The maximum slope for a ramp is 1:12.
This means that for every 1 inch in rise (the height of the ramp), the ramp should have 12 inches of run (the length of the ramp).
The optimal slope for a ramp is 1:20.
The Center for Universal Design at North Carolina State University has a publication titled, "Wood Ramp Design: How to Add a Ramp." You can find it at: http://www.design.ncsu.edu/cud/pubs_p/docs/rampbooklet296final.pdf.
