How short can a chimney be?
Short answer. Under the residential code's 3-2-10 rule, a chimney must rise 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. On many roofs the second part controls, so a chimney low on a steep roof must be much taller than 3 feet.
Applying the rule
- Measure 3 feet up from where the uphill side of the chimney leaves the roof.
- Picture a 10-foot horizontal line out from the chimney. Wherever any part of the building within that distance, a higher roof plane, the ridge or a wall, rises, the chimney top must be at least 2 feet above it.
- The taller of the two results is your minimum.
A chimney that exits right at the ridge can often meet the rule with about 3 feet. A chimney exiting low on a steep slope may need several more feet, because the roof climbs so quickly within 10 feet.
Why height matters
- Draft. A chimney that is too short sits in the turbulent air around the roof, so smoke can spill back into the room.
- Fire safety. Height carries sparks and hot gases away from the roof.
- Downdrafts. Wind over a taller nearby roof section can push air down a short flue.
Factory-built chimneys and gas vents
Factory-built chimneys follow the same principle, but their listing and instructions also govern height, bracing and the cap. Gas vents have different termination rules based on roof pitch, set out in the fuel gas code and the appliance instructions.
Fixing a short chimney
- Add a listed extension or a section of matching flue and cap.
- Rebuild the top of a masonry chimney higher, which also means new flashing at the roof if the work disturbs it.
- Use a draft-improving cap only where a sweep recommends it.
See chimney leaks and crickets and roofing permits in Georgia.