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REVIT can instantly turn MASS object Faces into Walls, Floors, Roofs and Curtain Systems with the out of the box 'Building Maker' tool set.
In case you are not already taking full advantage of these 'Building Maker' tools you should have a look at video tutorial below..
(click to play or right click and 'save target as' to download)
http://www.cadclips.com/downloads/RAC-2008-MASSING-1-BLDG-MAKER.wmv