Mary's eye and trowel. We have added new plants – the Agroforestry Research Trust's catalogue is just too tempting – and the list has now grown to over 185 species.

A booklet about the garden, including recipes and selected plant list with uses is available from RISC's World Shop.

download a low resolution version of the roof garden booklet

download the full roof garden plant list

download a list of herbs

listen to an interview with dave on the alternative kitchen garden