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Fire/Smoke Damage to wood structural members.

RESENG2 (Structural) (OP)
21 May 08 11:06
What is the overall consensus on fire/smoke damage?  I am often consulted to evaluate fire damaged roof members when the insurance company does not agree that the roof should be replaced.  Charring is obvious, but smoke damage is difficult.  Do any of you know of a resource that outlines the levels of fire/smoke damage to the decrease in strength of the member?  In my opinion, the exposure to extreme heat leaves the members brittle.  NDS notes that prolonged heat exposure above 150F can cause permanent loss of strength, but the percentage of loss and the definition of "prolonged" are left to interpretation.

Obviously, the CYA approach is to say that all members should be replaced.  This is actually the request of the contractor and homeowner.  However, the insurance company shouldn't have to pay for un-damaged, or lightly damaged members (I guess?).

Any opinions/suggestions?
Thanks!
csd72 (Structural)
21 May 08 12:10
A new joist between every second pair of joists will reduce load on the existing joists by 33%. Will this be sufficient to negate your fears regarding invisible smoke damage?

 
civilperson (Structural)
21 May 08 13:39
Wood is a forgiving material, once the char and black is removed everything that remains is full strength capable.  Measure the loss of dimension on timber members and calculate the remaining strength.  Smoke has no strength effect, just requires cleaning for odor control.  Prolonged exposure would be measured in years such as chimney enclosures or boiler room walls.
RESENG2 (Structural) (OP)
21 May 08 14:03
Thanks for the replies.  I agree that adding members to compensate for the loss in strength is justified, or checking the new dimensions of the effective "good material" left.

This particular house is fairly old and the county is requiring the contractor to bring it up to code.  It is located in a coastal region of Florida, therefore uplift connections have to be made.  A daunting task even with a new roof, but to have to work with a weakened roof system is the last thing I'd like to have my name on.  My concern is that the rafters have lost a significant amount of their original (in place) moisture content, more so than would be expected with new construction.  This would cause the members to shrink and consequently have the nails attaching the roof sheathing to loosen.  I could have them re-nail the sheathing, but likely the rafters will regain moisture and swell causing the fasteners to loosen once again.

To put it in perspective, if you have a house fire and your window curtains receive smoke damage, the insurance company will replace them.  However, if your roof system has smoke damage, they want you to paint them and everything is OK???
 
MiketheEngineer (Structural)
21 May 08 15:07
Start another fire and finish the job....

Your points are well thought out.  In order to meet FL code - it may well be easier/cheaper to remove and replace the roof.  Smoke and moderate heat will not hurt the rafters/trusses - if short term.  However - any charring should call for replacement.
csd72 (Structural)
21 May 08 18:19
You may consider giving 2 options:

1. to replace the whole roof
2. Outline the investigative work required to justify keeping the existing roof along with any work that has to be done anyway. Also state that there are no guarantees in this option.

get quotes for the investigative work and get the contractor to do an estimate for each - I am sure that replacing it will come out cheaper.
jheidt2543 (Civil/Environmental)
21 May 08 23:08
I agree with civilperson, smoke has no effect on strengh of wood members.  If you are concerned about moist levels, check them, however I've measured moisture content in 100+ year old industrial buildings and found it 5-10% and the lumber (heavy timber constrution) is still servicable.

Make an assesment of the reduction in cross-section due to charing and reduce the member capacity accordingly. You can always reinforce it.

Just painting is not enough to cover smoke smell, it should first be sealed with a product similar to "KILZ" and then the finish coat of paint.
dik (Structural)
23 May 08 12:05
A couple of issues... the material should be encapsulated to prevent/reduce the smoke 'odour' and that material subjected to a fire is more flammable in the next fire (pyrolisis sp?).  Should have a sign-off by a structural engineer in the employ of the insurance company.  What is the name of the insurance company?

Dik
RESENG2 (Structural) (OP)
23 May 08 12:44
No idea.  I wrote a report conveying my concerns.  I guess the insurance company could have another engineer back their case.  We'll see.

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