How to elevate a GDL object with one end fixed to the ground floor?

[Partha02]

I am facing an issue putting the gradient of a GDL object.  

For this, I use the SLAB element and also the ROTX command to put a certain amount of gradient of the object, which works fine for the fixed no of points.  But when I draw some lines on the ground floor of Archicad and pass their information to create the object, then there is an extra elevation to the object which is unexpected. Can anyone tell me how this extra elevation is happening?