Use this procedure to cut a slab or roof slab into two or more slabs or roof slabs.
You can use any 3D object or polyline to define the cutting geometry. If you use a 3D body, such as a wall, slab, mass element, or closed polyline, the slab or roof slab is cut along the intersection lines of the surfaces.
The slab or roof slab is cut only when the 3D body or closed polyline intersects its perimeter line. If you use an open polyline, the slab or roof slab is cut along the projection to its surface.
In either case, when the intersection line crosses the slab or roof slab multiple times, multiple slabs or roof slabs are created. If the intersection line has multiple edges, multiple edges are added to the new objects.
The smoothness (tessellation) of curved edges is controlled by 2 AEC display resolution settings called Facet Deviation and Facet Maximum. Beginning with Autodesk Architectural Desktop 2007, you can adjust these settings after the slab or roof slab is created. Also, the individual segments or facets making up a curved edge are no longer treated as separate edges; each arc on the perimeter of a slab or roof slab is treated as a single edge.
- Draw an object or polyline for defining the cutting geometry.
- Select the slab or roof slab to be cut.
If the object selected is… Then… a slab click 
. 
a roof slab click . 
- Select the objects or polylines that represent the cutting geometry, and press Enter.
- Enter n (No) to keep the geometry, or enter y (Yes) to erase the geometry.