Vent Height Above Roof

The 6 you describe should be just fine.

Vent height above roof.

This reduces the actual required height by 12 inches from our previous calculation of 64 inches. Open vent pipes that extend through a roof shall be terminated not less than 6 inches 152 mm above the roof. Above the roof surface with a listed cap or listed roof assembly. A vent stack is not like a chimney which requires 2 higher than any roof within 10.

Gas vents 12 inches 305 mm in size or smaller with listed caps shall be permitted to be terminated in accordance with figure g2427 6 5 provided that such vents are at least 8 feet 2438 mm from a vertical wall or similar obstruction. Vent pipes should extend to outdoors above the building roof and should terminate vertically not less than 6 above the roof surface nor more than 24 and must be at least 12 from any vertical surface such as a nearby sidewall. So 28 inches plus 24 inches is a chimney height of only 52 inches. Above the roof plumbing vent height.

I have a main vent and a sink vent and both are only 6 high above the roof. The slope is about 4 high for every 12 horizontal. Per the 10 and 2 rule once you are at the level of the peak you only need to be 2 feet above it. You are letting air into the plumbing system of the house.

All open vent pipes that extend through a roof shall be terminated at least 6 inches 152 mm above the roof and not less than 2 inches 51 mm above the invert of the emergency overflow except that where a roof is to be used for any purpose other than weather protection the vent extensions shall be run at least 7 feet 2134 mm above the roof.