An 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 angle.

At 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.

Where 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.

Installing Siding

Siding 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 wood sheathing.

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 in temperature.

Pieces 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

Vinyl 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 house.

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 sag.

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.