Originally Posted By: rmeyers
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The phenomenon found in several configurations of standard type roof trusses where the bottom chord of the truss if buried in insulation and maintains stable temp & moisture content while the balance of the truss members are exposed to temp & moisture variations that cause the elements of the truss to work in such a way that the truss “Arches” or Lifts" off of the intermediate walls below causing tape or joint separation along the tops of the interior walls. (Can even lift interior walls!)

Had 2 severe cases in the last month! One pending litigation due to inaction of contractor to address issue on new home - 1 year warranty.

Second case discovered in 7 year old house I inspected in late May 2002. Freshly painted, looked great, BEWARE! Last week the current owners, my clients, called and wondered what could be causing all these cracks to develop. One look and my first thought was that we'd been had - how far away did the sellers move? Illinois does have a disclosure law and I don't honestly see how the previous owners could not have been aware of the condition since it obviously took a very concerted effort to conceal it!
(In their defense maybe they thought that was the fix?)
Current owners seem focussed on proper fix and not retribution, good people!

Proper fix includes nasty attic work cutting drywall loose from trusses and adding blocking for proper independent drywall nailing on tops of walls as needed. Then usually extensive drywall refinishing and painting.

Any additional comments, experiences, research, solutions, etc on this topic? Also known in it's earlier days as CFPS or Ceiling-Floor, Partition Separation!

Russ Meyers

Originally Posted By: Rusty Rothrock
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I read your informative post concerning the arching of trusses. Very interesting, excellent post.

Normally (maybe nothing is normal in the building construction trade) when trusses are installed the bottom cord is spaced 24" OC (or whatever the designed on center dimension is) and then nailed into each partition wall that the truss comes into contact with. Here in Richmond we also ran a perpendicular 2 X 4 stiffener over the bottom truss member.

My question is this... Will or could truss arching occur even if all of the above (that I just talked about) were done at original installation of the trusses ?

Thanks. Rusty

Originally Posted By: rmeyers
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Hey Rusty, Hope You’re Having a Great Day!

In response to your question, it does seem that truss lift can occur even if the bottom chords are nailed to the top plates and the truss bracing is properly installed. The forces that cause it seem to be at work in most trusses but just not to the extremes that are present when the cracks start appearing.

In the cases I mentioned, the trusses were nailed to the top plates. There were signs of pulling apart at that connection, between top plates and even as previously mentioned, a short wall section being pulled off the floor. (Gap between baseboard and vinyl.) That's what makes it so interesting for us, as inspectors, the failures show up at the weakest points of connection.

The forces at work here appear to be rather powerful and fortunately the phenomenon requires a combination of extreme conditions to create the movement required to open significant cracks. It seems to take humidity and temperature extremes, certain manufacturer characteristics in the truss (wood species & grains) and even installation related conditions that may aggravate the situation. (Insulation, ventilation & vapor control.)

Unfortunately we seem to have the conditions here in central Illinois that lend themselves to creating "truss lift". The drywallers in this area have dealt with this issue for years and recommend letting the trusses "float" over the non-bearing walls. Nailer blocks or clips are used on the walls in order to secure the ceiling drywall and the ceiling drywall is then not nailed to the trusses within 18" of the walls.

Early research appeared to indicate it was a mid-west phenomenon but I suspect that was because the data was gathered adjacent to the study area
so I'm not sure of the extent of the problem in the Richmond area.

Let me know if you come across any suspected cases in your area and I'll post any significant developments in the two cases I cited.

Russ Meyers