Although slow-cooking a tri-tip roast wrapped in foil is typically done in the oven, you can just as easily slow-cook it on your grill, ideal on a hot summer day. Tri-tip is a tender, boneless roast, sometimes cut into steaks. As a roast, it benefits from long cooking at a low temperature, allowing the heat to slowly melt the connective tissues in the muscle to make it extremely tender. Wrapping it in foil ensures that the moisture stays in the meat. You can also include vegetables if you choose, which will slowly cook in the steam inside of the foil.
Cover the tri-tip with a prepared dry rub seasoning of your choice and place in the refrigerator loosely covered with plastic wrap for up to 24 hours. To make a dry rub, start with equal parts salt and sugar, then add additional seasonings, such as garlic powder, chili powder, black pepper and dried herbs by the teaspoonful until you get the flavor you're looking for.
Remove the tri-tip from the refrigerator 30 minutes to an hour before you start cooking to allow it to come to room temperature.
Prepare your grill for indirect grilling by piling the coals to one side of a charcoal grill and placing a drip pan filled halfway with water on the other side. Light the coals and close the lid to preheat. If you are using a gas grill, turn all the burners on high for 15 minutes. Then, turn off all the burners on one side of the grill and turn the other burners down to a medium-low temperature, about 250 degrees Fahrenheit.
Place the tri-tip directly over the flames on your grill to sear on all sides until well browned and slightly charred. Use heavy metal tongs to turn and flip the roast.
Remove the tri-tip from the grill and wrap it in a sheet of aluminum foil so that it is completely covered. You can opt to add vegetables that you would like to steam cook in the foil with the tri-tip, such as sliced bell peppers and onions, potatoes and carrots. Seal the foil around the meat and vegetables, leaving a little bit of room for steaming.
Place the foil packet on the grill away from the direct heat of the flames. Cover the grill and allow the tri-tip to cook for roughly 2 1/2 to 4 hours, depending on the size, until the tri-tip has reached an internal temperature of at least 145 F. Use a meat thermometer to determine the temperature.
Remove the tri-tip roast from the foil and place on a serving platter. Surround the roast with any vegetables you included, as well as the drippings. Allow it to rest for 10 to 15 minutes before slicing the roast into thin slices, across the grain.