ponding

Your thoughts please Provided suitable packing and longer fixings are used, will fixings on the ridges of box profile sheet (instead of the recommended valleys) cause problems ? Reason for asking – I want a detached garage roof slope to be less than the recommended minimum for box profile...

Flat asphalt roof dips/ponding between each joist. Could I cover with Torch on Felt, or should I level the dips with some material so felting on level surface? The flat roof covering could be felt I will upload a pic asap. But same question applies.