Using underlays for plans

On any floor plan, by default, the storey's plan immediately below the active storey is displayed as an underlay. The underlay can be used to trace out upper-storey plans. You can snap edges or vertices of rooms to those in the lower storey; you can also snap walls and slabs to objects on the lower storey.

To turn the underlay off, you can turn the lower storey off. You can restore the underlay anytime by turning the lower storey back on.