sedum roof for a small shed

I found a great site which I'm hoping to order from: http://www.enviromat.co.uk/home?gclid=CNHj9f6ChrECFaEhtAodERtMAw

...but my question is what exactly I need to buy, with a roughly 2msq roof that slopes to one side.

Will I need a water retention mat so the sedum doesn't dry out, or rather a drainage mat so the sedum doesn't get waterlogged? It is quite an enclosed garden and the roof will get plenty of sun (when it finally decides to emerge, this Summer!) 

No idea, as never had a living roof before, but it looks great for covering an ugly bit of the garden.

Thanks for your thoughts!

Sedum is dry-tolerant so it wouldn't want any water retention mat, but from the website  it looks to me like it comes complete with growing medium. There's also a video and files to download to help you

thanks yes, it seems to come fully planted on its own substrate which apparently contains a lifetime supply of nutrients - wonder how that works!