Installing ridge vents are much more difficult on high pitched roofs. I installed one on a 12/12 roof once and I thought I would have to call the fire department to get me down off the roof. You can use a carbide tip blade on a circilar saw and cut away the area neccessary. What one has to watch for is trying to hang on straddling the ridge and not cut away so much material that the roof vent will not cover. It not very difficult to cut away too much. Also one needs to use the screws with rubber gasket. The ones like they use on Tin roofs. Roofing tacks could back out and driving them in on unsupported plywood could damage the plywood.