These are the item at Lowe's...Not sure if the composite panels are as durable as steel which also comes in several colours FIL had a porch off the back of the house that used the composite panels - sprung a leak after only a couple of years. It was tough chasing down the source of the leak. Your shed roof would be significantly smaller but with it being under trees, I would think steel would hold up better to the potential of branches dropping from above.
http://ondura.com/products-ondura.shtml