What does 3-2-10 actually mean?

Three feet above where the chimney passes through the roof, two feet above anything within a ten-foot radius. You take the larger of those two heights.

Does a ridge 12 feet away count?

No — only structures within a 10-foot horizontal radius count. A ridge 12 ft away is ignored, even if it is taller than the chimney.

My chimney meets the 3-foot rule but still downdrafts. Why?

Almost always a taller ridge, wall or tree inside 10 ft is casting a wind shadow. Apply the 2-foot rule to that object; you likely need a taller stack or a specialty cap.

Can a chimney be too tall?

Yes — a very tall chimney can over-draft and burn fuel fast. That is managed with a damper or a key damper, not by cutting the chimney below the 3-2-10 minimum.

Do I measure the 10 feet along the roof or horizontally?

Horizontally — the plan distance, measured level. On a steep roof an object 10 ft away up the slope can sit well inside the 10-foot horizontal circle, so it still counts against you.

The 3-2-10 chimney height rule explained

A chimney that is too short, or shadowed by a nearby ridge, will not draft. The 3-2-10 rule fixes the minimum height with a simple max of two clearances.

By Francesco Zinghinì

How tall must a chimney be above the roof? The answer is the 3-2-10 rule, and it is not one number but the larger of two: the top of the chimney must sit at least 3 feet above the point where it passes through the roof, and at least 2 feet above any roofline, ridge, wall or structure within a 10-foot horizontal radius. Whichever of those two is higher, that is your minimum. The 3-2-10 chimney-height calculator takes both inputs and returns the max.

Why two clearances, not one

The 3-foot rule keeps the outlet clear of the roof surface itself, where snow, debris and the roof’s own boundary layer would otherwise choke it. The 2-foot-within-10-feet rule handles the sneaky failure: a chimney that clears its own penetration but sits in the wind shadow of a taller ridge, a second-story wall, or a big tree. That shadow creates positive pressure and downdrafts — smoke rolls back down the flue and into the room. The rule forces the outlet up out of that dead-air pocket.

Worked example: chimney near a ridge

Take the roof surface at the penetration as the datum, height 0. Suppose the ridge is 9 feet away horizontally — inside the 10-foot circle — and stands 4.2 feet above the datum. Now run both clearances: 0 + 3 = 3 ft from the first rule; 4.2 + 2 = 6.2 ft from the second. The max is 6.2 feet above the penetration. The ridge, not the roof, sets the height — and if you had built to the 3-foot minimum you would have a chimney that never drew right.

Flip it: a chimney on a gable end with the ridge 14 feet away. That ridge is outside the 10-foot radius, so it does not count — even if it is tall. Now only the first rule applies: 0 + 3 = 3 feet. Same house, very different answer, purely because of the 10-foot horizontal test. Measure the horizontal distance honestly; that is the number people fudge.

How to run it every time

Pick a common datum — the roof surface where the flue exits.
Clearance A = penetration height + 3 ft.
Clearance B = (height of the tallest thing within 10 ft) + 2 ft.
Required top height = max(A, B). Build to that or taller.

What the rule does not cover

3-2-10 is a minimum for draft and a code baseline, not a guarantee. Steep terrain, a hillside above the house, tall trees just past 10 feet, or a complex roof can still cause downdraft — sometimes you go taller than the rule, or add a specialty cap. A very tall chimney can over-draft and burn wood too fast, which is a different problem you solve with a damper, not by shortening the stack. And the rule is about the chimney outlet; clearances to combustibles inside the structure are a separate requirement (see clearance-to-combustibles).

Measuring the 10-foot radius on a real roof

The rule lives or dies on the horizontal 10-foot measurement, and roofs make that awkward. The distance is measured level, not along the slope — so on a steep roof, an object that is 10 feet away up the pitch may be well inside the 10-foot horizontal circle. Picture a dormer whose nearest wall is 7 feet horizontally from the flue and rises 3.5 feet above the penetration datum. It counts: clearance B = 3.5 + 2 = 5.5 ft, versus 3 ft from the first rule, so the chimney needs 5.5 feet above the roof even though the dormer looked minor from the ground. People miss dormers, parapets and second-story walls constantly because they eyeball the slope distance instead of the plan distance.

Two more field notes. First, snow country: in heavy-snow regions builders often add height beyond the code minimum so the outlet clears drifting on the roof — a stack buried to its cap does not draft. Second, the rule counts anything solid, not just other chimneys: a wide gable wall, a large HVAC unit on a flat roof, even a mature tree limb hanging within 10 feet can throw the wind shadow that stalls draft. When in doubt, treat the tallest object inside the circle as the one that sets the height, and build to it or above it — going a foot taller is cheap insurance against a lifetime of a smoky room. Remember, too, that the outlet height is a minimum for draft, not a licence to skip the flashing, cap and clearances that keep the rest of the chimney watertight and safe.

The 3-2-10 rule is written into NFPA 211 and adopted through the residential building code; see the National Fire Protection Association and the International Code Council. For how height ties into draft physics, the U.S. Department of Energy has good background on the stack effect. This is a planning rule — confirm the exact heights and the 10-foot measurement, follow NFPA 211 and local code, and have a mason or CSIA-certified sweep verify a marginal chimney. See also how draft works and the chimney anatomy reference.