Why chimneys must clear the roof
Wind blowing over a roof creates turbulence and a high-pressure zone on the downwind side of the ridge. A chimney outlet inside that zone can lose its draft, or even have air pushed down the flue, sending smoke and carbon monoxide into the house. Building codes therefore require the top of a chimney to rise clear of the roof and of anything nearby.
The US 3-2-10 rule
The International Residential Code (section R1003.9 for masonry chimneys) and NFPA 211 set two conditions, and the chimney must meet both:
- It must extend at least 3 feet above the highest point where it passes through the roof.
- It must be at least 2 feet higher than any part of the building within 10 feet horizontally.
On a sloped roof, the second condition usually decides. The calculator works out how much the roof rises within 10 feet upslope of the chimney, stopping at the ridge, and adds 2 feet.
The French rule
In France, the outlet must be at least 40 cm above any part of the building within 8 m, and at least 1.20 m above the roof when the slope is under 15°. You can select it to compare.
Worked example
A chimney passes through a 6/12 roof, with its upslope face 6 feet from the ridge. Within 10 feet, the highest point is the ridge, which is 6 × 6/12 = 3 feet higher than the chimney’s upslope side. The chimney must therefore rise 2 + 3 = 5 feet above the roof on that side, which puts its top 2 feet above the ridge. A 2-foot-wide chimney is 1 foot taller on its downslope face, 6 feet in total.
If the same chimney were 15 feet from the ridge, the roof within 10 feet would rise 5 feet, and the chimney would need to stand 7 feet above the roof on its upslope side, still below the ridge.
Practical points
- Place chimneys near the ridge whenever possible: they need less height above the roof, are less exposed to downdrafts and look better.
- Tall chimneys above about 5 feet generally need bracing or a support kit, especially factory-built metal ones.
- Factory-built chimneys must follow their UL 103 listing and the manufacturer’s height table, which may be stricter.
- Caps and spark arrestors keep out rain, animals and sparks, and some local codes require them.
- Inspection: the NFPA recommends an annual chimney inspection, and sweeping as needed.
Local codes can add requirements, for example for nearby trees, taller neighboring buildings or wildfire zones, so check with your building department before you build.