The best roof for ventilation is a peaked roof with slope of each side between 4:12 and 6:12. This slope allows fresh air to enter on soffit and mix with coop air before exiting gable vents. If the pitch is too steep the air shoots through without mixing and if too shallow a pitch doesn't create a vacuum to pull air in.
Anywho, without boring you too much with the physics I can say that if a pitched (gabled) roof with 4:12 to 6:12 pitch and being ultra conservative allowing for no wind you would only need 2.5 square inches of venting in each eave and total gable vents per chicken. That's to say if you had 4 chickens you'd need a vent in each eave end of 10 square inches (i.e 1X10, 2X5, etc.) and each gable end of 5 square inches.
A slant roof requires significantly more venting area.