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- Discolored paint or wall coverings.
- Smoke oozing or pushing from around baseboards or door/window moldings.
- Continued smoke generation after all visible fire has been knocked down.
- No abatement of heat conditions though the fire appears to be extinguished.
Areas of hidden fire can sometimes be detected by listening for crackling sounds of fire still burning, or touching walls or hidden spaces to feel for hot spots. If not found, a hidden fire can smolder for a long period with the potential for developing into a serious problem. Firefighters could leave the scene only having to return to extinguish the rekindled fire. The primary causes of rekindles are incendiary fires or careless firefighting.
A concern of the incident commander during every phase of firefighting is whether the building is stable enough to continue interior operations. This ongoing size-up must continue during the overhaul and salvage stages.
Structural damage caused by the fire must be thoroughly investigated to ensure that firefighters can operate safely in the building. Are collapse indicators present such as wall cracks, or walls out of alignment? Some floors in older frame buildings may have sagged. Are they safe to operate on? Are there water tanks or air conditioner units on a fire weakened roof that pose a threat to firefighters operating beneath them?
A fire-weakened building should not be subjected to jarring impacts from axes. Saws should be used to open floors and roofs. This minimizes shocking jolts to the building that could create a collapse situation.
Lightweight building components should be checked for stability and damage. Extreme care should be made to prevent saws from cutting through these supports creating conditions for a possible failure.
What are the building's contents? The weight of merchandise can affect the building's stability. Is there a heavy load of stock or machinery that could affect the structural integrity of the building?
Bales of absorbent material can absorb great quantities of water. This will drastically increase the floor loading. Additionally, the bales can expand and push out walls and weaken building supports. When piled one atop another, they can shift and fall. Many firefighters have been injured and killed by falling bales.
In attached structures that have common masonry bearing walls, a problem often found is that wooden floor and roof joist abut in the wall sockets of these walls. Fire attacking the wooden beams in the original fire building can transmit fire to the adjoining beam through conduction of heat. The fire can smolder at this location for long periods.
All buildings require thorough examination. Type of building construction can help determine what overhauling may be needed.
The fire-resistive building usually requires minimal overhaul. The investigation should concentrate on checking shafts and poke-through construction. The components of a non-combustible building will not add fuel to a fire but an investigation must be made to determine if any structural damage has occurred.
Frame and ordinary constructed building have components that will contribute fuel to the fire. There can be many concealed spaces that need to be checked. The heavy timber constructed building usually withstands the effects of fire, or is totally destroyed. Flooring may need to be opened to expose hidden fire.
Parts of a building damaged by the fire may create a dangerous situation and need to be removed such as a wall or roof sign, a window or roof air conditioner unit, or a hanging cornice. Once the emergency has stabilized, a consideration in opening a location to check for hidden fire should consider how the area will be repaired. An example would be checking an attic for fire extension. Ripping off roof shingles and placing a hole in the roof would provide access but expensive and time-consuming to repair. A better method would be to investigate the area from the interior. A trap door may exist through which a ladder can be raised into the attic, or the ceiling can be opened from below.