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...
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