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Home / Code Guides / Chimney Height

Chimney Termination Height — The 3-2-10 Rule Explained

Chimney height comes down to one memorable rule — 3-2-10. A masonry chimney must clear the roof it passes through by at least 3 feet, and be at least 2 feet taller than any part of the building (or neighboring structure) within 10 feet. The rule exists for draft as much as fire safety: terminate too low and the fireplace smokes.

RequirementModel Code StandardSection
Minimum heightMasonry chimneys must extend at least 3 feet above the highest point where the chimney passes through the roofIRC R1003.9
The 2-in-10 ruleThe chimney must also terminate at least 2 feet higher than any portion of a building within 10 feet, measured horizontallyIRC R1003.9
Spark arrestor specsWhere a spark arrestor is installed, its net free area must be at least four times the flue outlet area, with a screen that rejects spheres larger than 1/2 inch and passes those under 3/8 inch, made of heat- and corrosion-resistant material and accessible for cleaningIRC R1003.9.2
When arrestors are requiredThe IRC sets the specs but generally leaves the mandate local — wildfire-prone jurisdictions (much of California, for example) require spark arrestors on all solid-fuel chimneysIRC R1003.9.2; local amendments
CleanoutMasonry chimneys need a cleanout opening within 6 inches of the base of each flue, with a tight, noncombustible door, so the flue can be inspected and cleanedIRC R1003.17
Factory-built chimneysMetal factory-built chimneys terminate per their listing and the manufacturer’s instructions — most listings impose the same 3-foot / 2-in-10 geometryIRC R1005.1

Worth knowing

  • Measure the 10 feet horizontally from the chimney to the nearest roof surface, dormer, or adjacent structure — on a steep roof, a chimney near the eave may need to be far taller than 3 feet to clear the ridge by 2 feet.
  • A chimney that satisfies 3-2-10 can still draft poorly if the flue is undersized for the firebox or the house is tight — height is necessary, not sufficient.
  • Very tall exposed chimneys need lateral support: masonry chimneys extending well above the roof may require bracing or reinforcing, especially in seismic and high-wind areas.
  • Rain caps and spark arrestors are often one combined fitting — just confirm the cap does not reduce the net free area below the four-times-flue-area minimum.

Common questions

What does the code say about minimum height for Chimney Height?

Masonry chimneys must extend at least 3 feet above the highest point where the chimney passes through the roof. (IRC R1003.9)

What does the code say about the 2-in-10 rule for Chimney Height?

The chimney must also terminate at least 2 feet higher than any portion of a building within 10 feet, measured horizontally. (IRC R1003.9)

What does the code say about spark arrestor specs for Chimney Height?

Where a spark arrestor is installed, its net free area must be at least four times the flue outlet area, with a screen that rejects spheres larger than 1/2 inch and passes those under 3/8 inch, made of heat- and corrosion-resistant material and accessible for cleaning. (IRC R1003.9.2)

Adopted edition by state

Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.

Related sections and guides

This guide is BuildSpec’s own summary, not code text. It cites the 2021 International Residential Code and the 2023 National Electrical Code as model standards. The IRC 2021 text itself is in the Code Library for signed-in members; BuildSpec publishes no NEC text, which is at nfpa.org.Neither is an adoption record: your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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