Trimming the Sides of the Window

With the Mitered method, you simply measure the side of the window from the top edge of the trim to the bottom and add the width of the J (the visible part). On the bottom end of the J you´ll cut a miter, like in this picture. Then make a notch in the other end like this. This will be used to help shed water with the top piece. Getting this cut right will just take a little practice.

Now make sure this end is flush with the top of the trim an that it will lay fairly flat. Sometimes you may have caulk clumped up or something else that needs to be removed or flattened. Also, don´t press it hard against the trim. Leave 1/8"-1/4" to allow for installing the metal later on. If it´s laying OK, nail the top slot like you did the posts, at the upper end, then nail it on down every 10". Keep in mind that you want to leave the nails out 1/16".

Putting on the Bottom Piece

Some manufacturers offer a special undersill trim, but most of the time it´s just as simple and good looking to use J. For the bottom piece, measure from the outside edge of each of the side pieces. This is your length. You may want it to be 1/8" shorter, but try how it fits first. You´ll notch each end to receive the bend from the side pieces. The bottom piece gets tucked behind the side pieces on the ends. With the side pieces mitered and over lapping, you will have a neat appearance and it will help shed water.

Make sure each end fits nicely and that it doesn´t press out the side pieces or have gaps. A lot of times you have to clip the curl of the edges to help it lay down better.

Finishing Out With the Top Piece

To cut the top piece, measure the outside edges on the sides at the top. Then cut a piece of J that length. Cut your miter like the photo. After you do this a lot, you´ll know how much of an angle to cut, but until then you can lay the piece up and mark where the two pieces intersect. You can also use your speed square to mark a 45 degree line from the end. You´ll fold the flange down and it will go into the side piece. This provides a flashing to shed water and channel it down the side piece instead of behind the J. Each level of protection like this helps insure that water does not have a chance to penetrate the exterior.

What about doors?