What Goes Up Must Come Down…Right? A Truss Uplift Discussion

By: Scott Warren, P.E
Trusses make use of the most efficient geometric shape — the triangle. Manufactured wood trusses are prefabricated assemblies that utilize lots of connected triangles. The typical roof truss features top chords (rafters) and bottom chords (ceiling joists). Web members between the top and bottom chords channel loads to the chords. The chords and members of the trusses are connected by gussets/truss plates. These metal plates are pressed into the wood mechanically by a heavy roller which creates a rigid joint, as opposed to a joint that allows the connected members to freely rotate. This creates a problem when the wood naturally elongates in the presence of moisture or shrinks differentially. Truss Uplift or Arching The nomenclature “Trussuplift1uplift” can be misleading. It denotes something that may be attributable to the effects of wind. A more appropriate term may be “arching.” In your typical residential home with an unfinished attic space, the bottom chords are covered in insulation and are, subsequently somewhat tempered like the interior when compared to the top chords of the trusses, which are not insulated. The top chords, or rafters, are located farther away from the heated living space. While the bottom chord remains warm and relatively dry in the winter, the top chord absorbs ambient moisture in the unconditioned attic air and begins to stretch. As the top chord “grows,” and the bottom chord “shrinks,” the truss changes shape and the bottom chord bows upward (Figure 1). The walls in the middle of the structure are then separated from the truss chords which are displaced upward. The drywall attached to the ceiling will separate from the drywall attached to the walls creating a gap. The joint is usually filled with rigid joint compound and is unable to accommodate this arching or flexure experienced by the bottom chord of the rafter. Ultimately, a gap occurs. The photo to the right illustrates the bottom chord of a truss that arched upward, separating itself from the wall approximately 5/32 of an inch. What then? [caption id="attachment_2984" align="alignright" width="300"]Figure 1: Truss Uplift Illustration[1] Figure 1: Truss Uplift Illustration[1][/caption]Interior walls may be oriented perpendicular to the truss; however, the truss may not have been designed to be supported by that wall. Usuallythe bearing points (the outside walls) are loaded to the point that uplift isn’t a problem. Slotted anchors accommodate for this upward arching movement at non-load-bearing walls. The slotted anchor is secured to the top plate of the interior non-load-bearing wall. A long slot in the vertical leg allows for a fastener to be driven through the bottom chord; however, it should not be fully driven flush with the anchor. This allows for vertical movement. The photo to the left shows a properly installed slotted anchor. The nail head protrudes out of the truss allowing the wood to freely move up and down. The drywall installer may or may not be aware of the effect of truss arching, however the installer can employ a simple technique to mitigate the damage to the drywall. The drywall attached to the ceiling should not include fasteners within approximately 18 inches of any interior wall where the truss spans across it. A piece of “deadwood” on top of the wall creates a lip for the drywall to catch when the truss arches. This allows the drywall to flex like a diving board. Another option for concealing the gap either from a retroactive standpoint or design concept at the beginning of construction is the use of crown molding. In this case, the crown molding is attached only to the ceiling and not the wall. The crown molding now floats up and down the face of the wall like a buoy (Figure 2). Problems may arise with low-quality wood molding as the attachment points are limited. More dimensionally-stable materials like plastic AZEK ® can prevent warping. [caption id="attachment_2986" align="alignleft" width="300"]trussuplift4 Figure 2: Truss Arching Remedies.[2][/caption]Final Thoughts The owner will often point out cracks between ceilings and walls during a wind or blasting claim. After all, wind forces are strongest upon the roof, and the insured is naturally concerned with the roof following strong winds. But it is important to always consider the phenomena of truss uplift or arching during the claims process especially in the absence of other damage like torn shingles, broken tree limbs, and detached siding that tend to be caused by strong winds. One plus one has to equal two—so, if you find no collateral damage from wind, the force was probably not sufficient enough to cause the roof to rise off of the walls. When you see a separation fairly consistent in width across the top of a wall where it meets the ceiling in the middle of the structure that you know is attached to a manufactured wood roof truss, look for slotted anchors between the walls and ceilings and take note of any measurable gap. And if not sure, consult Donan.

[1] Carson Dunlop and Associates, Ltd.: Essentials of Home Inspection: Home Reference Book © 2003
[2] Carson Dunlop and Associates, Ltd.: Essentials of Home Inspection: Home Reference Book © 2003