trapezium roof shape... which way to run seams

Can anyone tell me the correct way to run panels on a shed roof-trapezium shape (24'x40'x36'x34') that has a 3:12 pitch. I am trying to build a new house and have to redo the metal roof because the old contractor installed without vapor barrier. He ran the panels from the 24' side and ended up on the 36' side with metal edges extended out over the side ... not under the rake? edge. I understand why he did this because if he had put the rake edge over the metal the water would have ran under the rake. I know this is confusing ... but can anyone help me here? I have had several contractors look at this and have gotten different answers from each one. The last contractor says to scrap the metal and go with shingles because this shape of roof will never work with metal. Thanks, Mige