The Split Face tool splits the selected face of the element; it does not change the structure of the element. 
 Video: Use the Split Face and Paint Tools
 Video: Use the Split Face and Paint Tools 
 You can use Split Face on any non-family instance. After splitting the face, you can use the Paint tool to apply a different material to this section of face. See 
  
Applying a Material to the Face of an Element. 
  
Note: The Split Faces tool does not work with walls that were created with an arc, ellipse, or partial ellipse.
  
To split a face
- Click Modify tab Geometry panel Geometry panel  (Split Face). (Split Face).
- Place the cursor on the element face to highlight it. You may need to press 
  Tab to select the desired face. 
- Click to select the face. 
-  
  Sketch the face area to split. 
  Note: The sketch must be in a closed loop inside the face or an open loop that ends on the boundary of the face. 
   
- Click 
   (Finish Edit Mode). (Finish Edit Mode).
 
Tip: You can split the face of a column. However, if you plan to have multiple instances of the split-face column in your project, create the column in the Family Editor and apply the split there.