Green roofs on Garden rooms have been around for a while and Anna Pavord in the Independent says “The advantages of green roofs have been drummed into us pretty thoroughly during the past couple of years. But we've still got plenty to learn about maintaining them. The self-sustaining ideal is not as easy in practice as it appears on paper.
“I'm a little bit in love with my green roof, which sits on a tiny shed in my garden, like a raised bed. The shed is so small that the roof is at head height. This gives me a wonderful view – I grow mainly spring-flowering woodland plants such as primrose and bluebell, which I can see close up without having to crouch down. I've watched tiny tortrix caterpillars chew holes in primrose petals, hairy footed flower bees zip between oxlips and large moths taking nectar from red campions. The image shows a Sedum roof on the roof of a Henley Stanza
and we are happy to help you with a price guide, however if you want to research them in depth then the UK's national centre of excellence for green roofs is located at the Department of Landscape at University of Sheffield - http://www.thegreenroofcentre.co.uk/ where they have loads of information and handy spec sheets to download - http://www.thegreenroofcentre.co.uk/green_roofs/downloads