Simple Window Trim – Easy How To

Easy How to Simple Window Trim

If you’re like me and have a builder-basic house, you might have noticed that your windows came with nothing—no sill, no trim, just bare drywall. That’s exactly what I was dealing with! But don’t worry, it’s an easy fix, and I’m going to walk you through how to add a window sill to give your windows that polished, finished look.

From this…

To this…

Shop the room here: You can find the materials I used on my Lowe’s storefront. (Please note that these are affiliate links, and I earn a small commission when you make a purchase through them. Your support is greatly appreciated!)

Materials You’ll Need:

1×4 Pine Board (Apron, sides, and top)
1×6 Pine Board (Stool) 4.5 inches longer on each side
Sherwin-Williams Pure White Semi-Gloss
Wood filler: For filling holes and imperfections.
Paintable caulk: To seal gaps and create a clean finish.
Brad nails: Various sizes for different parts of the build.
Painters Tape
Paint roller

Tools You’ll Need:

jig-saw
Brad nailer: Makes assembly faster.
Miter Saw
Tape Measure (I also used a digital tape measure)
Nail Punch
Caulk Gun
Safety Glasses
Level

Step 1: Measure the Window & Sand

Start by measuring the width and depth of the inside window frame. I measured and built each window individually for the best fit.

Next, lightly sand the area where the window stool will sit using 220–grit sandpaper. This will smooth out any rough spots and help ensure the stool sits flat against the surface. Check that the area is level. If it’s not, use shims during installation to create a flat, even surface before attaching the stool.

Step 2: Install the Window Stool

The stool is the bottom shelf-like piece that sticks out under the window.

Cut your 1×6 board to the width of your window plus 4.5″ on each side. For example, my window measured 34.5″ wide, so I cut my board to 43.5″ long.
Next, mark 4.5″ in from each end of the board. Then, using the depth measurement of your window frame, mark the sections that need to be removed. These cutouts will allow the board to fit around the window opening while leaving an overhang on both sides. (see photo below)
Clamp the board down. Use a jigsaw to cut out the notches, then test fit the board.
Once you’re happy with the fit, check that the stool is level and secure it with brad nails. If the stool isn’t level, add a shim underneath to raise the low side. Don’t worry about any small gap created by the shim—the apron installed in the next step will cover it.

Detailed image of a window trim with measurements, featuring 1x4 and 1x6 lumber dimensions and a 1/4 inch reveal. — Screenshot

Step 3: Attach Trim to the Sides of the Window

Measure the height from the top of the window stool you just installed to the top of the drywall.
Cut two 1×4 boards to this length using a miter saw. (Mine were about 60″ long.)
Before attaching them, leave a 1/4″ reveal along the inside edge of the window frame. You can use a laser level to keep the reveal consistent, or simply measure and mark 1/4″ along the length of the window.
Once everything is lined up, attach the boards with brad nails.

Step 4: Cut and Level the Apron and header

The apron is the trim piece directly below the stool. This will align with the width of your window.

The two 1×4 boards that you just installed, measure the width of the two.
Cut the new 1×4 board to that length with a miter saw, or use a circular saw.
Line up with the vertical 1×4 boards above. (see photo below)
Check that it’s level.
Attach it with brad nails.

Step 7: Fill, Caulk, Tape, and Paint

Caulk any seams for a clean, seamless look. Finally, paint everything. I chose SW Pure White in semi-gloss to match my baseboards, but you can go with whatever color works best for your space.