I'm writing a paper about flat roofs. I wanted to use the term "walkable roof" to describe a roof that can be used on a daily basis (e.g. a roof terrace), but I'm not sure if this term exists in English language? What name would you suggest for this type of a roof?
As a former building contractor (and native speaker of North American English) I would say that the term "walkable roof" does exist, but does not have the meaning that you want.
"Walkable roof", particularly in trade jargon would mean "a roof that is not too steep to walk on"; ie. slope is less than 8 inches rise (or so) over each horizontal foot. More likely to be used in reference to a roof with some slope than a totally flat one.
eg. "The roofers are replacing our roof; as it is not walkable, they are building scaffolding all around our house!"
I would call the type of roof shown in the photo a "rooftop patio".