[COcyclist]

Ecky, I reroofed my house some years back and added a full length ridge vent and added soffit vents as well. The house previously had just gable vents on each end. My understanding is that the soffit vents allow cooler air to enter between the rafters and create a chimney effect out the ridge. I did this primarily to try to extend the life of my shingles and help prevent water damage from ice dams in winter.

FWIW, it is very easy to retrofit the ridge vent but I would buy some cap shingle and long nails to cover the cap so the color matches your existing roof. Cutting the rectangular holes for the soffit was less fun with the dust from the circular saw covering me as I worked over my head.

Also, I have a sun porch the full length of the west side of the house with a shed roof on it. That attic would make the sun porch unbearable in summer and even heat up the second story of the house on that side. I found a solar attic fan at the HD big box store and put it into that attic. It works for free and is so silent that I have to check to make sure it is still runs. It only runs during afternoon sun in summer but it has made an amazing difference in how cool the house and fully glassed-in porch stays so it must be working.