When I clad the hidden side of both my garden room and garage I used polyester painted steel sheets, which you can order cut to length (because they are a PITA to cut yourself). Screwed to battens with special screws they are cheap and very durable, and actually look nice too. https://www.roofingsuperstore.co.uk...pvc-plastisol-coated-07mm-goosewing-grey.html
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