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When a newer house shows signs of interior ceiling corner cracking at the top floor in the winter, this can normally be associated with truss uplift.
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. The bottom chords of the truss is buried in heavy insulation (normally 10"). 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.
This cracking is upsetting to a homeowner, as most homeowners might assume that there are structural problems with the house. In reality, the ceiling has lifted up, creating a gap at where interior wall framing meets with the ceiling trusses. The truss expansion/arching literally lifts the truss off the interior walls.
SOLUTIONS - REPAIRS
The solution to an existing condition, the tops of the interior walls should be un-nailed from the trusses, remove the nails from inside the attic and them secure the tops of the interior walls with wood blocks or truss clips. (L shaped metal clips with a long slot on the upper leg) . The clips attach to the tops of the interior wall partitions and are then nailed through the slots to the trusses. The nails are not driven tight to allow for movement of the truss. Then take a nail set and drive all the ceiling nails within 16 inches of the corners at the center partitions and all the wall nails within 8 inches of the ceiling through the dry wall. The holes in the drywall should then be patched with dry wall joint compound and the walls and ceilings repainted.
Another possible solution would be to install crown molding around all the second floor ceilings, nailing the trim only to the ceilings. this procedure would cover the cracks and maintain a good looking ceiling corner. Note: The trim is difficult to install on vaulted ceilings. Also, when installing the crown molding in this manner , remember to paint the trim in the winter months so that there is no paint stripe at the bottom of the trim when the trusses lift next winter.
When building a new house the framing contractor should secure the interior partitions to the trusses with truss clips. The clips attach to the tops of the interior wall partitions and are then nailed through the slots to the trusses. The nails are not driven tight to allow for movement of the truss.
The drywall installer must then install the drywall properly to prevent cracking. They should not screw or nail the drywall to the trusses within 16 inches of an interior wall and within 8 inches of the ceilings on the interior walls. They should fasten the drywall to blocks of wood nailed between the trusses or use metal clips that are attached to the top of the interior walls. Most truss movement occurs over a period of time, this installation technique will allow the drywall to flex near the corners preventing cracking.
Trusses are pre-manufactured structural roof components which support the roofing and carry the top floor ceilings. (see sketch) They are a less expensive and faster method of building than stick built roof framing. (rafters and ceiling joists) Roof trusses span the upper floor, making all the upper floor walls none structural. They are a combination of wood members connected together with metal plates.
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