Christ you don’t need a torched felt roof on a garden shed, it’ll probably cost you more than a new shed. A decent quality roof felt, properly fixed should last at least 10 years; my current shed roof felt has been on 7 years with no sign of problems & the roof on a shed at my previous property only needed re-felting after 15 years. Last year I re-felted another 25 year old shed I inherited at my current property & I expect that will now outlast me!