Chimney Height and Clearance Calculator
Calculate the minimum chimney height and clearance requirements based on the 3-2-10 rule and NFPA 211 standards. Ensure your chimney meets building code requirements for safe operation.
Formulas Used
3-2-10 Rule (NFPA 211 / IRC R1003.9):
- Rule 1: Chimney Top ≥ Roof Penetration Height + 3 ft
- Rule 2: Chimney Top ≥ Highest Roof Point within 10 ft Horizontal + 2 ft
- Rule 3: Chimney Top ≥ Chimney Base Height + 10 ft
- Governing Height: max(Rule 1, Rule 2, Rule 3)
Roof Height at Distance d (ft):
H(d) = Roof Base Height + d × (Pitch / 12)
Clearance Above Ridge:
Clearance = Chimney Top Height − Ridge Height
Effective Draft Height:
Draft Height = Chimney Top Height − Chimney Base Height
Assumptions & References
- Calculations follow the 3-2-10 Rule per NFPA 211 (Standard for Chimneys, Fireplaces, Vents) and IRC Section R1003.9.
- Roof pitch is expressed as rise per 12 inches of horizontal run (e.g., 6:12 = 6 in rise per 12 in run).
- The 10 ft horizontal radius is measured from the chimney centerline in all directions.
- Minimum flashing clearance of 2 inches between chimney masonry and roof deck per IRC R1003.19.
- Results represent minimum code requirements; local amendments or specific appliance manufacturer requirements may be more stringent.
- This calculator does not account for nearby trees, adjacent structures, or prevailing wind effects — consult a certified chimney sweep (CSIA) for site-specific assessment.
- For gas appliances, additional clearances per NFPA 54 may apply.
- All heights are measured above finished grade (ground level).