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Arching roof truss uplift: here we provide a definition of arching roof truss damage or roof truss uplift and we explain why truss uplift occurs, where truss uplift cracks will appear, and how roof truss uplift cracking can be avoided or corrected.
Roof truss uplift can be diagnosed as the cause of nail pops and cracks or separation between the ceiling and wall top.
Arched roof trusses, moving in response to moisture & temperature variations across the truss, can lift the building ceiling sufficiently to cause nail pops or actual tears at the ceiling-wall juncture. This article explains why arching trusses happen, where the damage will appear, how to recognize it and how to prevent it from recurring.
Roof Truss Uplift and Cracks or Ceiling Nail Pops: Details of Truss Uplift Cause and Cure of Interior Wall/Ceiling Nail Pops, Drywall Gaps or Cracks
According to Carson Dunlop Associates, this cosmetic problem in homes in cold climates with roof trusses may result in significant
cracks between interior walls and ceilings, or between interior walls and floors. The cause is
upward bowing of the roof trusses to which the ceilings are attached.
[Click to enlarge any image]
The ceiling/wall juncture cracks typically open in the winter and close in the summer. These ceiling cracks can be very alarming, but are not a structural issue.
As we also discuss both
at CEILING FINISHES INTERIOR
at WALL FINISHES INTERIOR, roof truss uplift is a well-understood cause of cracks at the wall/ceiling juncture at building walls located under the center (usually) of certain roof trusses.
Roof truss uplift occurs when the bottom chord of the truss is exposed to significantly different moisture or temperature conditions than the rest of the roof truss.
For example if the bottom of the truss is buried in insulation while the remainder is in the open attic air, this condition can occur.
Photo at left: this is the type of separation crack that you may see at the ceiling-to-wall drywall joint over a center partition that runs at right angles to the bottom chord of rising roof trusses.
Builders avoid truss uplift cracking or ceiling nail pops by using special truss uplift clips to connect the ceiling and wall drywall while avoiding nailing or screwing the ceiling to the bottom of the trusses within 24" of the building walls. [See the roof truss uplift remedial action sketch just below.]
Why does truss uplift damage appear at interior partitions in the center of a building?
The differences in temperature and perhaps humidity that we just described can cause the roof truss to arch upwards at its center, often seasonally as attic temperatures and moisture conditions vary.
Because the truss ends are secured to building exterior walls - a location that resists outward thrust, as the truss bottom chord wants to expand along its length, the force pushes it upwards into the attic space.
Therefore roof truss uplift is most likely to be observed at the ceiling-wall juncture of central interior wall partitions that run at right angles to the direction of the roof trusses, or in other words, interior partitions that are parallel to the house front and rear eaves.
That's where truss uplift cracks may occur (typically in the center of the truss).
Builders avoid truss uplift cracking or ceiling nail pops by using special truss uplift clips to connect the ceiling and wall drywall that avoid nailing or screwing the ceiling to the bottom of the truss within 24" of the building walls where uplift may occur (typically in the center of the truss).
The drawings of roof truss uplift and corrective measures for truss uplift shown here are provided courtesy of Carson Dunlop Associates and appears in their Illustrated Home.
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