To create a sloping slab panel you must first define a Plane to align it to. Once the plane has been defined the slab can simply be moved into the new plane.  This method can also be used to model inclined / slanting beams.

We will use a simple table top model as illustration  (as shown below).

1. Press “Plane Definition”   button in the top Modelling tab
   
2. The “Plane Properties” form will be loaded.
   
3. Left-click in the region of the slab and the plane will  be inserted.
   

Initially, as shown below, the plane is placed horizontally at the storey level - the level and inclination being controlled by three node points at the corners of the plane (“Point: 1 “, “Point: 2” and “Point: 3” fields).  A plane is completely defined by 3 points so the 4^th point is calculated internally & not shown in the dialog.

When plane member is first defined, all the values in the “Z” fields will be zero. This means that plane is parallel to the storey level and at the same level. If you enter a positive value to the “Z” field of a corner point related corner point will be raised by that amount relative to the storey level. A negative value will lower the point.

1. Input Point 1 Z = -1000, while leaving Point 2 and Point 3 = 0mm > Update
   This effectively means Point 1 (& invisible Point 4 ) is lowered by 1000mm
2. Right-click > Move member to Plan Definition > Yes to confirm.
   
   

As shown below, all members contained within, or at the edge of the plane are adjusted to the plane.

If you bring out the Beam Properties of the slanting beam > 3D tab, the values of the del Z-I = -1000mm & delZ -J =0 are auto calculated.  These values are associated (locked) to the plane P1 and cannot be changed. If required, either I or J end can be dissociated from the plane by clicking the X sign.

Similarly, the Column Properties > 3D tab of the column will reveal del z (Top) = -1000mm &  it is locked to plane P1. Similarly, it can be dissociated from the plane by clicking the X sign.