text: poly is for reainwater guttening explain the reasons? why guttening made from iron why polythene a better? poly there is used for reainwater guttening. explain the reasons for it's use? some olther guttening may be made from iron explain why polythene would be a better choice? the 'poly' you are referring to is polyvinyl chloride. it has a number of advantages over iron. 1. iron corrodes (rusts) and requires painting every few years to protect it. 2.  iron is heavy and so heavier stronger brackets and support is needed and will be physically more difficult to put up. 3.  polyvinyl chlride is much easier to cut to any required length. 3.  polyvinyl chloride is cheaper than cast iron