In article ,
says...
Following up to Charlie Pridham wrote:

so I would be interested in how the weight would be distributed and
supported

two layers of felt above PVC sheetings on a metal framework, weight
is 30kg per square metre. Costs between ?600 and ?1200. book "vertical
gardens: Bringing the city to life"

That makes sense, so all the weight is not hanging from the top.
I think its a good idea and along with green roofs would help insect
populations in towns which in turn helps all the other wild life.
--
Charlie Pridham, Gardening in Cornwall
www.roselandhouse.co.uk
Holders of national collections of Clematis viticella cultivars and
Lapageria rosea