If the slabs are in different storeys then you need to set their floor plan display to 'Show one storey up' or 'Show one storey down'.
Then you will be able to select the walls and slabs all from one storey.
You may need to use the TAB key to cycle through overlapping selections.
Or place a all-storey (fat) marquee and view in 3D.
Then select the walls and slabs and then 'view selected in 3D' so only the selected items show.
Barry.
One of the forum moderators.
Versions 6.5 to 27
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
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