You can associate soffits with other elements, such as walls and a roof. If a wall or roof changes or moves, the soffit adjusts accordingly.
You can also create soffits that are not associated with other elements. To create non-associative soffits, use the Lines tool while in sketch mode.
Create a sloping soffit by drawing a slope arrow or changing properties of the boundary line. See Sloped Surfaces.
This tool creates a locked sketch line.
To see the soffit better, create a section view through the plan view that shows the wall meeting the roof.