3) Light the grill and bring to medium heat (between 350 – 375 degrees). Preheat for 7-8 minutes. This is the ideal heat to cook the tri tip fully in 35-40 minutes, depending on the size of the roast.4) Place the meat fat side up in between the burners.heck out our video about grilling a tri tip roast and see the extra notes below…. Here is a recap of the video (extra notes in blue): 1) Before you light up the grill, take the tri tip roast out of the fridge and bring it to room temperature (or about 70 degrees).