How to Install interior Window Trim

Ask This Old House general contractor Tom Silva helps a homeowner trim a window to match the rest of the house.

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Time: 3-4 hours

Cost: $100

Skill Level: Moderate

Tools List for Installing Window Trim:
Table Saw
Miter Saw
Hand Plane
Speed Square
Nail Gun
Shopping List:
Window Stool Molding
Extension Jamb Molding
Window Casing
Window Apron Molding
Wood Glue
Minimally expanding foam insulation

1. Remove any existing casing, stool, or apron molding.
2. Use the table saw and miter saw to cut a piece of stool molding to fit between the windows. Leave the ends long.
3. To determine the finished length of the stool, hold the extension jamb and casing in place, leaving a ¼” reveal between them. Then set the distance from the face of the casing to the face of the stool and transfer that measurement from the side of the casing to the stool.
4. Ease the edges of the stool with a block plane and sandpaper.
5. Use shims to make the stool square to the window.
6. Use the nail gun to nail the stool in place.
7. Rip the extension jamb pieces to the correct width, then cut them to length.
8. Cut the casing to length, mitering the corners at 45 degrees.
9. Make a mark ¼” from the edge of the extension jamb where the casing will sit.
10. Assemble the extension jambs and window casings as a single unit on a workbench or sawhorses using screws, wood glue, and nails.
11. Dry fit the trim assembly in the window opening. Shims may need to be added or the plaster may need to be taken down slightly.
12. Use minimally expanding foam in any joints around the window and around the extension jambs.
13. Nail the trim assembly in place.
14. Attach the window stool to the casing with a screw.
15. Cut the apron to the correct length, which is the distance from the outside of the casing to the outside of the casing.
16. Glue and nail the apron in place.

Tom installed 3½” Colonial casings and used stock lumber for the remaining pieces.

All of the wood and tools used can be found at a home center.

Special assistance with this project was provided by Cleveland Lumber Co (

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Homeowners have a virtual truckload of questions for us on smaller projects, and we’re ready to answer. Ask This Old House solves the steady stream of home improvement problems faced by our viewers—and we make house calls! Ask This Old House features some familiar faces from This Old House, including Kevin O’Connor, general contractor Tom Silva, plumbing and heating expert Richard Trethewey, and landscape contractor Roger Cook.
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Keywords: This Old House, How-to, home improvement, DIY, tom silva, ask this old house, window, trim, install

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  1. Bandgazebo
    August 6, 2018

    We have a new home that came with many builder-grade finishes, including windows without casing (just sheetrock). We added full trim and sills to our windows and it made a HUGE difference for not a lot of work or money. We did it the way Tom said he doesn't like as much lol, but they look so much better still. I totally recommend this type of project!

  2. A Wideman
    August 6, 2018

    Wouldnt you put on a lynseed oil or something like that to seal up that stock?

  3. Tony Kaye
    August 6, 2018

    That 1/4 “ reveal by hand tho

  4. Uncle Chris
    August 6, 2018

    Always enjoy watching your videos. Could you recommend what kind of wood is good for interior window sill?

  5. Rick T
    August 6, 2018

    Wow, another Tommy special; most people would build it piece by piece around the window…

  6. Mark Warrington
    August 6, 2018

    I thought it looked ok to begin with. Save the money and buy more beer 🍺

  7. Mareds
    August 6, 2018


    Parabéns pelo trabalho e um grande abraço do MAREDS

  8. ericlogos
    August 6, 2018

    Only 22 more windows to go

  9. WhatIsKenDoing
    August 6, 2018

    End grain showing instead of returned corners, beating the hell out of the plaster instead of scribing the jamb extensions? Come on, Tommy. That's elementary carpentry at best. Also, screwing the casing together is kind of overkill.

  10. Neil M
    August 6, 2018

    1:00 it's too shaaat

  11. David Drake
    August 6, 2018

    Assembling the whole unit first looks great but would scare me because I would imagine the window opening wouldn't be perfectly square (what is?). Were there measurements to check the square of the opening beforehand? I would love to be able to get my miters to join like that.

  12. Goran Pujic
    August 6, 2018

    Was he saying “short”??? 😂

  13. Anthony Marzella
    August 6, 2018

    C'mom Tom, exposed end grain is now very craftsman like. You should have miter retired the ends. I will give you props on building it as a whole unit. Best way.

  14. Uncle Chris
    August 6, 2018

    Thank you. What kind of wood showing in the video, it looks thicker and stains showing wood grins well?

  15. Michael Lebert
    August 7, 2018

    Despite what the haters say I like the way all the guys do their jobs, If I had the money I would pay them to build me a house

  16. erwin benally
    August 7, 2018

    Wow, excellent job!

  17. Andrew Holdaway
    August 7, 2018

    Good use of the fine adjustment tool (aka hammer)

  18. bigcat56308
    August 7, 2018

    "There's nothing wrong with this window, there's beautiful plaster work all the way around it, and you really won't notice any changes because we have these rods and curtains obscuring the view, can you help me waste time and money doing a completely unnecessary 'repair'?"

  19. Paul Watterson
    August 8, 2018

    Nice Job Tom

  20. Barb Heinze
    August 8, 2018


  21. Barb Heinze
    August 8, 2018

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