Truss Uplift – by the Barrie Home Inspector

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Truss UpliftPre-engineered roof trusses provide the frame for your roof system. These trusses are usually fabricated from 2-by-4 lumber into a shape that resembles a triangle with the letter W inside of the triangle.  Trusses spread your roof load to exterior walls which is continued down to footings. The bottom chord of the roof truss forms the ceiling joist and the bottom chord is immersed in insulation while the rest of the truss is not, stresses that set up in winter within the truss cause the truss to lift upward to some degree.

Truss uplift is a phenomenon common in newer homes built with roof trusses and is normally due to moisture differential between the bottom chords and the top chords of the trusses. Top floor ceiling will rise up in the winter and drop back down in the summer. Needless to say, this is a little disconcerting to most homeowners.

When wood shrinks, it shrinks differently along the grain than perpendicular to the grain. It shrinks much more at right angles to the grain, than along the grain. Studs don’t get shorter, but they get thinner in thickness and in width.

At first glance, one might assume that the floors have settled. Actually the ceiling has gone up – sometimes creating a gap as large as an inch where interior walls meet the ceilings.  In the winter the warm air from the ceiling below and the thickness of the insulation keeps the bottom chord dry, causing them to shrink. While the top chords are absorbing moisture and being kept damp with the high humidity in the attic. The dampness of the top chords of the trusses cause them to expand. This differential movement in the top and bottom chord of the trusses causes them to arch up in the center. When the trusses arch up it causes cracks in the ceilings at the center of the building. As the trusses dry out with the warm summer air they drop back down closing most of the cracks.

To help combat damage to drywall from truss uplift Drywall manufacturers recommend that when attaching drywall to the bottom of a truss the attachment should be about 16 inches away from an intersecting wall.  Another option is to attach trim to ceiling and not nail it to wall, allowing it to move up and down with the trusses.

When building a new home there is a framing clip is available today that’s designed to stabilize interior walls to the truss bottom. The clip allows the truss to move up and down without dragging the walls along with it. Most subdivision homes will not be using these as the framers get no extra money for installing “luxury items”.

 

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