If your using a ridge cap you can either use vented closures or build a vent to sit on the ridge cap or both
http://shop.bestbuymetals.com/colle...cts/10-foot-flex-o-vent-metal-roof-ridge-vent
http://shop.bestbuymetals.com/colle...cts/10-foot-flex-o-vent-metal-roof-ridge-vent