How To Trim A Window And Add A Sill

Of course, being so old, the house also needed tons of updates, and honestly, we renovated almost everything right away. But some of the smaller tasks have taken us years to tackle. 

Like replacing the worn octagon window in our family room.
That one was on our to do list for decades...and it was the last project in the family room to wrap that space up. So I was thrilled when my husband woke up one morning and replaced it.

While we tackled this carpentry project on a unique octagon window, the basic framing steps, tool list, and installation methods are exactly the same for a standard rectangular window. 

The old window was out, the new one was in, and from the outside, everything looked wonderful. Unfortunately, finishing the inside was another story. Big surprise.Our new window sat differently in the opening than the old one, and the original sill was suddenly too shallow. Skimpy is probably a better word. It didn't fit with the rest of the house and it definitely wasn't deep enough to hold any kind of decor. Nope, it just wouldn't do.

So, to make me a happy girl, my handy husband built a brand new, deeper window sill from a pine board, trimmed it out, and created a showstopper of a window that I adore.

DIY Window Sill and Trim Tutorial

What You'll Need:

• Pine board (for sill extension)
• Molding/Trim for the outer surround (choose a style that matches your home)
• 1 1/2 and 2 inch Finishing Nails 
• Hammer 
• Cordless Drill (for pilot holes)
  
• Miter Saw*
• Nail Setter
• Shims
• Caulk
• Wood glue & Wood filler
• Sandpaper & Paint

*A miter saw will give you straighter cuts and the ability to retrim a piece if you need to cut off small amounts vs a miter box.  

Step 1. Measure And Cut The Sill Board

Once the window was installed he measured from inside the window frame to the outside edge of the sheetrock to determine his depth.


He also measured the perimeter of the window. For our project, he used a clear pine board (1 inch thick by 6 inches wide and 8 feet long), because clear pine has no knots for a perfectly smooth finish. 

He then trimmed the width of the wood plank to the proper width of 4.5 inches.

For a standard window: You would cut one single piece of wood to fit across the bottom of your window opening for your sill. There are stock window sills you can buy, or you can make one. Be aware that depending on your window style, you may want the sill to either be flush with your wall or extend off the wall.

For our octagon window: He cut the board into 8 seperate pieces of equal length. To frame the octagon shape, he made 22.5 degree miter cuts on the edges. (If you're working with an unusual window shape, using an online angle calculator is much easier than doing the math yourself!) 

Step 2. Fit and Fasten The New Sill

To fit the wood inside the window opening, he loosely preassembled the pieces to create a box, using wood glue and finishing nails.


He drilled pilot holes in the wood, before adding the nails, to lessen the chances of splitting the wood.

Next, he slid the box into the window frame and added a few shims to ensure that everything was level and snug. Fortunately, since our new window was the same size as the old window, very little shimming was required.

Once level, he secured it into the framing with finishing nails.

Step 3. Add The Trim Around The Window

To give the window a beautiful, finished look, he trimmed the outer edge of the window with more pine.


This board was 1/4 inch thick x 2.5 inches wide and 8 feet long. It complemented the plain sill wood, with its smooth low profile.

Of course, you can choose any trim wood that fits your home.

For a standard window: You would cut 4 pieces of trim to surround the window. You'd make standard 45 degree miter cuts at the corners to form a perfect 90 degree frame.

For our octagon window: This pine board was also cut into 8 pieces and mitered at 22.5 degrees to match the window's shape.

And again, pilot holes were drilled to prevent damaging the wood, before the nails were added to secure the trim pieces to the wall studs around the perimeter.

Step 4. Fill, Caulk, Sand and Paint

With the nails in place, he used a nail set to gently tap and recess each head slightly below the wood surface. Then wood filler was added to cover the tiny nail head holes.


A clean bead of caulk was added around the all the wood seams, corners and the gap where the trim met the sheetrock. That last detail gave the window a flawless finish.


Once the wood filler and caulk were dry, he lightly sanded everything and then primed the raw wood.

He completed this DIY by painting the window sill and window trim with two coats of a good interior white semi-gloss paint.

When he was done, I had a fabulous upgraded window and a brand new view.

Have the opposite problem?

Is your window ledge sticking out way too far into the room instead of being too small? Yeah, this old cottage is filled with issues, so we had that problem, too. In our kitchen the sill was just too deep. Check out our step by step guide on how to cut back a deep window sill here.

In this room however, a deep sill is just what this decor obsessed girl ordered.


Now there's tons of room for decor in that space.

Like flowers, or frames.

Or...well you get the idea.

😉

What's on your window sills?