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Re: GBlist: Engineered Truss Problems



greenbuilding Conference @ txinfinet.com wrote:
> 
> Hi All!
> 
> One of our member builders has been experiencing a phenomenon known as "truss
> lift," in which the vertical and diagonal bracing chords of the roof truss
> system shrink over time to the point where they are able to raise the bottom
> chord enough to separate it from the interior non-load-bearing walls up to an
> inch.  Has anyone else experienced this situation? Causes? Solutions? (Our
> builder has gone back to standard framing with 2x6 rafters.)
> 
> Thanks!
> 
> Doug Seiter
> Austin Green Builder Program
> 

Wood shrinks very little with the grain, it shrinks perpendicular to the
growth rings. What may be happening is the floor is deflecting, and/or
the floor joists have shrunk and compressed so that the floor has
dropped relative to the truss.

A solution to this is a "flexible" drywall joint. Do not fasten the
drywall on the ceiling within 12 inches of the wall. Instead, use clips
to hold to the wall. A more expensive way is to fasten a wider plate at
the top of the partitions to which the drywall is fastened, but not the
truss. As the partition drops, the drywall bends a bit rather than
cracking. 

I don't think this would work for a 1 inch gap though. There must be
some other mechanism causing that much change, and I would seriously
investigate. (Maybe sinking footings for interior lally columns?)

John Bunzick, CCS, CCCA
HKT Architects.
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