I have a shed that was built on the back on my carport. the carport is on a concrete slab but the shed's foundation was 2x6 with plywood on top. There were a few concrete blocks under the 2x6 foundation to prevent it from sinking, but it didn't work. The shed had sunk about 4 inches on the back wall. The carport is leaning in the direction of the shed because the concrete slab the carport is on is not sinking so the shed is pulling the carport as it sinks. I planned on just jacking the shed up and putting more blocks under the back wall, but the 2x6 floor joist were all rotten. I had to run a double 2x10 beam under the roof joist along the top of the back wall and then support it with 4x6 post. I was then able to remove the old rotten floor.