A.Chuck Green,
NARI-certified remodeler and owner of Four Corners Construction
in Ashland, Mass., responds: Truss framing can lead to
major separations at the intersection of ceilings and interior
walls. If your crown molding is assembled from at least two
components that can slide independently, it should help
disguise some truss uplift movement, rather than be part of the
problem. The aim is not to resist the movement but to hide any
gaps as much as possible. Where truss uplift is a major problem
— that is, where a truss-framed ceiling moves seasonally
an inch or more — the solution suggested here may need to
be scaled up.
I typically install crown molding with a rough base
underneath it. This nailing base is triangular in section, with
the outer face fitting between the back of the crown molding
and the wall. I also usually install at least one additional
piece of flat trim stock — which can be square edged,
beveled, rounded, or molded at the bottom — against the
wall, behind the crown molding. This additional trim improves
the look of most simple moldings. When truss uplift is a
concern, two pieces of flat trim will be needed.
Start by installing the first piece of flat trim stock to
the wall. Then, before installing the triangular nailing base,
nail or screw the second piece of flat trim to the back of the
triangular nailing base. Next, install the triangular nailing
base by nailing it or screwing it to the ceiling. Finally, the
crown molding can be installed, using longer finish nails at
the top of the crown, and very short brads at the bottom. Since
the crown molding is fastened to the ceiling not the wall, some
truss uplift movement can be accommodated between the two
pieces of flat trim stock.
All of the pieces except the triangular nailing base will
need to be prefinished, to allow movement to occur without
revealing a line where the finish or paint stops. The crown
molding will look best if the second piece of flat trim has a
molded bottom, to help disguise the unsecured sliding joint
between the two pieces of flat stock.