In week beginning 07-Sep-2015 I did an internal renovation of an outhouse on my property.
This outhouse had a roof but all the walls were constantly damp, because of trees and other planting of my property.
I decided to "tank" the walls and floor using plastic sheeting. Into of that I screwed battons to the walls and created sub floor.
The roof should still be replaced as it ends inside the outside of the external walls (meaning rain drops into the top of the walls and you cannot install guttering).