How to Install Interior Window Trim | Ask This Old House

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

Time: 3-4 hours

Cost: $100

Skill Level: Moderate

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

Steps:
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 _” 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 _” 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.

Resources:
Tom installed 3_" Colonial casings and used stock lumber for the remaining pieces.

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

About Ask This Old House TV:
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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How to Install Interior Window Trim | Ask This Old House

[Jacobhopkins]

Watching Tom draw lines freehand is too satisfying.

[Hollyrock]

Tommy’s the best. I watch his videos all day…

[williamsmith]

I gave it a thumbs up because I learned 2 things even though ive done this quite a few times.

If I may chime in.

Keep a clean rag and clean water handy to wipe away glue squeeze out thoroughly when it happens. Stain wont take to glue and u will be miserable.

If you are going to foam things in then also apply adhesive or even caulk to the back of the piece. Then only use the amount of nails it takes to hold it in.place until it dries. Less holes to try and hide makes the finishing easier and better looking.

[matthewbrown]

Tom can doing anything i can watch these videos all day!!

[richardtaleo]

This is by far the best video on window trim

[Bandgazebo]

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!

[andrewholdaway]

Good use of the fine adjustment tool (aka hammer)

[mikefrank]

love watching tommy - he is just too good

[SharkRangler]

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.

[larrymoore]

I would have never thought of making the trim adjustment to the wall by using a hammer. I enjoy watching Tom at work.

[totosugito]

I so much learn about house improvement from you thanks

[seanb]

Sounds like Tom's feelin a little undah the weathah

[bottlefed]

When I trim out windows I assemble the entire frame and stool prior to installation, of course that's on new windows where the stool buts against the window instead of under it.
Also very often the windows are not the same depth to drywall side to side or even top to bottom so to get the jamb tight to the window and drywall you can do a whole bunch of drywall smashing and angled trim cutting (when the trim sits at an angle to the wall you will have to duplicate that angle on your miter saw to keep the cuts tight) its a lot easier and nicer to simply cut the jamb boards to the desired width and taper. To cut stool or jamb boards say 3 1/2" on one end and 3 5/8" on the other with a standard table saw,  simply mark the board you are cutting at those distances then lay another board on top 1/16" past the mark and nail to the board you are cutting with four 1" 23g set the nailed board against the fence you your table saw and set the fence so that the blade is aligned with the first mark. After making your cut you can pull the boards apart by hand and pull the 23g pins with a pair of needle nose or conventional pliers to ready the "guide board" for the next cut, , you will not be able to see the marks from the pins on the stool or jamb board and the cut will be as perfect as any cabinet makers cut.

[bcask]

I replaced and trimmed 11 windows last week, and what am I watching? Yep. I could watch Tommy work all day.

[jlech]

I’ve never seen anyone in person ever use a protractor on a job before watching Tommy’s videos. Now I wonder how anyone gets by without em.

[miketanner]

That crown molding is anything but beautiful

[WoolyBuggerPicker]

If DIY I would have stained it after the dry fit prior to final fixing.

[bigk]

Tom is using "Great Stuff" foam WITH OUT gloves? I learned that lesson the hard way. I throw on some disposable gloves when I use that stuff.

[albertleppo]

tom is becoming roger

"the window stool is too shAAWT and the apron is too shAWWT"

[Thumbhit]

Wonder a tiny bit if Tommy did the other window or she was on her own to get done.