How High Should Sewer Vent Be Above Roof

I have a main vent and a sink vent and both are only 6 high above the roof.
How high should sewer vent be above roof. 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. Vent pipes shall terminate not less than 6 inches above the roof measured from the highest point where the vent intersects the roof. Plumbing books vary in their advice. Some builders recommend a minimum of 30 cm 12 inches.
How high should the plumbing vents be above there roof. Recommended minimums make sure the stack to protrude a minimum of 15 cm 6 inches from the roof. Does anyone know what legal height requirements there are for vent pipes from memory i m pretty sure that it must only be a minimum of 300mm above the gutters that s to ensure storm water does not get into the sewer system. A vent terminal and depending if the roof is strictly for weather protection it terminates about 24 above the roof.
Where a roof is used for any purpose other than weather protection vents shall extend at least 7 feet above the roof and shall be properly supported.