So far we’ve looked at two techniques for creating your cathedral ceiling. You can install rafter ties (of sufficient strength and at specific prescribed intervals) to replace the conventional ceiling joists. Or you can use a ridge beam of sufficient strength for the particular plan.
Your third method is to use scissor trusses. As you probably know already, a roof truss is a framework in the shape of your roof slope made of wood (such as 2x4s) fastened together by metal connector plates. It replaces a pair of rafters, which must be made of larger dimension lumber and are installed individually, and the ceiling joists — in one strong, rigid framework. A standard truss is shaped like a triangle, with the bottom chord (where the joist would be) running along the bottom, and the top chords defining the slope of the roof. In a scissor truss, the bottom chord slopes upward and downward, usually at a slope of 1/2 of the top chord.
This method involves a straightforward process of installing the trusses at their prescribed locations. It also has the advantage of giving you lots of space for insulation in the area below the roof. The chief disadvantage is that your cathedral ceiling will be half the slope of the actual roof, so you compromise on the overall ceiling height and slope.
A word about insulation: No matter what method you’re using, cathedral ceilings by definition don’t allow you to insulate the attic floor — since there is no attic. You must install the insulation in the space available. Therefore, you need to allow enough room for sufficient insulation as well as an air channel to allow air to circulate up from ventilated soffits to roof vents or a ridge vent. Consider using rigid insulation, at least in part. While it costs more, it gives you more R-value per inch, and can therefore help you save on lumber.