Installing Window Trim in a Wrap-Around Style

Once you have confirmed the proper window jamb depth, or have installed extension widow jambs, you can then begin the actual process of installing the wrap-around window trim.

Installing Window Trim

Start by measuring the top inside edge of the window jambs, or window frame, and add ¼” of an inch to this measurement. The extra ¼” will provide a 1/8” reveal on each side of the window. The reveal is an aesthetic technique employed by finished carpenters.

Next, using a miter saw cut one end of a piece of trim molding at a 45o angle.

Now measure from the inside edge (shortest edge) of the piece of molding you just cut, the same length you measured on the top of the window frame. Make a small pencil mark on the far end of the molding on the inside edge.

Next, adjust your miter saw blade so that it cuts another 45o angle cut where the mark you just made on the piece of molding is at the short end of the angled cut. Now place this piece of molding over the window frame so that it is approximately 1/8” higher than the bottom inside edge of the window jamb. This will enable a 1/8” reveal to show up on the top of the window trim, as well as along the sides. Use your level to make sure the trim molding piece is perfectly horizontal and level. Use a couple of 6d or 8d finish nails to tack the molding in place. Do not sink the nails.

Next repeat the process for the side trim pieces and again line them up and tack them in place with a couple of nails on each end of the boards. Note that your vertical measurements should be made from the bottom inside edges of the installed trim molding piece to 1/8” below the bottom of the inside edge of the window frame. Again, this is to achieve a 1/8” reveal along the bottom of the window frame.

Now repeat the process for the bottom trim molding piece, measuring the distance between the inside edges of the two vertical trim molding pieces. Once you have cut the bottom molding piece, place it in position underneath the two vertical trim pieces and make sure the mitered seams line up. If they do not line up, adjust the placement of the side moldings accordingly by pulling out the tacked nails and shifting the trim pieces as necessary. Once the pieces are square, tack the side pieces back in and then the bottom piece.

Finally sink the finished nails into the window trim molding using a hammer and nail punch. In addition, install additional nails on both the inside and outside edges near the corners of each trim piece. Also install additional nails ever 12 to 16” along the length of the trim pieces, both on the inside and outside edges.

Note: you may want to touch up the cut window trim edges with stain or paint prior to nailing them up to the window frame.