dang. thanks.
again - this fill isn't for a wall, but for a floor/ceiling in a relatively unfinished (interior) single story building. i have a floor of 2x10s plus plywood decking. i wanted to use the same fill and reverse its direction to be used as a ceiling (plywood decking instead of, say, gyp). incredible that something that simple isn't an option.
on a side rant, it's incredible that i need to use the roof tool to create a sloping slab (for a driveway). i don't really understand the benefit to having a specific tool for slab, or roof, or wall. it's really all the same, just defined by directionality and 'fill'. archicad would have a much friendlier interface and would be a much more useful product if a simple mindshift like that took place at graphisoft.