Prime rib roast needs to be grilled using indirect heat in order to avoid burning it, according to the recipe provided by Char-Broil. Remove the roast from the refrigerator, and allow it to sit for approximately one hour before applying the dry rub mix.
Mix together 1 tablespoon each of salt and black pepper, 1 teaspoon each of minced onion and garlic, and 1/2 teaspoon each of parsley and red pepper flakes in a small bowl. Rub the roast with the dry rub mix. Make sure to coat the area between the ribs as well.
Preheat the grill so that its temperature is between 250 and 300 degrees Fahrenheit. Place the roast bone-side down on a roasting rack that is inside a pan. The pan catches the meat juices, which can later be turned into gravy or au jus sauce.
Put the pan on the grill away from any direct flames. Cover the grill, and allow the roast to cook until the inner temperature is 115 degrees Fahrenheit for a medium-rare roast. For medium, remove the roast once the internal temperature is 125 degrees. Cooking time is about three hours.
Once the roast has reached the desired temperature, remove it from the grill and let it sit for 20 minutes so that the juices coalesce before carving it.