Speaking from experience if you intend to stay there then the best way is to
have a pitched roof. You may find when you go to replace the felt that the
wood underneath is rotten. Yes it is dearer bur with our peculiar UK climate
a pitched roof is the answer. It is all right in middle Eastern Countries to
have flat rooves where they have little rain and even less snow. Any way
when I lived in the Far East metal on the roof was far too hot but it did
stop the roof being eaten by insects. As others have said the pikeys would
have the roof away in no time. They are already nicking the lead from Church
es and drain covers and even road signs
Robbie