Every place where vinyl siding joins something
else—a window, a door, another wall—requires the
appropriate trim piece. The trim pieces loosely
hold the siding in place, cover any cuts, and
provide a small space where the siding can expand
and contract. All lines of vinyl siding come with
several standard trim pieces, plus some extras.
Most of the time the bottom of each piece of siding
locks into the piece below it. But for the first
row on a wall, the bottom locks into a special trim
piece called a starter strip.
outside corner post is used for outside corners.
There is a channel molded into each side of the
post to receive the end of the siding.
There are several options for trimming out inside
corners. The easiest is an inside corner post. It's
also possible to use two piece of J-channel
installed back-to-back to create the same effect.
Many professional siding contractors bend a piece
of vinyl or aluminum flashing for the inside corner
and then use a single piece of J-channel. However,
this requires a sophisticated tool called a brake.
J-channel is the most common piece of siding trim.
Besides being used for inside corners, it's used
for trimming around windows and doors. It is also
used where siding meets a soffit or roof at an
the top corners of windows and doors, the two
pieces of J-channel should be mitered and
overlapped carefully so that moisture is directed
down and around the window and doesn't get a chance
to seep behind the siding or trim.
siding runs up into a horizontal surface (like
under a window or at a soffit) you need a piece of
undersill trim. In these situations the top edge of
the siding will usually have to be cut to fit. The
undersill trim is designed to grab this cut edge
and hold it in place.
pieces generally come 12 feet long and between 10
and 12 inches high. The bottom of each piece hooks
on to a lock molded into the top of the piece below
it (except the first piece on the bottom, which
locks into the starter strip). The top of each
piece is loosely nailed into the studs, or into
The ends of the pieces are hidden by corner posts
or J-channel. There should be about a ¼" gap
between the end of the piece and the inside of the
channel, so that the piece can expand with changes
should overlap by about an inch. Again, this is so
that the material can expand and contract without
exposing the sheathing underneath.
If nailing too tight it the MOST common
do-it-yourselfer mistake, the SECOND most common
do-it-yourselfer mistake is to not pull each row up
properly as you nail it. You need to pull up each
row firmly and consistently so that it locks
solidly into the piece below it. That way, you're
getting the full height out of each piece of siding
and the siding has a consistent vertical spacing
all the way around the house.
Where you need to cut the siding horizontally to
fit or under a window in the space left at the top
of the wall, you need to create small tabs along
the cut edge with a snap lock punch. These tabs
will lock into the the undersill trim and hold the
cut edge in place.
Installing Soffit and Fascia
soffit material has a different profile than
siding, but installs according to all the same
principles. The first step is to install trim
pieces. If there's wood framing already in place
for the soffit, use a piece of J-channel to hold
the ends of the soffits along the house and along
the short sides of the soffit. If there's no
framing, use a trim piece called F-channel along
The soffit pieces slip into the trim and get nailed
up into the wood subfascia
with short, small-headed trim nails. The end
along the house sits loosely in the F- or
J-channel. If the soffit is wider than about 12
inches, it may be necessary to install additional
wood support pieces down the middle and nail the
soffit panels into those, too, so that they don't
Once the soffit is installed, you finish the job by
installing fascia to cover the outside edge of the
pieces and to protect the wood subfascia. We tend
to use aluminum fascia material (although vinyl
fascia IS available).
The top edge of the fascia can be held in place
with a piece of undersill trim, or it can be hidden
behind a piece of drip-edge or other roof-edge
flashing. Often, you have to trim the top of the
fascia so that it fits between the edge of the roof
and the soffit.