One must be cognizant that flashover is a transition not a condition. I know of no fire patterns that could be definitive of flashover,

921-2014 “3.3.83 Flashover. A transition phase in the development of a
compartment fire in which surfaces exposed to thermal radiation
reach ignition temperature more or less simultaneously
and fire spreads rapidly throughout the space, resulting in full
room involvement or total involvement of the compartment
or enclosed space.”

However, the condition of full room involvement does, produce a number of fire patterns by which its occurrence could be reasonably opined.

921-2014
“3.3.89 Full Room Involvement. Condition in a compartment
fire in which the entire volume is involved in combustion of
varying intensities.” [See Figure 5.10.2.7, below]

“5.10.4 Flashover.
5.10.4.1 Flashover represents a transition from a condition
where the fire is dominated by burning of the first item ignited
(and nearby items subject to direct ignition) to a condition
where the fire is dominated by burning of all items in the compartment.
This transition is generally characterized as the transition
from “a fire in a room” to “a room on fire.” It is important for
investigators to be aware of the fact that flashover is a triggering
condition, not a close-ended event. The postflashover condition
is called full room involvement. The onset of flashover occurs when
the hot gas layer imposes radiant energy levels (flux) on unignited
fuels sufficient to ignite them. A heat flux from the hot gas
layer of approximately 20 kW/m2 at floor level is generally considered
sufficient to cause flashover. At this heat flux, crumpled
newsprint will ignite in seconds. Flux levels in full room involvement
are considerably higher than at flashover. Heat fluxes at a
floor level of 170 kW/m2 have been recorded.

“5.10.4.2 Once flashover conditions have been reached, full
room involvement will follow in the majority of fires unless the
fuel is exhausted, the fire is oxygen deprived, or the fire is
extinguished. In full room involvement, the hot layer can be at
floor level, but tests and actual fires have shown that the hot
layer is not always at floor level. Full room involvement may be
achieved by fire growth that does not involve flashover.”

6.3.2.4 Full Room Involvement–Generated Patterns. If a fire
progresses to full room involvement (see 5.10.2.1 through
5.10.2.8), damage found at low levels in the room down to
and including the floor can be more extensive due to the
effects of radiant flux and the convected heat from the descending
hot gas layer and the contribution of an increasing
number of burning fuel packages. The radiant heat flux
has the greatest impact on surfaces with a direct “view” of
the hot gas layer. As the hot gas layer descends to floor level,
damage will significantly increase. Damage can include
charring of the undersides of furniture, burning of carpet
and floor coverings under furniture and in corners, burning
of baseboards, and burning on the undersides of doors.
Full room involvement can result in holes burned through
carpet and floor coverings. The effects of protected areas
and floor clutter on low burn patterns should be considered
(see 6.3.3.2.8). Although the degree of damage will increase
with time, the extreme conditions of the full room
involvement can produce major damage in a few minutes,
depending on ventilation and fuels present.


[www.kennedy-fire.com]

Pat Kennedy, CFEI, CFPS, MIFireE
Fire and Explosion Analyst
Sarasota, Florida
[www.kennedy-fire.com]