IRC 2003. G2427.6.5(503.6.6 Gas Vent termination.
Above the roof surface with a listed cap or listed roof assembly. Gas vents 12 inches in size or smaller with listed caps shall be permitted to be terminated in accordance with figure G2427.6.5 provided such vents are at least 8 feet vertical wall or similar obstruction. All other gas vents shall terminate not less than 2 feet above the highest point where they pass through the roof and at least 2 feet higher than any other portion of the building within 10 feet.

Jon the way I interpret this, the last paragraph is your situation.

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Dave