A. Henry Spies responds:Truss uplift is caused by differential shrinkage between the upper and lower chords of a truss. In a well-insulated house, the bottom chord is buried in ceiling insulation. In the winter, that chord is kept much warmer, and tends to dry to a lower moisture content than the top chord, which is exposed to the ventilated attic air. This dry bottom chord shrinks. Most of the shrinkage takes place across the grain, but there is some lengthwise movement as well.
In a triangular structure, such as a truss, if the bottom member of the triangle is shortened while the two top chords remain the same length, the peak of the triangle rises, pulling up the bottom chord, which is attached by webbing or a king post. As the ceiling rises, unsightly corner cracks may open up. If the partition is firmly attached to...
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