Try using the Mesh 3D V2 library part from the object depository, this should give you the opportunity to specify the thickness of the ramp, which the standard mesh will not. I have not used this mesh 3d v2 extensively, so am not 100% sure of what it can do, maybe someone else here can confirm or deny that it would be possible.
-There are 3 ramp library parts under concrete.
-using stair maker you can create a ramp rather than a stair
-Objective allows you to tilt the ramp sideways as well as sloping it up; see recent post here
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"Implementing Successful Building Information Modeling"