I did a similar PT ramp out an 8' x 8' side garage door dedicated for the tractor. I use 2x10s and made a "diagonal" cut for the slope starting off the tops. So one end is the full width and the other is whatever is left based on the slope. For the 8' entrance I used 5 supports. I then screwed the diagonal pieces centered on a PT 2x6 which is sitting on the ground, basically forms an inverted "T". The 5 supports were then spaced accordingly and a 2x10 screwed to the tall ends to be mounted to the building. I screwed a 2 x 4 to the front ends to basically secure the assembly. I then drilled anchor holes into the foundation to mount the assembly to. I used 2 x 6 boards for the decking.
The key is to get the ramp area graded (height) for the assembly to sit tight to the building. Once graded I ran the tractor back and forth to compact the area before securing it to the building. The nice thing is all the weight is just on the ground. I didn't take any pictures when I built it but hopefully you get the idea.