[jentron]

You want to cut the chimney, so set the roof as the knife. In this example I manually deleted the 4 chimney surfaces inside the roof.

Also, set your surfaces to single sided so you don't get the bright lines around the edges.

Ron

P.S., real chimneys go through the roof, they aren't sliced to sit on top. And they are sealed to the roof with tin patches and tar.