Is this a 'Roof Sweat' problem and what is the answer?

[alwaystryitonce]

Any vapour barrier should always be fixed to the warm side of any construction. The problem is almost certainly a case of condensation. Pictures will confirm it if you manage to upload them. Mould spores are constantly in the air, for mould to germinate there must be a source of moisture nutrition and oxygen. Obviously, oxygen cannot be removed and nutrition for the mould to feed off will be found in most building materials. The only thing to resolve the problem is to remove the moisture , which has appeared from condensation. Whenever any item is stored in such a way that it will block a flowing heat source from reaching a wall it will make this area cold. When any warm moist air does reach the cold spot, it will meet a temperature at it's dew point and condense. Causing moisture. If the problem is a roof leak, the problem will get a lot worse in rain. My advice, keep a free flow of air wherever possible, a loft space is, believe it or not, not for storage. Upload pics when you can.