Allow roast to stand at room temperature for at least 1 hour before roasting.
Preheat the oven to 190 degrees C. Mix the salt, pepper and garlic powder in a small bowl. Place the roast on a rack in a roasting tin so that the fatty side is up and the rib side is on the bottom. Rub the spice mix onto the roast.
Roast for 1 hour. Turn off the oven and leave the roast inside without opening the door. Leave it in there for 3 hours. 30 to 40 minutes before serving, turn the oven back on at 190 degrees C to reheat. The internal temperature should be at least 62 degrees C. Remove from the oven and let rest for 10 minutes before carving.