http://www.homebuildingandrepairs.com/engineering/index.html Click on this link to learn more about structural engineering, roof framing and remodeling. This video should provide you with a simple explanation of why you cannot remove ceiling joist and rafter ties to convert a flat ceiling to a vaulted or angled one. In other words, if you know anyone who's going to try and convert their old flat ceiling to a cathedral type, it wouldn't be a bad idea to have them watch this video, before making any modifications.