Have a look at this topic over th help forums: http://www.google.com/support/forum/p/sketchup/thread?tid=6c575f696884e0af&hl=en It seems that if you sign up for the trial of IES VE-WARE, you get a promo code worth $ 100. It's an old thread but someone posted that it still worked yesterday.

TIG, Thanks! I will do as you suggest! Makes perfect sense and should work intersecting two roofs first then cleaning up....