Trimming Out a Window

This story shows you how to trim out a window.


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.

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.

Prestart Checklist

About 2 hours to install basic window trim; more time is required for more complicated situations.

Tape measure, hammer or power nailer, nail set, hand miter box with backsaw or power miter box, jigsaw, level, combination square, drill

Careful measuring and precise cutting

Install the window and the surrounding wall surface.

The casing and stool material of your choice; 3d, 6d, and 8d finish nails; wood putty

Installing the stool: Step 1

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.

Installing the stool: Step 2

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 (top). Turn the casing on edge and make a mark one casing thickness away from the first mark (bottom). Do this on the other side as well. The distance between the two outer lines is the length of the stool.

Installing the stool: Step 3

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

Installing the stool: Step 4

Use a jigsaw (shown) 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.

Installing the stool: Step 5

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.

Attaching casing: Step 1

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.

Attaching casing: Step 2

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.

Attaching casing: Step 3

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.

Attaching casing: Step 4

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

Comments (3)
sbird21 wrote:

thanks for the help. article on window stools very helpful,but pictures helped alot. you have the vest website for home repair,best of all your there 24 hr a day 7 days a week .

8/16/2011 10:04:50 AM Report Abuse
Add your comment

You must be logged in to leave a comment. Register | Log In
Wish-list Projects

Making these dreams come true is simpler than you thought -- print these instructions to begin!