I just had a roof replaced. The salesman who sold me that I didn't have enough ventilation. He suggested more openings in the soffit area. We did install three front and back. I have since had more work done in the back yard and the contractor suggested that I still did not have enough ventilation in the roof. On the top of the roof is a ridge vent approximately six feet in length. This new contractor suggested that we install a whirly type of vent on the roof to let the hot air out. He claims that this is the most sensible device. My wife does not agree and my neighbour doesn't know for sure.
Answer:
If I were to make any adjustment to your ventilation system, I'd extend the ridge vent to go from gable to gable and leave it at that. Warm air rises and there is no better place to exhaust it than at the ridge it rises to. Even the turbine vents you speak of demand that air move horizontally toward them to be exhausted. In that process there are all kinds of wooden beams and such for moisture to soak into.
