Avoid Drywall Cracks from Truss Uplift

Key Highlights:

• Truss uplift occurs when moisture causes the bottom chord of a truss to swell, leading to upward movement that can crack drywall joints.
• Seasonal changes in temperature and humidity are primary factors influencing truss movement, especially in insulated attics.
• Floating drywall corners and backing angles are effective strategies to allow for movement and prevent visible cracks.
• Using a truss backing angle provides drywall support that accommodates seasonal bowing while maintaining drywall integrity.

TRANSCRIPT:

Let’s look at truss uplift for a minute because it confuses a lot of people.

The bottom truss chord is "inside" and the top chord is "outside"

Insulation in the attic means the bottom chord of a truss is warmer and moister than the rest of the truss. In winter, that causes the bottom chord to swell and the top chords to shrink.

And because the bottom ends of the truss are nailed in place, it can only swell upward. When drywall is fastened to the walls and ceiling, this seasonal movement leads to cracks.

Floating drywall corners hides framing movement

One way to avoid these seasonal cracks is to float the corners, keeping fasteners a foot and a half or so away from the end of the sheet.

A stronger way is to use a truss backing angle, which allows the movement, and holds the angle together tighter than paper tape and joint compound. The ceiling drywall is fastened into the backing angle at the edge and into the truss about 18 inches back from the edge, where the movement is less pronounced.

Now, when the truss bows upward in the winter, the backer angle can flex, but the drywall stays put. To quote the great Dr. Joe,

“Truss uplift isn’t a problem if the client doesn’t see it."

More to explore:
Dr. Joe on Truss Uplift