A.Truss chord uplift is caused by the dimensional instability of the lumber used to make the trusses. When the bottom chord is covered in insulation, it tends to expand and contract with seasonal climate changes at a different rate than the top chords. This causes the bottom chord to bow and lift periodically.

Some builders may add a king-post to the truss to eliminate the problem. However, this solution is not supported by our research at the Building Research Council at the University of Illinois. A project sponsored by the Truss Plate Institute, concluded that the most important factor is for the truss fabricator to use bottom chords cut from the mature wood on the outer part of a log. "Juvenile wood," formed while the tree is young and located in the center of the log, expands and contracts with changes in moisture much more in length than the mature wood at the outer portion of the log.

For builders, I recommend using clips such as the Stud Claw (5370 Chestnut Ridge Road, Orchard Park, NY 14127; 716/662-7877), which connect the trusses to interior partitions but allow the trusses to move up and down. Also use drywall clips to hold the edges of the ceiling drywall to the studs rather than nailing it to the truss chords. Together, these allow the truss chord to move without causing finish problems.