Then we will shingle it, as that is cheaper then metal...
This surprises me -- unless you mean metal installed over plywood instead on purlins with no plywood underneath.
With the cost of plywood we saved a lot of money by using metal on purlins instead of shingles. Saved a lot of time too -- the metal roof went up in a day with a crew of two men and a boy (15, man-sized and very handy).
I *think* you can see the underside reasonably well in this photo.
Note: We blocked between the rafters to make installing wire easier because that ventilation was not needed in our Open Air design. When soffit ventilation is needed you don't put in those blocks. @aart has photos of how to install the wire in that case.