My roofer wants to install a GAF liberty SBS peel and stick roof on top of my existing bitumen roof. His guys already made the mistake of installing asphault shingles so they partially cover the old bitumen roof. He says he wants to cover the old one because it slips under the cedar shingle siding and I guess he thinks he'll have to remove them. I read the GAF Liberty instructions and it calls for a smooth surface (bitumen definitely not smooth) and for the removal of the existing roof. Can't he just tear off the old roof and slip the new sheets under the space under the shingle left by removing the old roof or will that create problems? Is covering over an existing bitumen roof acceptable? How should a roofer be doing this job? Any ideas on how to deal with this would be appreciated. If you're a roofer would you please say so in your reply so when I show this to my guy he'll know it's coming from pros? Thanks!