A Level II chimney inspection is performed at the time of any cleaning, for real estate or insurance purposes, or as an annual check requested by a homeowner. A fire investigation is completed after a structural house fire has occurred, and is usually requested by an insurance or forensic company. This inspection could be considered Level III, but goes much further than that and is called a C&O (cause and origin) by the insurance and forensic companies.
My husband, Gene, is a State Certified Fire Investigator and does the onsite investigation. I write all the reports, so we both get to see what happens in the real world after there has been a house fire. Some fires only cause monetary losses to the structure, but others result in injury or death to the occupants.
Reasons for fire
Most house fires related to chimneys are due to an installation error, or problems not discovered by someone doing an inspection, maintenance, or repair work. Rest assured, the last person doing anything to the chimney is the first person the insurance company goes after. Then everyone else from the builder, to the supplier, to the manufacturer, might be involved. In our experience, about 60% of house fires that occur around chimneys happen at the smoke chamber area.
The smoke chamber is the transition area in a masonry chimney that starts just above the damper and continues to where the first flue tile is installed. It serves as the support for a clay tile flue liner. Chambers are usually corbeled brick or block work, starting wide at the base and narrowing down to accommodate the flue liner. Most chambers are 3 ft - 4 ft in height, but can be shorter or taller and out of proportion.
Most chamber problems occur due to improper construction behind the facial wall and in front of the chamber. The finished firebox, chamber, chimney, flue, and facial wall may look fine from both the outside and inside the chimney. The facial wall is not a structural part of the chimney, but a decorative front that is independent of the chimney (if it was built correctly).
The area behind the facial wall cannot be seen during a normal inspection. A wood header and studs may be installed incorrectly, with inadequate clearance to the chimney. Some builders do not follow clearance requirements, and place the combustible wood framing right next to the smoke chamber. Other combustibles that may be found in this area are plywood, paper-covered sheet rock, or insulation.
Pyrolization is the chemical change that occurs to combustibles as they are exposed to heat over time. Any wood in the vicinity of the chimney may be exposed to heat. Pyrolized wood can ignite at 200° F - 225° F (or possibly lower) instead of its normal ignition temperature of 450° F - 500° F.
It is important to note that only heat is required to ignite wood or other combustibles, not a flame. We have seen cases where the wood headers ignited when exposed to heat from gaps in this area, even though they were more than 2 in. from the chamber.
There could be voids in the smoke chamber that are unseen by the human eye, but that can allow smoke and heat to transfer through the masonry. Also, as the masonry warms up during a high-heat event, cracks can open even further. It is usually safe to assume the worst, even if you can't see it.
The smoke chamber may need to be swept or cleaned to remove creosote before being inspected. Any glazed creosote must be removed by power cleaning.