Step 1: Mark the length
Start at the top
Cut a 45-degree angle on one end of the trim and hold it so the short end of the angle overhangs halfway, or 3/8 in., onto the jamb. Then mark the other end flush with the inside of the jamb. That’ll give you a 3/16-in. reveal.
Step 2: Get the spacing right
Place the top piece
Hold the trim 3/16 in. away from the jamb at both ends and along the base of the trim. Nail the trim to the jamb with 1-in. brads spaced about every 6 in. Nail the thick part of the trim to the framing with 2-in. brads.
Step 3: Check the fit, then cut to length
Cut the side
Cut a 45-degree miter on one end of the trim board. Adjust the miter as needed for a perfect fit. Then scribe the cut length 3/16 in. past the bottom of the jamb. Nail the trim onto the jamb first and then to the window casing framing, as you did with the top piece.
Step 4: Glue and pin for a solid miter
Glue and pin together the miter from both directions with 1-in. brads. Wipe the glue squeeze-out with a damp rag right away.
Step 5: Trim the other side
Finish the sides
Repeat all the same steps on the other side of the window, fitting first the top miter, and then marking and cutting the bottom one. Nail the window casing trim into place.
Step 6: Fit the first bottom miter
Start the bottom piece
Cut an overly long piece of window casing trim and cut a miter on one end. Overlap the far end to check the fit. Mark and recut the miter as needed for a perfect fit.
Step 7: Fit the opposite miter
Adjust the final miter cut
Cut a test miter on the other end and check the fit. Adjust the miter as necessary until you’re satisfied with the joint.
Step 8: Scribe for length
Mark the final cut
With the saw still set for the previous miter, flip the trim over and scribe the length for the end that has that miter. Transfer the mark to the front side and make the cut.
Dealing with problem drywall
Projects more than 1/8 in.
If the drywall projects more than 1/8 in., crush in the drywall with a hammer. Just be sure the crushed area will be covered by trim. In this situation, your miters won’t be 45 degrees. You may need to go as low as 44 degrees to get a tight miter.
Projects less than 1/8 in.
If the drywall projects past the jamb 1/8 In. or less, and is close to the window jamb, just chamfer the edge with a utility knife. Check to see if you’ve pared off enough drywall by holding a chunk of trim against the drywall and jamb. If it rocks and won’t sit flush against both surfaces, carve out some more.
Drywall too low
If the drywall’s recessed behind the jamb, don’t nail the trim to the framing at first. Only nail it to the jamb and pin the mitered corners together. After the window is trimmed, slide shims behind each nail location to hold out the trim while nailing, then cut off the shims. Caulk the perimeter of the window casing trim to eliminate gaps before painting.
If you have drywall that’s ‘proud’ (sticking out past the jamb) or recessed behind the jamb, you have to deal with it before trimming or the window casing trim won’t lie flat. Here’s what to do:
Avoiding Trim-induced Headaches
Here are a few tips to help you avoid a few trim hassles:
- Whenever you can, cut with the thick side of the trim against the miter saw fence. You’ll be less likely to tear out the narrow tapered edge that way.
- Cutting right up to the pencil mark almost always leaves pieces too long, so remove the pencil line with the blade. You’ll most likely still have to shave off more.
- Sneak up on cuts by starting long and dipping the saw blade into the wood while you work your way to the cutoff mark.
- Trim out the biggest windows first. That way, you can reuse miscuts for the smaller windows and not run out of material.
- When nailing 3/4-in.-thick trim, use 15-gauge 2-1/2-in. nails for the framing and 18-gauge 2-in. brads for nailing to the jamb.
- To prevent splitting, avoid nailing closer than 2 in. from the ends.
Required Tools for this Project
Have the necessary tools for this DIY project and other trim projects lined up before you start—you’ll save time and frustration.
- Air compressor
- Air hose
- Brad nail gun
- Caulk gun
- Miter saw
- Safety glasses
- Utility knife
- Wood glue
Required Materials for this Project
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.
- Extra pencils
- Wood shims