A.David Utterback responds: If your
floor is framed with green lumber, shrinkage of the floor joists could
contribute to the problem you describe. But the primary cause
is probably truss uplift. Lower truss chords that touch the
warm ceiling dry out and shrink during the heating season,
while the cold upper chords gain moisture and expand. The whole
truss then curves like a bow, rising at the center. If the
truss is nailed to the wall, it pulls the wall with it. The
moving truss can also create drywall finish cracks in the
corners where walls and ceilings meet (see illustration,
below).
Whether floor joist shrinkage or truss uplift is causing the
problem, the way to prevent it is the same: Instead of nailing
trusses to wall plates, use truss clips as shown in the photo
below (available from Simpson Strong Tie, 4637 Chabot Dr.,
Suite 200, Pleasanton, CA 94588; 800/999-5099). The clips allow
each truss to flex up and down freely, while preventing it from
moving side to side Also, don’t screw the ceiling drywall
to the truss where it passes over the partition. Instead, hold
your screws 12 to 16 inches back from the partition and screw
the edge of the ceiling drywall to 2-by nailer blocks that have
been fastened to the wall top plate between the trusses. This
gives the drywall enough room to flex at the joint and prevents
a crack from forming.
We usually see these movement problems in the first couple
of heating seasons. After that, the houses seem to settle down
and the problems lessen. In any case, the wall should drop back
down in the spring, so wait until then to fix the problem.
Whatever you do, don’t shim under the wall during winter
when the crack appears — if you do, when the roof settles
back down in spring, you’ll have created a bearing
situation in the center of the truss where there isn’t
supposed to be one.
Dave Utterback is a district manager and code expert for
Western Wood Products Association. Before joining WWPA, he was
a contractor specializing in residential and commercial
wood-frame construction.