Chimney Height Calculator (3-2-10 Rule)

Enter where the chimney exits the roof and the roof pitch. You get the minimum height above the roof and whether a planned height passes.

How the numbers work

The first rule is simple: 3 feet above where the chimney exits the roof. The second rule looks 10 feet out horizontally from the chimney and finds the highest part of the building in that circle, which is usually the roof sloping up toward the ridge. The chimney top must be 2 feet above that point.

The calculator finds that point from the roof pitch. If the ridge is closer than 10 feet, it uses the ridge distance, because the roof stops rising there. Use the extra height field for a dormer, a taller roof section or a ridge cap within 10 feet. The larger of the two rules is your minimum.

Worked example

On a 6 in 12 roof with the ridge 6 feet away, the roof rises 3 feet in that distance. Add 2 feet and the chimney must be 5 feet above the penetration, which beats the 3 foot rule. A 4 foot stack would be 1 foot too short.

This is a code geometry check only. Always follow the appliance and chimney manufacturer's installation manual and your local building department, which may have stricter rules for snow country.

Questions

What is the 3-2-10 chimney rule?

The chimney must extend at least 3 feet above the highest point where it passes through the roof, and at least 2 feet higher than any part of the building within 10 feet horizontally. It comes from NFPA 211 and the International Residential Code, section R1003.9 for masonry and the listing for factory built chimneys.

Does the 3-2-10 rule apply to metal chimneys?

Yes. Factory built class A chimneys follow the same height rule in their installation instructions and UL 103 listing, though some manufacturers add their own requirements.

What if the chimney is too short?

A short chimney can draft poorly and spill smoke. Add listed chimney sections and, if the stack is tall and exposed, a roof brace kit.

How do I measure the roof rise within 10 feet?

Use the roof pitch. A 6 in 12 pitch rises 6 inches per foot, so within 10 feet toward the ridge it rises 5 feet. The calculator does this math.