FIRE STOPPING at CHIMNEY PASSAGES - CONTENTS: What are the Fire stopping or fire-block details needed where a chimney passes through building floors? Why we need fire blocking & combustible clearances. Chimney inspection, diagnosis, and repair
Fire stopping at Chimney Passage Through Building Floors
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.
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). Sketches here were provided courtesy of Carson Dunlop. [Click to enlarge any image]
Other Common Examples of Indoor Fire Clearance Safety Hazards
We 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.
Fire Stopping where Chimneys Pass Through Closets, Walls, Floors
Where a metal chimney passes through building walls or floors fire blocking or use of a fire-blocking foam insulation may be required by local building codes.
At left you can see the orange fire-block foam spray insulation that oozed out from the partition wall that was sealed with this material. The building inspector also required us to trim away the excess foam.
NHFireBear, a frequent contributor to InspectApedia commented:
Ideally, there should be no unsealed openings from your closet to the attic, since any fire in the closet would naturally convect into the attic, possibly spreading the fire in an area where there are not smoke alarms or heat detectors.
That would create a life-threatening hazard to anyone in the building, if that were to happen. The ceiling should have a pass-through "ceiling mount" thimble/support, listed for the purpose (connector below, chimney above), and the chimney pipe through the roof should have shielding around it (metal hardware cloth, solid metal or solid wood), where it passes through the attic, to prevent combustibles from coming into direct contact with it.
A chimney fire should preferably stay in the chimney (even at 1,500 degrees) at least long enough to get the fire department there to save the roof.
Combustion air may be drawn into the burner either from a louvered closet door or from a properly installed air pipe from the outside or the attic, terminating in the burner or within a foot of the closet floor. Some local jurisdictions do allow gas-fired burners to have an unsealed overhead vent OUT to the attic.
NFPA 54 (2009): FIGURE A.18.104.22.168(1)(a).
I would prefer not to have that hazard for the citizens and firefighters in my town. The same is not allowed for OIL-fired burners.
NFPA 31 (2011): 22.214.171.124.
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.
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(Feb 1, 2013) Doug G. said:
Thank you for writing about this... Please elaborate further on the details of the sentence "The chemical change of pyrolysis lowers the temperature at which a substance will catch fire." Please describe or document a research source that describes the details of the chemical change and subsequent lowered ignition temperature. I believe your statement to be correct, at least largely so, but I am able to find only small amounts of peer reviewed data that supports the statement. I am trying to understand the details of the process so I feel comfortable in explaining it to others. My current understanding is similar to what you have written but the "show me" people in my audience are (and rightly so!) asking "what are the chemical changes and how can the wood possibly ignite at a lower temperature?"
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