Everything looks fine, but I would tie the 2x6 into the into a double 2x10 on the high and low side. Depending on where you live will be the big reason for determining pitch and spacing on the rafters . Once all that is tied in, I would mount 1x4s to the top of the 2x6 no more than 3 ft apart going up the rise. You can let the tin stick out 6 inches over the boards length wise. They can also width wise that much as long as there is a 1x4 under it.
Will the 2x6 span that 18 feet? I am doing something similar for a lean to and thought 12-14 ft was as far as I could get.
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Have you considered steel?