I think the background for sketch render is treated more like 'paper' that's being drawn on.
You will have to mask out the parts you don't want in a photo editor like photoshop or similar.
An easy way to do this might be doing a white model render without shadows and combining this with the sketch render.
Erwin Edel, Project Lead, Leloup Architecten
www.leloup.nlArchiCAD 9-26NED FULL
Windows 10 Pro
Adobe Design Premium CS5