If I understand your question, you’re looking for a way to enclose an area under your deck, which is 9’ off the ground, so you can have a screened-in porch which would be dry when it rains. Am I right?
I have done this on both my last house and my beach house. In the case of my last house, I wanted to build a shed under my deck to store the usual stuff, but it needed to be dry. Like you, I did not want to rip up the existing deck to do it. My deck was also about 9' off the ground.
What I did was to use corrugated fiberglass panels. I attached them underside the deck joists. At the end by the house, they were basically attached to the underside of the joists, while on the outer end they were attached to the bottom of the supporting beam (my deck was cantilevered) which paralleled the wall of the house. In this way, the fiberglass panels sloped from the house to the outside of the deck.
After that, I built my shed walls more or less free-standing from the panels. I did add a support down the middle of the panels (they were 12’ long) to keep them from sagging. The shed was there 7 years before we moved, and it never leaked.
Now, in my case, there were no posts in the middle since the deck was only 14’ wide from the house, and about 12’ from the house to the support beam.. This meant the panels had no obstructions in the middle to work around. I built the shed about 10’ wide so that the edge of the panels hung out from the side of the shed. It was my intention originally to run gutter along that edge, but I finished around the perimeter of the shed with a planter box, and that worked fine. I would suspect you will want a gutter.
I used white panels in the hopes that it would add light to the shed, but the panels were too close to the joists, so I added windows. They (the panels) also soon looked dirty from the inside of the shed. I would use dark colored panels. The shape of the corrugated panels allowed for easy interlocking, so to speak, and it never really built up large amounts of debris, which was a pleasant surprise. They kind of cleaned themselves.
The beach house was a different plan. I have a 2-story deck, and all I wanted to do was keep the lower deck somewhat dry so we could sit out there during summer rains without raindrops running into our chips and beer. The panels are working fine there as well.
If I did not do a good job explaining, let me know and I will try again. Good luck.