the "select all" and then "shift and select" option but that doesn't work
Shift+click to deselect something from a set of selection. 
I can't select a surface that is INSIDE a room. when i trie to select it is always the wall of the room that is selected.
By default there is no selectable floor, you need to draw a floor using Create rooms tool. Later by modify rooms section apply the texture.How do i attach my workfile (sh3d) in this reply?.
Follow the instructions by hansmex to attach a file.By simply : Upload the file in mediafire.com
Copy file link(file location). Paste in post when you replay.
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