Swayback Gable Roof?

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Yeah, if this will be a tear down, then all you really need is an exterior render of it. So you should simply use a symbol for it and place it on top of the walls. Could do it fast and realistic in Sketchup and import it. Another thing that popped into my head would work really well if you don't do your final renders in Chief.

I do the basic 3D render in CA and then use Thea/Blender/CrazyBumps for final renders instead of CA raytrace. If you go that direction, you can simply create a normal straight roof and create the sway in a program that is better suited for it.

Finally, if you aren't like me and prefer the less time consuming process, you can always create your raytrace and then import into photoshop and warp the roof. Obviously that won't work well if you are doing a walk-through or sitting down with someone and manipulating the view in real time.