Sizing stack vents
Stack vents are sized by the number of fixture units that are being served by the soil or waste stack and also by the length of the stack vent. There may be special requirements for sizing stack vents included in your local plumbing code; such as in the NPC 2005(National Plumbing Code of Canada), one stack must be a minimum of 3" to assist in venting the public or private disposal system. Stack vents will always be at least half the size of the stack at the bottom. IE: Any stack serving a water closet(toilet) will have at least an 1.5" stack vent.