In most homes the ceiling is constructed with traditional roof framing consisting of two slanted roof rafters that set on the wall plate and connected to the wall plate and a horizontal piece of lumber called an overlay which is also the ceiling joist. These pieces of framing lumber create a triangle that can be exceedingly strong, depending on the roof pitch and size of the lumber. The roof strength relies on each piece of lumber.

A cathedral ceiling, often called a vaulted cathedral ceiling, is another matter. They are built similar to the traditional roof except for the overlay (ceiling joist) which is positioned at an angle to give the ceiling a high pitch. Cathedral ceilings can be a dramatic look, especially if they have skylights installed.

A commonly phrased term in the construction industry is the “scissor truss” that is used to form a cathedral ceiling. The scissor trust consist of the roof rafter and the ceiling joist that are attached together to form the high pitched ceiling we call a cathedral

ceiling. Two pieces of lumber on each truss set on opposite wall plates and intersect the roof rafter at the desired angle to form the cathedral ceiling. These trusses can be built in place or a pre-made truss can be purchased that will do the job. In modern construction, builders usually use pre-made trusses that are specifically designed for the room span to support the roof load and snow loads.

If you are building a DIY home and want a cathedral ceiling in one or more rooms your material can provide you with the appropriate truss that is designed for your home. Most of these suppliers have structural engineers on site that will determine the specifications of the truss and order it from their supplier.

I recommend that if you are going to build a cathedral ceiling that you use the pre-made roof truss rather than building a truss in place. In most cases the cost for the pre-built truss comes out cheaper than what it would cost to build a truss in place.

The challenge is the insulation for cathedral ceilings, especially those with skylights. Do not fill the entire cavity above the ceiling with insulation. If you use traditional fiberglass or cellulose insulation you need to leave at least a two inch air space between the insulation and the bottom of the roof sheathing. Ridge venting is an absolute must so that the closed-in space above the ceiling can be properly vented in summer and in winter. Install the skylights before installing the ceiling insulation and insure that the area around them is heavily insulated using sheets of Styrofoam insulation to prevent cold spots where condensation can collect during winter weather.

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