In another thread Boss Hog said: “BTW – Lack of attic ventilation is the worst culprit in truss uplift, IMHO. But that’s another subject………..”
I’d be interesting in reading anything BH has to say about the subject: how to spot it (I would guess there are subtle clues sometimes ina ddition to the obvious); what other culprits there are, how insulation might affect it, etc.
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I did a heck of a long post back in the webx days about truss uplift, but can't find it be searching the archives. Guess I'll have to start from scratch.
First of all, I think it's important to know what truss uplift is. The most widely accepted theory is that it's caused by a moisture difference in the top chord and bottom chord during cold weather. The bottom chord is covered in insulation, and is warm and dry. It dries out a great deal and shrinks some. The top chord is cold so moisture condenses on it. Since its damp, it expands. Since the top chord expands and the bottom chord shrinks, the truss arches upwards. This typically manifests itself by causing cracks where the ceiling drywall meets the drywall at the top of the interior walls.
In my experience, lack of attic ventilation is a prime cause of this. Absolutely EVERY house I’ve been to with truss uplift problems had poor attic ventilation. Let me run a couple of cases by you:
Case #1. A DIY builds his own house, and has uplift problems. He used continuous soffit vents, and ridge vents. He had never heard of proper vents (the styrofoam chutes) so he didn’t use them. When he blew his insulation he crawled out to the edges of the house and blew the soffit full of insulation.
Case #2. A guy builds a $450,000 spec house. It has an indoor pool and a huge hot tub in the master suite. No vapor barrier in the ceiling. Bath and kitchen fans are exhausted into the attic. In one spot in the house, a girder truss had been securely fastened to the top plate of the wall under it. The wall had been picked up unevenly, and caused a HUGE wandering crack in the drywall. Cracks were evident at other locations as well.
Preventing truss uplift is fairly simple during construction. Fixing it in an existing house that’s occupied is a messy pain in the neck.
Your truss manufacturer should be able to provide you with info about truss uplift. You’ll note that they refer to it as “partition separation phenomenon” instead of truss uplift. That’s mainly just smoke and mirrors to try to deflect some of the blame for drywall cracks. To some degree, I think this is justified. I think contractors and homeowners have learned just enough about truss uplift that they think ANY drywall defect is caused by the trusses. (I know of one guy who was called out to a house that had the tape pulling loose from vertical inside corners)
Here are a few links regarding truss uplift:
http://www.woodtruss.com/faqpartitionsep.htm
http://www.woodtruss.com/images/publication_images/ttbpartsep.pdf
http://trussnet.com/articles/TrussNetfeatureArticles.cfm?ID=21
I’d be happy to discuss it more, if any of you are interested.
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o.k., the top chord absorbs moisture,,, how about steps taken in the plant to minimize this, like painting something on it, ? That's along with the ventilation.listening for the secret.......searching for the sound...
no, wait, ..attach top chord and web with <g> sliding tenons. problem solved.listening for the secret.......searching for the sound...
I've never heard of anyone trying to paint or otherwise seal the top chords to prevent truss uplift. I doubt most builders would be willing to pay the extra cost.
But it's an interesting dea.
Sliding tenons? Not sure that would work. The webs have to be in either tension or compression, or there wouldn't be any point in having them there. Sliding tenons would make them useless. (I know you were kidding, but someone else might have thought it would work)They say television is making people dumber. What do they mean by that?
Quit pulling Boss' chord, er cord, er string, er leg ... yeah that's it - quit pulling his leg.
;).
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"The first rule is to keep an untroubled spirit. The second is to look things in the face and know them for what they are." --Marcus Aurelius