It could be automated. All it takes is the right equipment. Obviously you have to avoid the loop. I've seen that in person, open doors right next to the exhaust fans. That assures that nether the doors nor the fans do much good. The trouble with a fan in the roof vent are at least a couple. Fans will be much smaller than what is needed for effective natural ventillation. My 1700 sq ft greenhouse has three 36 inch exhaust fans. Mostly I run only two. That area of roof vents would be totally inadequate. You'd need 10-20 times that area of passive roof venting. Also a fan in the vent area would impede air movement when the fan wasn't operating. To me roof vents are too expensive for what they do. With roll up sides I'd rather have a means to uncover the ends all the way to the peak. This would allow air into the sides and out the peak at both ends. Neither roof vents nor open ends sound cheap or easy. But I'd rather jury rig the ends than the top for venting. Roll up sides by themselves will work during cool weather. Then during the freeze free period, uncover the ends and replace with bird netting if critters might be an issue.

Thanks for the great replies. I should have mentioned the climate. I'm in Western Oregon (Portland) so overall things are pretty mild here most of the time. We typically get a week or three of 90-100 degree weather in the summer and maybe touch the high teens every few winters with little snow. While it rains a lot we basically never get hot and humid conditions like you see in some parts. We're apparently "zone 4c". Since we have the finished attic (where we sleep) I assumed that ventilating will be a benefit in the summer when the sun tends to bake the roof and make it pretty hot upstairs, but that's open to discussion. @maingrower - no, the existing decking seems to be in good shape, but 2 out of 3 roofers want to replace it anyway. I'm not clear on why. SIP panels look interesting, as does the spray foam... but how does the roofing go down on top of that? Another layer of decking? Any suggestions on relevant Building Science sites? What I've found on the first pass seems mostly relevant to new construction. Thanks again!

all snark aside... OP if there were no issues caused by existing ventilation...don't add any. simple, don't 'fix' what isn't broken. from reading your first post...this is an insurance re-roof? metal roof? in any case...getting excited & defensive doesn't do anything but add to the chum in the water. GW isn't always like this...but it gives back what it is given. I've been here for years & have never made a dollar off of anyone here.. best of luck.

A gable fan would be hard to install because the gables are up there behind the ceiling and walls. The only access to the eaves are the kneewall doors and there isn't much space behind them. We'd have to cut the ceiling to get up there to the gables. The extra roof is an interesting idea. Would the sleepers run up and down the roof and create the pathway for the air? Or would they go across the roof? If they go across, wouldn't they then block the air pathways? Would the extra weight of another roof be a possible problem? How could I tell if the structure could handle sleepers and an extra roof? The present roof sheating is 1x4 boards, not plywood. I'm told there are some soft spots and some of the sheating is likely going to have to be replaced. There was a leaking problem in 1998 and roof repairs were done then, but I don't think they did anything to the sheating then. Can baffles be put in from on top the roof after the top is cut off for the ridge vent? Could they slide in from there or do they tear up too easily? Are baffles ever made of anything besides foam? The house has been there 56 years and the present roof has lasted 29, all without roof ventilation except for the gable vents, so I'm wondering why a new roof would start to have problems because of lack of ventilation. It's a pretty steep roof, if that makes any difference in this issue. I'm going to use a light-colored shingle. There are mature shade trees in the yard, and one in particular shades it from the west. Virginia is hot in the summer, but in the winter it seldom gets below 20.