NFPA 54 12.7.2. Gas Vent Terminations.
A.........
B Gas vents that are over 12 inches in size or are located less than 8 ft. from a vertical wall or similar obstruction shall terminate not less than 2 ft. above the highest point where they pass through the roof and not less than 2 ft. above any portion of a building within 10 ft horizontally.

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Dave