[Trapezoidal Staircase]

Very interesting! I made my staircase with the Simple Stair tool and I did not get the empty geometry. I tried the same thing with the Stair tool and got the same result as you. It seems as though the Simple Staircase automatically resolves this issue!

[Trapezoidal Staircase]

Hi!

 

I created a regular staircase using Simple Stair. From a Top/Plan view, I used the Polygon tool to draw triangles over the parts of the staircase I didn't want. Next I extruded them to be the same height as the top tread. Finally, I used Subtract Solids to remove the triangular prisms from the staircase. 

[Trapezoidal Staircase]

Figured it out. Thank you for your help!

[Trapezoidal Staircase]

Does anyone know how I can make a staircase that has wider width at the bottom and a narrower width at the top so that it forms a trapezoid from an aerial view?