Chimney Termination Height (IRC R1003.9, the 3-2-10 Rule)
How tall a chimney must stand above a pitched roof under the 3-2-10 rule of IRC 2024 R1003.9.
Example
You enter
- Roof pitch (in of rise per 12 in; 0 for flat) 6
- Horizontal distance from the chimney's upslope face to the ridge (ft) 14
- Height of any other building portion within 10 ft, above the penetration (ft, 0 if none) 0
- Chimney width along the slope (in) 32
- Measured height above the high side of the penetration (ft, blank to skip) 6
You get
- Roof rise (ft) 5
- Required (ft) 7
- Same height measured on the low side 8.33 ft
Checked against published worked examples from: ICC.
Details, formula, and sources
Scopeat least 3 ft above the highest point where it passes through the roof, and at least 2 ft higher than any portion of the building within 10 ft. On a pitched roof the roof itself, rising upslope, is usually that portion: within 10 ft it rises the pitch times the distance to the ridge or 10 ft, whichever is less. A chimney 14 ft below the ridge of a 6 in 12 roof faces roof 5 ft higher 10 ft away and must stand 7 ft above the high side of its penetration, not 3; at 12 in 12 with the ridge 6 ft away it is 8 ft. A dormer, parapet or taller wall within 10 ft is entered as its height. It also gives the same height measured on the downslope face, which the chimney's own width raises (8.33 ft for a 32 in chimney in the first case), and checks a measured height. A factory-built chimney follows its listing; draft is the chimney height for draft calculator's job.
Formula or tableroof rise within 10 ft = pitch x min(10, distance to ridge) / 12; required above the high side of the penetration = max(3, 2 + roof rise, 2 + other portion height); low side = required + pitch x (chimney width / 12) / 12.
Source and editionIRC 2024 (International Residential Code). Section R1003.9 (the same section and values in IRC 2021), read at codes.iccsafe.org on 2026-10-19.
Free accessFree to read at codes.iccsafe.org.
What governsEstimate. AHJ and licensed structural engineer govern.
API field namespitch_in12, ridge_distance_ft, other_height_ft, chimney_width_in, measured_ft, roof_rise_ft, required_ft, low_side_ft
- 10 ft measured horizontally from the chimney's upslope face the roof's highest point within that reach is at the ridge or 10 ft upslopereading of IRC R1003.9 on a pitched roof
- Masonry chimney factory-built chimneys and appliances with listed terminations follow their listingsIRC R1005 and the listing