Installing Window Trim with Apron and Window Stool Cap

One common way to install window trim is to use window stool cap and an apron along the bottom of the interior window sill. There a couple of advantages for this method of window trim installation. First, the use of window stool cap and apron help to emphasis the woodwork within a room. This is often desirable in homes where there is an interest in accentuating craftsmanship woodwork. Second, installing window trim with an apron and window stool also provides extra shelf space for displaying knickknacks and other small items. Summarized below are instructions for installing window trim with an apron and window stool cap.

Checking for and/or Installing Window Extension Jambs

Examine your windows to make sure the window jambs extend flush with the adjacent drywall surface. If they do not, you will need to install window extension jambs so that the window jambs do extend flush with the drywall surface.

Installing Window Stool Cap

First, it is important to determine how wide of a window stool you desire. Window stool cap is milled in a number of different widths and styles so first visit your local home improvement store and determine what window stool cap you want to use on your interior window trim project. In addition, don’t forget to pick up additional trim casing molding for the apron, top and side trim pieces, as well as 6d and 8d finish nails and some wood filler.

Also, if you need window extension jamb material make sure to pick up some 1xN lumber for creating the extension jambs.

One you have your material on site and have confirmed the proper window jamb depth, or have installed extension widow jambs, you can then begin the actual process of installing the window trim with stool cap and apron. To begin this type of window trim installation project start by installing the window stool cap. Consequently start first by measuring the distance between the side window jambs, near the bottom of the window and add ¼”. The extra ¼” is to provide a 1/8” reveal between the side window trim pieces and the edge of the window jambs. The reveal is an aesthetic technique employed by finished carpenters.

Add to this measurement two times the width of the side window casement trim + 1/2”. This will provide the overall width of the window stool cap, and allow the window stool cap to extend beyond each side of the side window trim by ¼”.

Once you have determined the overall length of the window stool cap, cut a piece to the measured length, making square cuts.

After cutting a length of window stool cap, you will next need to notch out both ends so that it rests on the interior window sill and butts up against the side window jambs and the bottom window sash.

To determine how much material to notch out measure from the interior window sash edge, to the edge of the window jamb. Make measurements on each side of the window frame near the bottom of the window. These measurements will determine the depth of how far in you need to notch the window stool cap. Next, measure in from each edge of the window stool cap the width of the side window trim pieces plus 3/8”. Make these measurements on either side of the window stool cap. Check that the distance between the two marks just made on the window stool cap is the same distance between the side window jambs. They should be. If not, check your measurements.

After notching the window stool trim with a jig saw, slide it into the window frame. It should fit snuggly. Note that you may need to add spacers underneath it to support it. If this is necessary add these spacers in first before fastening the window stool cap to the window frame. Use finish nails to install the window stool cap.

Installing Window Trim – Side, Top and Apron Pieces

For the side window trim pieces start by measuring the top inside edge of the window jambs, or window frame, and add ¼” of an inch to this measurement. Again, the extra ¼” will provide a 1/8” reveal on each side of the window frame.

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

Next 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.

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.

Next 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 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 the top surface of the window stool cap.

To install the window trim apron, measure the length of window stool cap and subtract 1/2” of an inch.

Next set your miter saw to a 15o angle and cut one end of a piece of window trim casing molding.

Now, measure from the longest length of the cut window trim casing the distance measured between the window stool cap minus the 1/2”. Make a mark on the window trim apron piece.

Rotate your miter saw blade so that it faces the opposite direction at a 15o angle and cut the piece of window trim relative to this mark.

Center the apron trim piece under the window stool cap so that you have a ¼” space on either side of the window stool cap. Nail the apron to the base of the window, underneath the window stool, so that it goes over the drywall. Make sure to place nails near the edges and in the center.

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.