A round gable vent.

Exhaust fan unit over a restaurant kitchen.

Useful Links:

1. Codes"Section R806 - Roof Ventilation" section of the 2015 International Residential Code. Available on the UpCodes website.

2. Technical: "Guidelines for Ventilating Attics and Cathedral Ceilings to Avoid Icings at Their Eaves" is a very technical article (lots of math and graphs) available on the website of the US Army Corps of Engineers Engineer Research and Development Center.

3. 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.

4. 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.

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

6. Manufacturers: You can find ventilation product manufacturers here.