Existing Wood Trusses

[STR04]

I'm working on a re-roofing project in FL with 30 yr old parallel chord trusses. The roof is flat and has been leaking for years. I viewed some of the trusses exposed to moisture but could not see any real damage to the wood or plates. I doubt I can find the original truss mfr. so how would I evaluate the existing conditions and make repair or refurbish recommendations?

TIA

 

[dik]

First, it's a matter of determining if there is any 'dry-rot' (ie brown rot). This can be done by checking for any soft areas and/or having a mitochondrial specialist examining the area. If there is dry-rot you may have serious problems and it may be more economical to re-do the roof framing.

If no dry-rot, then if the plates haven't worked out; sometimes, they work their way out then the trusses are likely good for their original design load. This may have changed and acceptance of the old loading may be up to the authority having jurisdiction.

Dik

 

[Structures33]

Assume that all members are sound (i.e. no rot). Then either analyze the truss by hand or create a computer model to determine the forces in the members. If they work, great and if not, determine what needs to be replaced to make it work.
If there are members whose strength is in question, determine from the analysis if the original size is adequate or if it needs a larger size - then repair accordingly.

 

[jechols]

I recently had a similar project except that on one end of the building part of the roof collapsed. The entire roof had been leaking for some time. Obviously we replaced the collapsed portion and the owner wanted us to examine the remaining roof trusses. Be thorough, at first glance the rest of the trusses appeared to be fine. The contractor then took a hammer (claw end) hit one of the trusses that had already partially failed and it was apparent the wood was in bad shape. He then went on to point out that the trusses that looked fine, upon closer inspection had hairline cracks. Then I looked at a chord member that had snapped and the way it broke looked more like low quality metal brittle failure (i.e. no strands, short jagged break). I recommended evacuating the rest of the building until the entire roof could be inspected or replaced. The owner angrily complied saying that I was an alarmist. A few weeks later after a good rain another portion of the roof collapsed. What I am trying to convey here is do not assume the wood material is ok. If it has transformed into what I had on my job, failure could be imminent.

j

 

[BPA827]

The trusses should have some sort of designation on them to indicate the manufacturer. With a little luck, you may find the original company that designed the trusses.

 

[STR04]

jechols,

What part of the county did this take place?

 

[youngstructural]

I have heard of very similar occurences to what jechols has reported, and know of a progressive collapse of a late 19th century truss system in Kingston, Ontario in which one failed truss brought most of the structure down like a stack of cards.

I would be curious where this was as well jechols; Was it indeed in the states as it seems STR presumes?

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...