In areas subject to frost closure, a vent extension through the roof must be increased to a minimum of 3 inches, and the change in size must be made at least:

Giải thích

Where frost or snow closure is a concern, the vent must be at least 3 inches in diameter through the roof, and the increase in size must be made at least 1 foot inside the building below the roof. Making the transition too close to the roof line (3 or 6 inches) leaves the enlarged section exposed to frost, and 2 feet is more than the code requires.