Originally published in the June 2015 issue of Sweeping Magazine
Question: Why do masonry chimneys have to meet the 3 - 2 - 10 rule?
by Mike Segerstrom
Because they do! It is widely known that masonry chimneys are required to meet the 3 - 2 - 10 rule. This rule means that they must extend 3 feet above the roof penetration on the shortest side, and the top of the chimney must be 2 feet higher than any portion of the building structure within 10 feet. This height requirement for masonry chimneys penetrating or adjacent to pitched roofs has been around for many decades.
But why is this? I personally have been working on chimneys since 1997, and have been actively involved in the industry and with industry professionals for the past 10 years. I have asked this question many times, of many different folks in the industry. I have discovered that there is something of a divide in the reasoning behind the 3 - 2 - 10 rule.
One school of thought says that it is for safety, to make sure anything hot coming out of the top of the chimney, including flames or flaming creosote in the event of a chimney flue fire, doesn’t catch the adjacent roof or building construction on fire. But if this were true, wouldn't there be stricter requirements for wood shake roofs, and maybe more lenient requirements for metal roofs? There aren’t different requirements. It’s the same, whether the pitched roof is rubber, asphalt, slate, metal or cedar shake.
Another school of thought says that it is for performance, that this minimum height ensures that the chimney is tall enough to provide draft. And also that it will help prevent other parts of the building from hindering draft. But as we see in the field, meeting the 3 – 2 - 10 rule does not always guarantee these. Take for example the 3000 square foot 2 story home with a single story family room addition in the back. There is a fireplace located on the gable end of this addition, and the chimney meets the 3 to 10 rule. But 15 feet away from the chimney, is the second floor and attic of the main part of the house, well above the top of the chimney. This chimney can be subject to severe down drafts and house pressure problems, even though it meets the 3 - 2 - 10 rule.
These are just a couple of examples where we can see that meeting chimney height requirements, regardless of our school of thought, may not protect wood shake roofs, or solve performance problems. Taller is obviously better, especially with a wood shake roof. Remember, the 3 to 10 rule is the MINIMUM height requirement. And sometimes we also have to take a forensic approach to solving house pressure and make up air issues. The CSIA offers classes on chimney physics, where these pressure issues are discussed at length.
We also have to remember that if we are relining a masonry chimney with a listed liner system, the chimney is required to meet the 3 - 2 - 10 rule. Chimney height is not grandfathered and often times it didn't meet the minimum height requirement when it was built. Listed solid fuel appliances that we install and/or connect to a masonry chimney, will require the chimney meet 3 - 2 - 10 rule. Many gas and pellet appliances that we install or connect to a masonry chimney will also have this requirement.