Rain water is like idle teenagers!
Rainwater is like a teenager, if you let it stand around it will find a way to get into trouble. The answer is to keep it busy and the best way to keep it busy is with a decent fall across your roof.
Different roof pitches mean you can use different roof coverings.
Really flat like 1 degree ( 18mm fall over every metre) needs a concealed fastner roof material like the splendid Kliplok . I am fairly sure this product name was started by someone called Jaymz.
Reasonably flat like 3 degrees ( 53mm fall over every metre) can use a screw fixed square profile roof like Trimdeck. If you have a few careless tradesmen on your roof you will discover very quickly why this prodiuct has earned the nickname Dintdeck!
Not at all flat
A roof of 5 degrees or more (88mm fall over every metre) can use corrugated iron. This wonderful material was invented by an Australian who was anxious to show a new iron product in England. He couldn’t afford the cost of freight in addition to his own fare, so he rolled the iron up and put it in his steamer trunk. When he got to Mother England it sprung forth like a big afro and corrugated iron was born.