How to Install J Molding. J molding is a type of molding used to present a finished edge on vinyl or aluminum siding. It is used around window casings and door casings and at the top edge and bottom of the siding to hold siding in place.
Prepare to install J molding by first choosing your nails. Roofing nails are best for installing J molding. If you use ½-inch styrofoam over the studs of the house, you need to use 1 ½-inch or 2-inch nails. These longer nails are necessary in order for the nail to be able to go into the thickness of the styrofoam and still go firmly into the stud. If you are nailing J molding directly into 4-by-8 foot plywood, ¾-inch or 1-inch roofing nails will be sufficient.
Use this example to begin to install J molding: If you have a 38-inch wide by 48-inch long window, the top piece of J mold would be cut 1 inch longer on each side of your window. This allows you to cut your sides to overlap your top and bottom pieces. Your sides would be 50 inches long. Cut your sides on 45-degree angles. This will give your window a finished look.
Finish wrapping your window with the J mold on all four sides. At the top and bottom of the window, your channel will be protruding out into the finished edge of your J mold. Cut that channel out, flush with the back of your J mold. This will give you a channel of 1 inch all the way around your window.
Complete each window and door casing using the same technique. To install J molding along the top and bottom edge of siding is done in the same way, but of course, you don't need any 45-degree angles for straight trim.