Fire stopping at a chimney (C) Carson Dunlop Associates Fire Stopping Requirements at Chimney Passages Through Building Floors
Why we need fire blocking & combustible clearances

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Chimney construction: fire stopping.

This article describes the basic requirement for fire stopping needed where building chimneys or flues pass through building floors.

Fire stopping is a measure taken to slow the spread of fire between building floors - an event that could occur at openings cut to permit a chimney to pass through from one floor to another.

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Fire stopping at Chimney Passage Through Building Floors

Sketches courtesy of Carson Dunlop. [Click to enlarge any image]

Due to the drying of lumber and movement in structures the chimney shaft must remain free of any ties into the framing of the building. The space between the shaft and the building is or can be sealed with "fire code" [fire-rated] sheetrock or metal flashing if a fire stop is required between floors.

Fire stopping at a chimney (C) Carson Dunlop Associates Fire stop at metal chimney (C) Carson Dunlop Associates

Chimneys that pass through several floor levels of a home may be required to be fire stopped at each penetration. Typical fire stops are sheet metal or fire code sheetrock.

Carson Dunlop's sketch shows fire stopping at the fire-clearance gap provided between the chimney and wood floor framing (above left) and fire stopping around a metal chimney as it passes through building floors (above right).

Other Common Examples of Indoor Fire Clearance Safety Hazards

Fire clearance too close to combustibles (C) Daniel FriedmanWe often find flue vent connectors routed too close to combustible wood framing or, as in our photo at left shows, too close to other combustible materials such as the foam insulation just a few inches above this flue pipe.

Many building fire safety codes specify that the fire clearance between a flue vent connector and combustibles needs to be that specified by the appliance manufacturer.

Indeed some modern heating appliances permit pretty close clearances, as little as a few inches.

In the absence of a manufacturer's specification, we want to see at least 18" between the flue and the nearest combustible surface.

The reason for Fire Clearances from Wood Materials

The reason that building codes specify a healthy distance between wood materials (or other combustibles) and flue vent connectors is not just that the heat from the flue will immediately set the wood on fire. Rather it is also that wood that has been heated over time, even to the relatively low temperature of 200 to 300F, will be chemically affected to become more readily combustible.

Details are at PYROLYSIS EXPLAINED - separate article

Readers of this article should also

This article series on chimneys, chimney construction, and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed.


Continue reading at FIRE & SMOKE DAMPERS, AUTOMATIC or select a topic from the More Reading links shown below.

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FIRE STOPPING in BUILDINGS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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