A round gable vent.

Exhaust fan unit over a restaurant kitchen.

Useful Links:

1. Codes"Section R806 - Roof Ventilation" section of the 2012 International Residential Code.

2. Technical: The principles of natural ventilation (as opposed to fan-driven ventilation) are presented well in this article at the Whole Building Design Guide site, which is maintained by the National Institute of Building Sciences.

3. Technical: "Ventilation"is a good short introduction to the ventilation of houses in cooler climates. This technical bulletin is available on the Canadian Roofing Contractors' Association website.

4. Technical: "Attic Ventilation Calculations Made Easy" is a useful little article from 2014. Available on the website of Air Vent Inc.

5. You can find ventilation product manufacturers here.