Folks looking to construct a chimney by themselves, according to local building and fire codes of course, often have trouble grasping this 2 foot, 10 foot, 3 foot standard. Not only are these measurements important to meet federal requirements, a chimney that is properly positioned on the home relative to the rest of the home will draw better and generally cause you less problems as a homeowner.
What is the 2 foot, 10 foot, 3 foot rule? This rule indicates that the chimney must be at least 2 feet taller than any structure within 10 feet of it and also at least 3 feet higher than the point at which it penetrates the roof. These specifications aren’t there to merely cause you problems when constructing your chimney, they are intended to make sure that the chimney will draw properly and that a fire hazard is not created. If, however, the manufacturer of your particular prefabricated fireplace unit, wood stove, or other appliance has other specifications, these must be followed.
A chimney that is too low to the roof (does not extend 3 feet above the roof line) is more likely to cause a roof fire from flying sparks and less likely to draw well. As a rule, 15 feet should be the minimum chimney height unless you have an extremely small firebox or other unique situation. Too tall of a chimney can result in overdrafting which can damage some appliances, so even if you plan on constructing the chimney yourself you should always consult with a professional on such technical specifications. If the chimney does not extend at least 3 feet above the roof line the roof may actually hinder the chimney’s ability to draft well.
Any peak that is within 10 feet of the chimney can compete with the chimney causing draft problems as well. This is why the rule calls for the chimney to extend 2 feet above any structure within 10 feet of the chimney. If your home is located at a particularly high altitude, you must consult a professional when it comes to these particular specifications because the high altitude and thinner air will cause necessary measurements to differ slightly.