How long you cook a tri-tip depends on its size and how well done you like your meat. Tri-tips can be whole roasts or strips. Whatever the size of your tri-tip, however, it will be juicier, more tender and more flavorful if it is cooked to rare or medium rather than well-done.
Tri-tip strips cook to rare in about 8 to 10 minutes, and to medium in 12 to 14 minutes. This is the total cooking time, so turn the strips over at the halfway point -- 4 to 5 minutes on each side for rare, and 6 to 7 minutes on each side for medium.
How long to cook a tri-tip roast depends on its weight and shape. For an average-sized, 2-lb. roast that is about 2 inches thick, cook the roast on one side for 20 to 30 minutes. Turn the roast over and continue cooking to an internal temperature of 140 degrees F as measured with a meat thermometer. Check its internal temperature after cooking for about 10 minutes on the other side. Allow the roast to rest for 10 minutes before slicing.
Gas or Charcoal
Use the indirect method if you're charcoal grilling. If you're using a gas grill, your grilling time may be shorter, so check the tri-tips to be sure they don't overcook. Move them farther from the gas flame if the meat is browning too much too quickly, and let them cook until they're done to your liking.
Every grill is different. How long your tri-tips take to cook depends partially on your particular grill and the way you cook. If you know you usually grill in less time than your recipes suggest, assume your tri-tips will cook more quickly, too. Check your meat before it should be done when you're cooking tri-tips for the first time. Note how long it took to cook the meat to your satisfaction, and follow that guide for your future tri-tip cooking.