Also worth noting because someone asked in the discord: The slope doesn't have to be constant but it has to always be positive (or negative). You're allowed to build variable slopes as long as the angle of the slope remains greater than 0 degrees and the full descent has a greater than 4:1 ratio.
This could be considered an extension of the measuring rules - ramps in loops do start on the ground or at the ceiling but they are counted from the ramp start.