Click Show/Alter State and choose Diaphragm from the Attribute list.
Slab items, roofs and nodes are color coded according to their Include in diaphragm setting.
In the Properties window, set Entity type to Slab Item or Roof.
Do one of the following:
Click an individual slab item or roof panel to switch between including and excluding the element from the diaphragm.
Hold down the left mouse button and drag a box from left to right to switch between including and excluding slab items and roof panels within the box from the diaphragm.
Hold down the left mouse button and drag a box from right to left to switch between including and excluding slab items and roof panels within the box or cut by the box from the diaphragm.