To add a slab, select the Slab icon from the Architecture ribbon. Once in the Slab Sketcher mode, either select the Pick tool and click on a room, to create a slab that takes the room's form; or, use the line tools to manually draw the slab's outline. Click on Apply to apply changes.

To delete a slab, click on the slab to be deleted. Then, either press Delete or select the Delete Slab option in the left panel.