If you are going to build to 2.5mts at one side anyway would you not be as well having a pitch roof? It might not be as much of a slope as you wanted but it would Still be there. The metal roof is not really that noisy and the roof sheets are screwed down on top of the outside sheet of timber anyway.
Photo below (left)shows my workshop roof. The photo on the right shows my nearly complete bike shed with the same type of roof. The roof has the outer sheet of osb, 100mm of poly insulation, then another sheet of osb inside so its well insulated for heat and sound.




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