How high must a B-vent stick out of a 4:12 roof? What the code table actually says

A homeowner in southeastern Michigan asked on Reddit's r/Roofing forum this week how far a Type B gas vent must stick out of the roof: 12 inches or 24. A rusted section is being replaced along with the roofing, and the pipe now stands 15 inches above a 4:12 roof with no vertical walls nearby.

The answer is in Section G2427.6.4 of the International Residential Code, which follows the National Fuel Gas Code, NFPA 54. A vent 12 inches or smaller with a listed cap and no wall or comparable obstruction closer than 8 feet follows a slope table: 1 foot from flat to 6/12, then rising in steps to 2 feet once the pitch passes 8/12 and 4 feet past 11/12. An older table in DuraVent's instructions also gives 1 foot for a 4:12 roof, and InspectAPedia notes the height is measured on the uphill side of the pipe. At 15 inches, this vent passes.

The 2-foot figure covers vents wider than 12 inches and vents with a wall or other obstruction closer than 8 feet. Those must clear the roof by 2 feet at the high side of the penetration and also top any part of the house within a 10-foot radius by 2 feet. DuraVent's troubleshooting guide says downdrafts usually come from a cap that sits too close to a wall, a parapet or another structure. The 3-2-10 rule roofers often quote is for chimneys, according to training material from the Michigan Training and Education Center, which also cites the NFPA 54 rule that a gas vent's top must sit 5 feet or more above the top appliance's flue collar or draft hood.

DuraVent calls for a listed cap, a 1-inch air space between the pipe and combustible roof framing, and a storm collar set in non-hardening sealant. In Michigan, the licensing agency LARA says mechanical work needs a permit except for minor parts. An owner who lives in a single-family home can do it unlicensed but pulls the permit; a hired mechanical contractor pulls its own.