What kind of roast do you have ? As BDSkelly stated, bark will occur during the smoking process IF given enough time. But, if you are smoking something like a ribeye, top round, or tenderloin roast that you only want to take to mid rare, it won't be in the smoker long enough for a good bark to develop. With something like that, you'd want to smoke first and then finish with a reverse sear to get a nice crust.