In cold climates vent stacks should be increased in size where they pass through the roof. If they are less than 4 inches in diameter they should be increased to 4 inches; and if they are 4 inches or larger in diameter they should be increased one size before passing through the roof. The object of increasing the size of vent stacks where they pass through the roof of a building is to reduce the possibility of the outlets becoming choked with hoar frost during cold weather. The increase in size should be made at least 1 foot below the under side of the roof, and a long increaser fully 16 inches long should be used. In cold climates vent stacks should not extend more than 12 inches above the roof, as there is greater probability of becoming choked with hoar frost.

Fig. 23