Decorative trim is an easy way to add style to a boring window. Learn how to install it yourself with this simple tutorial.

Yellow window with white shade

Trimming a window is well within reach of most homeowners. The project is comprised of two main parts: installing the stool and attaching casing. We'll walk you through both steps, plus offer valuable tips along the way.

But before you start trimming a window, finish the surrounding walls. If you cut out a large opening for installing a new window, that probably means you need to install new drywall. Attach the drywall and apply three coats of joint compound, sanding between each coat.

If you will stain the new trim, paint the wall first. Apply stain and finish to the trim before installing it, so you will only have to touch up the cut ends and nail holes later. If you will paint the trim, you can paint the wall after the trim is installed.

Also keep in mind that the jambs must be flush with the wall surface. If the jambs do not extend out to the wall surface, install strips of wood to extend the jambs outward. If the jambs are proud of the wall, plane them down.

You'll need about 2 hours to install basic window trim; more time is required for more complicated situations. Make sure you're able to accurately measure and precisely cut.

What You Need

  • Tape measure
  • Hammer or power nailer
  • Nail set
  • Hand miter box with backsaw or power miter box
  • Jigsaw
  • Level
  • Combination square
  • Drill
  • Casing
  • Stool material
  • 3d, 6d, and 8d finish nails
  • Wood putty

Before You Begin: Pick a Design


Before installation, you'll need to choose an ensemble of casing, stool, and other molding pieces. You can use single pieces of trim for the casing and apron or build a richer look by combining two or more pieces. There are designs that reflect traditional, country, and Victorian styles. Aprons can be made of any knot-free stock; round the edge with a sanding block. If you plan to paint the trim, you can take advantage of the many style options available in medium density fiberboard (MDF) and plastic trim. The molding profile numbers shown are widely used.

How to Install the Stool

Step 1: Draw the Reveal


Use a combination square to draw the reveal on the edge of all the jambs. This is the width of the jamb edge—usually 1/4 inch—that will not be covered by the casing. Set the square and hold the pencil tip against it as you slide the square along the jamb.

Step 2: Measure and Mark


To determine the length of the stool, hold a piece of casing flat on the wall, one side aligned with the reveal line. Make a light mark on the wall. Turn the casing on edge and make a mark one casing thickness away from the first mark. Do this on the other side as well. The distance between the two outer lines is the length of the stool.

Step 3: Cut to Length


Cut the stool to length. To mark for the notch at each side, first mark for the depth of each notch and then mark for their length.

Step 4: Cut the Notches


Use a jigsaw or handsaw to cut the notches. Adjust the jigsaw blade so it slightly undercuts—that is, so it cuts slightly more from the bottom of the piece than from the top.

Step 5: Fit and Attach


Set the stool in place and check for a tight fit all around; you may need to plane or saw it to make slight adjustments. Drill pilot holes and drive 8d finish nails to firmly attach the stool to the rough sill.

How to Attach Casing

Step 1: Cut Head Piece


Cut one side of the head (top) casing piece to 45 degrees. Hold it in place, the cut edge aligned with the reveal line on one side, and mark for cutting on the other side. Tack (drive nails partway) the head casing piece into place.

Step 2: Mark for Side Casing


To mark for a side casing, cut one end to 45 degrees. Hold it upside down, with the tip of the cut end resting on the outside edge of the sill, and mark the other end for a straight 90-degree cut at the top edge of the head casing.

Step 3: Drive Nails


Hold the casing pieces together. Install the casing with 6d finishing nails into studs and 3d finishing nails into the jamb. When you drive a nail near an edge, drill a pilot hole first to prevent splitting the wood.

Step 4: Drive Nails Horizontally


To help ensure a tight and durable fit at the miter, drive a 3d nail horizontally to lock one piece against the other.

More Trimming Tips

How to Finish an Ornate Apron


A straight cut to both ends of an ornate apron looks amateurish. Here's a better approach. Hold the apron stock across the opening and mark where it intersects at the two outermost edges of the side casing. Miter-cut both ends. Next make miter cuts on a scrap of the same material. Cut the miter first, then make the straight cut so the resulting wedge is as long as the apron is thick. Glue the wedge piece in place to make a return—a finished end—for the molding. Fasten the apron by drilling pilot holes and using 6d finish nails every 12 inches.

How to Install Block Casing


Block casing, which features a square rosette at the upper corners, adds a decorative touch and is easy to install because there are no miter joints.

Draw the reveal lines. Squirt a dab of construction adhesive onto the back of each rosette, carefully place it at the corner of the lines, and hold a square against it as you partially drive a nail to hold it in place.

Cut casing pieces to fit between the rosettes and the sill. Install with 6d and 3d finishing nails.

How to Fix Joint Problems


If a miter joint is not tight, use a plane or knife to shave the miter; undercut so the bottom of the cut is deeper than the top. It sometimes helps to slip a small shim under one or both of the pieces. Or start installing the side piece with the miter tight but the piece going at an angle; straighten the piece as you drive nails farther down. If a jamb is proud of the wall, use a plane to shave it down.


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