Place rib roast onto a large cutting board. Trim off any straggling fat or meat pieces hanging off the sides of the roast. Let meat stand at room temperature 30-45 minutes before grilling.
Preheat grill. Combine rub ingredients in a small bowl and thoroughly coat prime rib roast with mixture. Transfer to grill.
Cook over indirect medium heat for 2-3 hours. Rotate roast occasionally for even cooking. Once roast reaches an internal temperature of 130-135 degrees F (medium rare to medium), remove from grill and let stand for 8-10 minutes before carving. Serve with horseradish sauce, and your favorite side dishes.