1 Close the hood of the grill and allow the roast to finish cooking using the indirect heat of the grill. 2  Cook until internal temperature reaches 140°F. Then remove the roast from the grill and allow it to stand for 15 minutes before carving. 3  During this time the temperature should rise to 145°F for proper doneness. Close the hood of the grill and allow the roast to finish cooking using the indirect heat of the grill. 2  Cook until internal temperature reaches 140°F. Then remove the roast from the grill and allow it to stand for 15 minutes before carving. 3  During this time the temperature should rise to 145°F for proper doneness.