Clay,
Not Sure What your locale requirements are. This is what I have from IRC 2003
M1804.2.3 Natural draft appliances.Vents for natural draft appliances shall terminate at least 5 feet above the highest connected appliance outlet,and natural draft gas vents serving wall furnaces shall terminate at least to an elevation at least 12 feet above the bottom of the furnace
M.1804.2.4 Type L vent. Type L venting systems shall conform to UL641. Such vents shall terminate with the vent manufactures installation instructions not less than 2 feet above the roof and not less than 2 feet above any portion of the building within 10 feet.
From the Book (not the lip)
