This works:
[area delete where item z = 50
This will delete all items that have a z value of 50 in the area you draw with the bounding box picker.
Also for your example about roof tiles and floor tiles
[area delete where floortiletype
With 'floortiletype' being the type of floor tile that you wish to delete.
Also if you only wanted to delete a certain type of floor tile on a specific z then you can use:
[area delete where floortiletype z = 50
That should be everything you need really
