Only the top surface of a mesh can have a slope.
The underside is always flat.
2 workarounds.
1 - ensure the mesh is deep enough to allow for the fall.
Duplicate (copy & paste) the mesh and alter the elevation of this mesh down by how thick you want the drive to be.
Then do a Solid Element Operation and subtract the lower mesh away from the higher mesh and hide the lower one on a layer that won't be seen.
2 - Make the mesh you require and then use the Mesh To Roof add-on to convert it to roof planes (then hope you don't have to edit it - keep a copy of the mesh in a hidden layer just in case).
Barry.
One of the forum moderators.
Versions 6.5 to 27
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