Here’s a guide:
Table of Type B-Vent Rooftop Clearance Requirements
The 1992 Vent Sizing Tables require that all Type B gas vents terminate above the roof with a listed cap or listed roof assembly in accordance with the manufacturer’s instructions.
Vent caps 12" and smaller may terminate a distance above the roof if 8 feet or more away from a vertical surface as follows:
ROOF PITCH - rise/run in inchesMINIMUM HEIGHT*
Flat to 7/12 - 1.0 foot above the roof surface
7/12 to 8/12 - 1.5 feet above the roof surface
8/12 to 9/12 - 2.0 feet above the roof surface
9/12 to 10/12 - 2.5 feet above the roof surface
10/12 to 11/12 - 3.25 feet above the roof surface
11/12 to 12/12 - 4.0 feet above the roof surface
- Continues to 21/12 pitch at 8.0 feet
Vent caps larger than 12" must be located at least 2 feet above the highest point and 2 feet higher than any portion of the building within a horizontal distance of 10 feet.
From: http://www.inspect-ny.com/chimneys/B-Vent_Chimneys.htm