I built a covered deck last year. For the base, I used deck piers that are available at most hardware stores. To level the deck I used a short section of 4X4 that was custom cut for each position. The deck is 2X8 spaced on 16" centers. At each corner I put in 4X4 posts for roof support. I used a total of 6 posts. One at each corner, and halfway down the length. The posts at the rear of the structure are shorter to alow for rain run off. For the roof structure, I used 2X6 also spaced on 16" centers. The roof itself is corrugated plastic that most people use on greenhouses. I put on the corrugated plastic to allow light through. I also put in a back wall to cover the dirt embankment behind the lot. The bottom portion of the wall is 1X6 fence boards capped with a 2X6 giving a small shelf for drinks etc. The top half of the wall is lattice, held inplace by quarter round trim pieces.
The whole structure is peessure treated and screwed together. I used screws so it can be taken apart and moved. I built it 8X16 because a friend has a float that he uses for heavy equipment, and the 8X16 will fit easily on this trailer. I have a photo, but not on a website that will let me post it on here. If interested send a PM and I will send you the photo.