How much chimney height do I need above the roof?
Chimney height is one of the critical factors affecting the performance of your wood stove, and whether your wood stove installation is safe.
For traditional construction, the "3-2-10 Rule" is used to determine how high a chimney needs to be above the roof line. For RVs, Buses, Vans, and Boats, we suggest using a somewhat shorter detachable chimney.
The 3-2-10 Rule
The chimney opening above the roof of a sticks and bricks house should be 3 feet above the roof line, or 2 feet above any part of the building within 10', whichever is higher.
For most small structures like small cabins and tiny homes, the tallest part of the structure is within 10' of the chimney. In that case, the chimney needs to be 2' above the peak of the roof, or 3' above the roof line, whichever is higher.
For larger structures, the result will depend on the roof pitch. For a building with flat or shallow pitched roof, the chimney needs to be 3 feet above the point where it passes the roof line. For a building with a steeply pitched roof, the chimney opening needs to be 2' above the highest part of the roof within 10'.
Why is chimney height so important?
Taller Chimneys Create Stronger Draft
One factor that dictates the strength of the draft produced by a flue system is the difference in air pressure between the firebox and the chimney opening. Because air pressure is lower at higher altitudes, and a taller column of rising air pulls harder on the air below it, increasing the height of a chimney generally strengthens draft.
Traditionally constructed homes typically have at least 15 feet of vertical height between the firebox and the chimney opening. Most tiny structures simply aren't tall enough to support a 15 foot flue system, so it's important to have as much height as possible within reason.
Taller Chimneys Prevent Backdrafts
When a chimney opening is too close to the top of a structure, swirling air currents caused by wind hitting the structure can push down on the chimney opening, causing smoke to back up in to the living space. This condition, called a backdraft, is hazardous, but generally avoidable.
To prevent backdrafts, chimneys following the 3-2-10 rule are typically high enough to stay above any swirling air currents. As a result, wind hitting a sufficiently tall chimney should all be lateral, and tends to improve draft rather than causing backdrafts.
In extreme conditions, shielded and wind-directional rain caps can help prevent backdrafts, but are not a substitute for adequate chimney height.
Taller Chimneys Keep Smoke off the Roof
If a chimney is too short, smoke can hit the cold roof and condense into a flammable creosote deposit. Not only is this condition unsightly, but it's a fire hazard.
Sparks from the chimney landing on a creosote deposit could cause it to ignite and start a house fire. A spark arrestor screen built into the rain cap can help limit the size of sparks that can escape, but is no substitute for adequate chimney height.