Installing J-Channel For Vinyl Siding Around Windows

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Log In How-To Installing J-Channel for Vinyl Siding around Windows

Follow these guidelines for cutting and installing J-channel around windows before fitting the vinyl siding panels.

By Andy Engel

J-channel around windows

One of the last steps before installing vinyl siding panels is trimming around all sides of windows and doors with J-channel (aka “J”). This channel hides the cut edges of the siding where it meets windows and doors, making for a clean look. The J pieces will lap each other to drain water, so add twice the width of the front of the J to each length. That’s usually 2 in.

Cut the J to length with a miter saw or snips. At both ends of the top and bottom pieces, cut along the inside of the channel the width of the front of the J to create a flap. 1 Make similar cuts to the side pieces but cut out the flaps. Nail the J in place, starting with the bottom, using roofing nails every foot. 2 Fold the bottom-piece flaps up along the side of the window, then nail the side pieces in place, lapping the bottom flaps. Then install the top piece of J. That goes over any drip cap that’s in place, and its flaps should lap into the side J-channel.

Cut the side J-channel
1) Cut the side channels to leave flaps that tuck between the upper and lower J-channel and the window.

nailing the bottom section in place with roofing nails
2) Start the J-channel installation around windows by nailing the bottom section in place with roofing nails.

Detailing the Top Piece of J-channel

The upper piece of vinyl always has to be cut at the soffit. A lot of carpenters snap it into the piece below, and tuck the top into the J above, and call it good. I’ve seen a lot of pieces of siding installed that way laying on the ground after a windstorm.

Another approach is to tack the top in place with painted aluminum nails. That works, but the vinyl can buckle in the heat. I use a snap-lock punch to cut small slots in the vinyl near its top edge and use these slots for the aluminum nails driven at an angle to get past the face of the J-channel.

Small slots punched in the cut edge
Small slots punched in the cut edge allow nailing in more visible locations (under windows or soffits) with small aluminum nails.

Hold the siding in place under the window to mark the width of the window. 3 Then measure from the buttlock on the course below to the top of the J (inside the channel recess) and deduct 1⁄4 in. for expansion. This is the depth of the cut you’ll make in the siding panel. 4 Extend the first marks to this depth and connect them using a straightedge. Using snips, make the cut on your marks. 5

mark the width of a cut to clear the J-channel
3) With the J in place, hold the siding in its installed location to mark the width of the piece you’ll have to cut out for the window.

Measure from the buttlock to the J-channel.
4) Measure from the buttlock of the lower course to the recess of the J-channel. This is the depth of the cut for the window.

cut the panel at the marks
5) When all your measurements are marked, use snips to cut the panel at the marks.

Using Undersill

An alternative to J-channel for below windows and soffits is undersill.

Undersill is meant to grip and hold the top of vinyl siding whose nail flange has been cut off. It’s a good product when it can be used, particularly under longer windows, where siding tucked into J would go without nails for a distance. But it won’t keep the siding in if the siding is cut near the point where it thickens at the center or the bottom.

Using Undersill

Carpentry CompleteExcerpted from Carpentry Complete: Expert Advice from Start to Finish, by Andy Engel

Available at Amazon.com.

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