shed roof question

I am finally getting close to putting up a shed, but the main question mark remaining is the roof. Originally I was going to build a rather large shed towards the back corner of our back yard. That would sort of ruin the view out of our kitchen window, though, and I really don't need one that large.
There is already an existing cement slab up next to the house where the former owners must have had a small shed. I have decided to expand this a bit, using 4x4" posts sunk into the ground. To make things quick, I'm just going to use treated 2x12"s to make the sides and sloped roof. After that, I can put on metal siding as I have time over the next couple of weeks.
The roof troubles me, however. As I said, it will be a single basic slope, higher on one side and lower on the other. Complicating things a bit is the fact that I want the wood posts to continue on up past the roof, so that next spring perhaps I can build a wooden deck on it. I could transition our master bedroom's window into a door, and then we could have a private walk out deck. The deck surface would be up above the shed's roof.
At first I thought I would just put on a metal roof. However, I would like the roof of the shed to stick out past the walls, which means that the posts would have to be sticking up through the roof. That would entail quite a bit of metal cutting, and I'm not sure I'm accurate enough with the tinsnips, especially with corrugated metal roofing.
I'm wondering what other options could be considered? Since the roof will be covered up by the decking, it would need to be very long lasting. (the roof wouldn't be accessible after putting on the deck above it)
Thanks for any suggestions!
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Ohioguy wrote:

Hi, Isn't that working things backward? I'd build deck first with roof and use the space enclosed below as storage. Here building deck requires permit.
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I built a deck about 12 years ago using composite material. I think it was called DEX. Still good shape. I laid all the decking with deck screws from top side. (deck is 13 x 42 and 12 foot above ground. This year I wanted to park our van under it and of course the rain goes through the spaces between deck boards and this made a mess on the van. Also wanted to build a 12 x 13 shed under deck. No problem. Removed deck boards to be able to add a roof under deck to cover van and shed. Then replaced the deck boards. If I had used the under mounting brackets to lay deck I could not have done this. WW
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