Written by: Sean Summers, National Fireplace Institute (NFI) Master Hearth Certified Technician - WoodStovePro.com
Questions about this rule come up a lot, and there seems to be a lot of confusion about it. Thankfully, this standard is actually quite simple. In most cases, with just two pieces of information, you can calculate the height of your chimney on paper without ever having to climb on the roof.
First, you’ll need to know the pitch of your roof. Roof pitch is easily calculated, and if you don’t know how, read my article Understanding and Calculating Roof Pitch. Also, you’ll need the horizontal distance between the peak and the edge of the high side of the pipe (see Figure 1) where you plan your roof penetration. We’ll go over the math a little further down; first let’s go over the 10 and 2 rule (see Figure 2).
Most wood stove and factory-built fireplace manufacturers (and local building codes) will require that “the chimney must extend at least 2 feet above any portion of any structure within 10 feet (measured horizontally) and must extend at least 3 feet above the highest point of its roof penetration.”**
Keep in mind that there is generally a minimum chimney height requirement for the overall system (usually 15 feet, which sometimes includes connector pipe) that can affect the finished height of your chimney system. Other factors can influence the necessary height of the chimney such as “house stack effect” and high altitude. “Chimney height may need to be increased by 2-3% per 1000 feet of elevation.”** Although added height generally equates to stronger draft, there is a point where the system can become too long and over-drafting might occur, causing increased fuel consumption, hotter burn temperatures, and damage to your appliance.
Now for the math lesson. Roof pitch is expressed as a ratio of rise over run. Rise is the length, expressed in inches, that the roofline travels upward over a 12-inch horizontal distance, which is known as the run. For example, a common pitch like 4/12 would mean that for every 12 inches of run, the roof rises 4 inches. So, if the 10-foot, 2-foot rule requires us to measure 10 feet to the nearest roofline, or structure, then we can use the rise to calculate a height requirement. Back to our example, over a run of 10 feet, the roof would rise 40 inches. Additionally, per our rule, we need to add 2 feet, or 24 inches, giving us a total height of 64 inches. So, take the top number of your roof pitch and multiply it by 10 then add 24, note Example 1. If your calculated required height falls below 36 inches (3 feet), then, as per our rule, it must be a minimum of 3 feet on the high side of the pipe penetration.
This calculation from Example 1 alone would give you a solid working height, but if you are close to your peak then you might have more pipe height than you actually need. If you can measure the horizontal distance between the peak and the edge of the high side of the pipe where you plan your roof penetration, you can use that to calculate your height in relation to your roof peak or ridgeline. Often you can measure this from inside the room where you are planning to install the stove or inside the attic space. Many people mistakenly think that the pipe always has to be 2 feet above the roof peak.
Let’s look at a couple of examples. You are close to your eave and you find that your measurement is 18 feet from peak to penetration on a 4/12 pitch (see Figure 3). For 18 feet of run, you would have a rise of 72 inches. If the previous calculation results in the required height of 64 inches, we can see that the top of the pipe will not rise above the roof peak but would still meet the requirements of the rule. For the next example, you have a shorter chimney height. If your peak to penetration distance is 7 feet, you would have a rise of 28 inches. Per the 10 and 2 rule, once you are at the level of the peak, you only need to be 2 feet above it. So 28 inches plus 24 inches is a chimney height of only 52 inches. This reduces the actual required height by 12 inches from our previous calculation of 64 inches.
Please note that this rule doesn’t generally apply to pellet and gas vent systems. Hopefully this article helped you understand the 10 and 2 rule but if you still have questions, please leave a comment or send us an email. We are always happy to help.
*This post is meant to be a guide so always consult with your appliance owner’s manual and local code before planning your system.
**Woodburning Hearth System Reference Manual. Hearth Education Foundation. Dec. 2002 pg. 92