Installing Window Trim

Installing Window Trim

Level: Experienced

Adding trim around your windows gives them that final touch, enhancing the look of any room. Whether you've recently had new windows installed or you want to update your old ones, this is a job that any experienced do-if-yourselfer can do.

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Project Steps
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    • Step 1: Prepare the Room
      measuring tape

      Measure the windows to determine how much molding you'll need. Gather all the required tools and clear the room so you can set up a convenient "shop." Roll up area rugs or cover carpeting with canvas drop cloths. Use plastic drop cloths to cover any remaining furniture. Set up a worktable, or plywood on a sawhorse, for your saw. Set a large window fan on "exhaust" to draw the dust out of the room and away from your work.

      Safety Alert!

      Always take the necessary precautions when working with a saw. Remember to wear goggles or safety glasses.

      Helpful Tip

      To avoid mistakes, make this your mantra: measure twice and cut once. Take your time and work on larger windows first. That way, if you do cut a piece of trim too short, you can use it on a smaller window.

    • Step 2: Cut Stool and Fit to Window
      DeWalt mitre saw

      Using a power miter saw, cut a length of window stool (the flat piece upon which the window shuts down) that's 1-1/2" longer than the outside dimension of your installed casing. Hold a piece of casing in place and lightly mark the outside edge on the wall. Repeat on the other side and measure between the marks. Mark the center of the angled sill and the stool. Then align the two marks for the next step.

      To determine the notch width, center the stool on the window and mark the edge of the stool at the inside of each side-jamb. If necessary, rip or plane the window-edge of the stool so it's parallel to the sash. To determine notch depth, measure from the stool to the sash at both jambs, subtract 1/16" and connect the two points. Mark and cut the notches. Then bevel or round the ends of the horns and tack the stool to the rough sill with 8d finishing nails 10" on center.

      Helpful Tip

      Because surfaces are rarely perfectly flat at wall openings, use a scribe (compass) to mark any necessary cuts on the back edge of the stool and at the horns. This will assure the stool fits nicely against the sash and the horns against the wall.

    • Step 3: Cut and Install Jamb Extension

      You'll have to extend your window jambs if they aren't flush to the interior wall surface. Hold a straight edge on the wall surface and measure the distance between the jamb and the straight edge to determine how thick the extension must be. Check three points on both sides and the top, then choose the largest dimension.

      Helpful Tip

      If there is more than a 1/8" difference between thickness measurements, you can split the difference by ripping the extension 1/16" thinner than your measurement. Or you can rip a taper and plane or sand it once it's in place.

      It isn't as common, but sometimes a jamb is wider than the wall. In this case, use a block plane to plane the jamb flush with the interior wall surface.

      Rip strips of 3/4" stock to the thickness you need. Cut a full-length head-jamb extension. Glue and nail it so that 1/16" to 1/8" of the head jamb will be left exposed—this is called a reveal. Then cut the side-jamb extensions to fit snugly between the stool and head-jamb extension.

    • Step 4: Install Head Casing

      Mark a 3/16" reveal on both the side- and head-jambs. Measure between the side-jamb marks and cut a 45-degree miter on both ends of the head casing so that the short dimension equals that distance. Then position the casing on your marks and tack it to the jamb with 4d finishing nails and to the framing with 8d or 10d casing or finishing nails.

      Helpful Tip

      You should always tack your casing in place initially. Wait until mating pieces are in place before driving the nails home. This way, if you must remove a piece to fine-tune a cut, you can do so without damaging the trim.

    • Step 5: Install Side Casing

      Measure from the stool to the top of the head casing at each side of the window and miter-cut each side casing so that its long point will equal that measurement. Apply glue to the miters and lock the joints with a finish nail from the top and side. Nail side casings to the jamb and wall as done with the head casing.

      Helpful Tip

      To avoid possible measuring errors, cut a miter on one end of the casing. Stand it upside down on the stool with the top touching the head casing and mark the length at that intersection.

    • Step 6: Miter-return and Install Apron

      Ideally, the end of the apron should have the same profile as the face. If you have nicely detailed trim that will be stained and want to do a first-class job, make a mitered return on the ends of the apron. Cut an outside miter on the ends of the apron and on two short molding scraps.

      Glue and nail the scraps to the apron to form a 90-degree corner at each end. When the glue dries, cut off any excess. Nail through the apron into the wall framing, then nail through the stool into the casing with 6d finishing nails.

      Helpful Tip

      If you cut in the other direction — pushing the saw into the face of the molding — you could use the back of the casing to guide your cut. You're less likely to splinter the face of the trim when you do this.

    • Step 7: Alternative Apron

      For less demanding applications, you can cut the ends of the apron to match the molding's profile. Cut an inside miter (long point on backside of trim), and cope the resulting profile straight back. Use a sanding block to smooth the cut.

      Helpful Tip

      If you plan to stain the end grain, apply a wood conditioner before staining so the end won't soak up too much stain.

    • Step 8: Final Touches
      installed window trim

      Use a nail set and hammer to drive all nails slightly below the surface. If you plan to paint, fill the holes with wood putty and sand smooth. If you plan to stain and varnish, do that first and fill nail holes using putty sticks.

      Congratulations! By trimming your windows, you've added architectural detail and aesthetic appeal to the interior of your home.

  • Here’s what you’ll need to complete this project successfully.

    Before you begin, use the shopping list below to uncheck the tools you already have to complete this project.

    Then, print or save your updated list and bring it to your local True Value hardware store, where an expert Hardwarian will give you the remaining tools and expert advice you need to complete this project.

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