wood rafters exposed to fire and heat 1

[charliealphabravo]

Hi all,

I am looking at a conventionally framed roof that has experienced a fire. I have identified the rafters, ceiling joists, and collar ties that are clearly damaged by charring. They are fairly localized.

I notice that nearly all the rafters are darkened by soot and smoke along the ridge. Also I notice that the rafters are shiny where they have been weeping sap.

I would appreciate any armchair opinions or references to literature that discusses this condition. I would not necessarily think that the strength of the rafters have been compromised by the weeping sap but it is indicative of exposure to heat and I recall discussions in other threads about the effect of heat on the strength and durability of wood.

In this particular case, the roof is rather simple and modest in size and I think that replacement with trusses might be most practical. I'm not inclined to nickle and dime the entire roof with a piece by piece evaluation. This case is in Alberta, Canada. The lumber is not stamped but is most likely DF or SPF.

Thanks again.
cab

 

[Ron]

CAB...In my opinion, the person who knows more about this subject than perhaps anyone else in the US is Robert H. White at the USDA, Forest Products Laboratory. He has written extensively on the subject and you can download many of his publications from the Forest Products Lab website

Here's a link...

Fire Damage

 

[charliealphabravo]

ahh...grass hoppa thank yoo

 

[dik]

Do a quick search of the site, and there are numerous threads on fire damage to wood structures.

Ron: Hear about the wooden cow?





























Wooden milk!

Dik

 

[msquared48]

Please... I just finished my cereal when I read that.










































Now I have to try to eat again.

Mike McCann
MMC Engineering

 

[charliealphabravo]

thanks dik, i am seeing them now. they didn't come up in my earlier search...

 

[dik]

Ron and I have posted a few comments on fire damage... and there's some detail. We've both been involved with doing reports for this.

Sorry Mike... I just remembered it... goes back 40 years... Have another bowl of fruitloops...

Dik

 

[RFreund]

Have guys discussed the sapping issue before or what that is indicative of?

EIT

 

[dik]

Sapping occurs in the early stages of pyrolysis and is more prominent with softwoods having a more prominent sap content and lumber that is normally greener. The fire heats the lumber and increases vapour pressure, and makes sap products more fluid like. The increased pressure tends to force the sap out. It's normally part of the initial stages of pyrolysis. There is normally (and initially) a minor increase in strength, hardness and stiffness. This accompanies a reduced toughness and is normally not recoverable.

Dik

 

[dik]

Catch the thread...

thread337-304619

 

[Ron]

dik...you need to provide warnings with posts like that!! Coffee on the keyboard...again.

...and dik, I'm way late on drafting our fire damage FAQ/Whitepaper. Been working on a book (not about fire damage...just construction), but will have the edits done for that in a couple of months. I'll try to be a bit more diligent.

 

[dik]

A book... Can I be in line to purchase a signed first edition... I'm pretty sure that JAE will want one also...

Dik

 

[Ron]

dik...it certainly won't make the best seller list! The publishing has been delayed until Jan 2013. It was originally supposed to be Jan 2012, but the publisher had some internal issues with "permissions"....new administrative procedure for validating photo and drawing permissions. Caused a lot of extra work for the production group and some extra for me as well.

I'm just taking over the authorship from another author who retired. I did some editing on the last 2 or 3 editions, so the publisher asked me to take it over for the next (9th) edition. Beats having to do it from scratch!!

Yes...you and JAE will get a copy. It's heavy enough that it will hold down paper on a windy day, so at least you can get some use out of it!

 

[dik]

I don't mind buying a copy... it'll likely be an interesting read... now if I can only find the time to read it <G>.

Dik

 

[RFreund]

I know this is very general, but when do you usually specify replacement for a typical 2x rafter or floor joist framing.

No Charring generally ok
Any charring needs replacements
Charring at bearing ends - questionable?


EIT

 

[dik]

I generally accept a small amount of charring (1/16" or so)... scraped off and the area encapsulated. Fire restoration contractors (in this area) are very good at encapsulating the charred/smoked area to prevent future odour. Slight charring at the bearing ends OK too; ends are often not as badly charred as mid-span areas.

Dik