Chimney Height Calculator
Calculate chimney height from roof pitch using the 3-2-10 rule.
Returns minimum height above the roof peak and flashing position per NFPA 211.
Required Chimney Height
Chimney draft is the natural upward airflow that pulls combustion gases out of your home. A taller chimney creates more draft, but height must follow strict safety codes related to roof geometry.
The 2-10-3 Rule (IRC standard):
- The chimney top must be at least 2 feet above any part of the roof within 10 feet (horizontal)
- The chimney must be at least 3 feet above the point where it passes through the roof
Minimum Height = Roof Pitch Rise + 2 ft (if within 10 ft horizontal)
Draft pressure formula:
Draft (Pa) = 0.0342 × Atmospheric Pressure × H × (1/T_outside - 1/T_flue)
Where:
- H = chimney height in meters
- T_outside = outdoor temperature in Kelvin (°C + 273)
- T_flue = flue gas temperature in Kelvin
Worked example: Chimney height = 6 m, outdoor temp = 5°C (278 K), flue temp = 200°C (473 K):
- Draft ≈ 0.0342 × 101,325 × 6 × (1/278 − 1/473) ≈ 30 Pascal
- 25–50 Pa is the ideal range for most wood-burning fireplaces
Standard chimney dimensions:
- Flue area should be at least 1/10 of fireplace opening area
- Minimum flue diameter: 8 inches for most residential fireplaces
- Minimum height above roofline: 2 feet (code minimum, taller is better for draft)
Factors that reduce draft:
- Low outdoor temperatures improve draft; high winds can cause back-drafting
- Obstructions (trees, adjacent buildings) within 10 feet reduce effective height
- Oversized flue relative to firebox area weakens draft