I
entered an attic of a 2000 sq. ft. property in Plano Texas last week and was
confronted with a burnt area covering around a third of the attic. A
disconnected flue that exhausts into the attic just below the roof decking
caused the burning. I was amazed just how much damage the discharge of fumes
into the attic had caused but found out later that the current roof had been
installed 14 years ago and the flue may have been disconnected then. It is
known that high temperatures have a dramatic effect on lumber’s load carrying
capacity and increases the probability of brittle failure at the weakest link in the piece of lumber
associated with a defect - notably, a "large" knot(Note 1). There was a failure present and visible in
the attic as the following picture shows. This raised my concern in three
areas. First there is a distinct possibility that the attic could catch fire,
second that the structural integrity of the attic has been so compromised that
it could collapse and last, but not least, the probability of carbon monoxide
entering the living area.
In
the picture below you can see some discoloration of the outer flue pipe. Steel changes
color as its temperature rises, which may indicate that the pipe achieved a
very high temperature at some point in the past. Since flue gases leaving a
flue can reach over 5000F(Note 2) it is possible that
these very high temperatures were reached.
The
temperature at which wood chars depends on multiple variables and I do not
intend to discuss it here only to note that it is generally above 150°F(Note
1), which is the temperature that lumber’s load-bearing ability starts to
drop dramatically. In addition 170ºF is the temperature cited in the 1959 UL as
a safe maximum for lumber. It is also known that long-term exposure to hot
pipes can cause lumber to burn3.
The
following picture shows a tar deposit running down the surfaces of the decking
and framing. The roof was covered with 20-year 3 tab shingles and this is
evidence the heat may be melting the tar in the shingles and it is seeping into
the attic.
Although
a burnt attic is an unusual find I have found this type of issue in the past,
which prompted me to write this short description of the problem. At the time
of writing I have had no adverse feed back from the sellers regarding the issue
other than the listing agent saying the attic had been inspected by two other
inspectors in the last month and they had not mentioned the burn damage at all.
In
my opinion this burnt attic represents a considerable risk to my clients and
the current occupiers of the property and I would be interested to know how
many times this has been found by other inspectors in the last few years. The
following is the call out that went into the report.
Call
Out
The
right elevation of the attic has extensive burn damage. This burn damage will
result in the weakening of the attics structure. The cause of the damage is the
central heating furnace that vents into the attic just under the roof decking.
In addition to structural damage caused by the missing flue to the furnace the
heating of the decking will have caused the shingles to degenerate. Finally there
is a possibility of Carbon Monoxide entering the living area. You should
arrange for a structural expert to access the damage to the attic and what
repairs need to be made. In addition you should
· Obtain an assessment of
the roof insurability from an insurance company
· Enquire if the building
is habitual under current City regulations
All
this should be completed before escrow.
You
can find out more information on
References.
1)
Modeling the reliability of wood tension members exposed to elevated
temperatures http://swst.metapress.com/content/h9h673482r173h6g/
2)
Flue Gas Temperature and Heat in the Flue Gas Flow (Btu/hour) http://ackerandassociates.com/computer_program_examples/23_flue_gas_temperature_and_heat_in_flow/
3)
‘Pyrophoric Carbon’ and Long-term, Low-temperature Ignition of Wood http://www.doctorfire.com/low_temp_wood1.pdf
https://drive.google.com/file/d/0ByvFfKK7TGciX3VMUDVlQ3NxbVU/view?usp=sharing
