What type of scissor roof truss construction would be best for a cathedral ceiling on a 4\12 pitch with a 22' span? Or would parallel scissor method be best for this pitch roof?

Use a manufactured truss unless you want to use strucural beams and cut the roof in.

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Depending on your Geo I would stick build it. You need Room for insulation and air flow. I use 2x12s to build those ceilings. You have the 9"+ for (R 30) and still have the air space needed to vent.

Are you locked in the 4/12 pitch or is that mainly for shingle specs your trying to meet?

What I usually do for a vaulted project this small is run a ridge board and set 2x12 rafters. I add the collar beam which gives some flat on the inside peak to mount lights or ceilings fans Also you want the 2x12s to run Ac out of the ceiling. Cooling a room with a cathedral ceiling is different then a flat ceiling.

On a flat ceiling you can run your 2 1/4 x12 wall stack on the side walls for the hi low in heat and A.C. and that works fine.

A cathedral ceiling is a bit different. The hot air gets trapped and becomes counter productive in cooling the room evenly. I run insulated duct up in the ceiling to drop the cold air through that trapped heat in the room to even out the room temp.

Without ceiling diffusers the heat is trapped and the cold air is trapped lower.