Updated: Feb 25, 2021
Have you noticed a horizontal gap along the intersection of your wall and ceiling? This may be caused by truss uplift. In Nova Scotia, we experience different seasons, and those seasons bring changes in temperature. The bottom chord of the truss is covered with a thick blanket of insulation, keeping it warm, while the top chords are above the insulation allowing them to absorb moisture from the cold winter air. The top chord elongates/expands while the bottom chord shrinks. This causes the bottom chord to arch in the middle, lifting it off the walls. Truss uplift is not a structural concern but may be cosmetically unpleasing. In the spring/summer the gap typically disappears as the cycle is reversed.
Diagram 1: Cracks and gaps along the ceiling/wall intersection caused by truss uplift.
Diagram 2 (Attic view): The bottom chord completely covered with insulation.
Diagram 3: The Components of a Truss