DIY Window Trim on Builder-Grade Windows

I added DIY window trim to our builder-grade bare living room windows. See how I decided to add the trim, as well as all of the steps I took to ensure a professional-looking finished product!

Adding DIY window trim on builder-grade windows

Man, it’s been a while since I’ve done a home DIY, hasn’t it? That tends to happen when you’ve lived somewhere for a while and just don’t have many projects to do. I can’t believe we’ve been in our townhouse for over 5 years now!

But with time can also come a desire for change. That’s exactly how I felt about my living room. We had the outsides of our windows cleaned a few weeks ago, and I decided to take down the curtains to wash for the first time in 5 years.

And wouldn’t you know it, after taking them down…I loved how it looked! I used to think that you HAD to have curtains in a room, even if they weren’t functional curtains. It was one of the first things we did in the house.

We actually used to have two sets of curtains on these double rods: a dark gray exterior curtain and a sheer white layer under those. It was sooo heavy. So when I took down the gray ones to wash last year, I just…never put them back up.

And these days I am even more of a minimalist. Basically if I have to clean it, it needs to serve a purpose. So when those white sheer ones came down too, I loved it.

Also—with such a huge plant collection—I need to maximize the light in our home. With the curtains down…whoooo boy. The light was great! 

So I made an impulsive decision to take down the curtain rods and list all of them (plus all of the curtains!) for free on my neighborhood Facebook group. It was a bargain for someone else—I just wanted them out of the house.

And since I’d committed to the project by giving everything away, it was time to get to work! =

Here’s what I used for our DIY window trim…

(Affiliate links below; read more about those here)

Building supplies

• 1” x 3” (actual: 3/4″ x 2 1/2″) for the main trim
• 1” x 2” (actual: 3/4″ x 1 1/2″) for the border trim
• Miter saw
• Nail gun
• Bar clamps
• Measuring tape and pencil
• Safety gear including glasses

Finishing supplies

• Latex (paintable) caulk and caulk gun
• Wood filler
• Painter’s tape
• Primer
• Pure white semigloss paint

And here’s how I added modern DIY window trim to our builder-grade windows!

Step 1: Take measurements for the main frame

I knew I wanted to do a simple style with trim the same width all the way around the window. I’d originally looked at craftsman window trim styles—the kinds that have the taller top/header pieces.

But I ultimately didn’t feel like that was the best choice for our home since it isn’t really a craftsman-style home. However, a simple window trim will work in any home, so I got started with some measurements.

The window sills are still in great shape, so I knew I wanted to work around those. I measured the distance from the windowsill to the top of the window on each side. Then I measured the width of the window. This is what I used to get started.

For some similar projects, check out my tutorial for re-caulking crack wood stair tread seams, how to DIY easy picture ledge shelving, and how to hang a planter from the ceiling!

Step 2: Cut the main frame trim pieces

Based on the size of my windows, I decided to run 1×3 around the three bare sides of the window. I cut the left and right pieces to be the same height as the window, and then I cut the top piece to be the width of the window + the width of each left/right piece. So, the width of the window + 5 inches.

Tip: This project requires precise cuts, so I measured, marked, and cut each piece as I went. Then I dry-fit it in place to make sure I didn’t need to shave off another 1/16” of an inch. Even though all of my windows are the same size, the exact measurements differ slightly.

Once I’d dry-fit all 12 pieces (3 on each of the 4 windows), I primed them. You can skip this step by buying pre-primed wood, which I did for the 1×2. However, our store was out of the pre-primed 1×3, boo.

Tip: As you are cutting and priming the pieces, you can mix them up. I recommend marking which window they belong to on the back. For example, I marked mine “1L” and “1R” for first window, left side and first window, right side.

Step 3: Nail the DIY window trim frame up

Once the primed pieces has dried, I nailed them up using our battery-operated nail gun. Make sure you wear safety gear—protect those hands and eyes!

Ahh, it was already looking pretty amazing! I could see my vision coming to life. But it was time to move on to cutting the exterior frames made of 1×2.

Step 4: Cut exterior frame, prime, and nail up

Next I repeated the measuring, marking, and cutting process for the exterior 1×2 frame. As with the 1×3 main-frame pieces, I cut the left and right 1×2 pieces to run the entire length of the window from the sill up to the top of the new trim.

Once I’d cut and dry-fit those pieces, I measured and cut the final top piece for each window. And when I’d finished that, I used my nail gun to nail everything into place.

Note: The location of the top outer-frame piece and the height of our ceilings made it difficult for me to attach those pieces with a nail gun. Therefore, I attached them using wood glue and clamps.

Step 5: Caulk the window trim…and caulk…and caulk…

They look so good! But they were about to look a lot better. The next step was to caulk the crap out of these bad boys using paintable latex caulk. 

I caulked all of the seams because I really wanted these to have a seamless look. Caulking is messing and I almost always use painter’s tape to help me. Here’s what I do:

• Apply painter’s tape on either side of the area I want to caulk.
• Cut off the tip of the caulk at an angle, making sure to keep the opening as small as possible.
• Use a caulk gun to run a line of caulk on the painter’s tape; tip a disposable-gloved finger in water and run it down the line of caulk to flatten it.
• Remove the painter’s tape. Dip your finger in water again and run it over the caulk line once more if you need to.

About halfway through, I was getting super tired of caulking and switched methods. Instead of using a wet gloved finger, I cut a rag up into small pieces, wet that down, and used that to smooth out all of my caulk lines.

Step 6: And finish off the DIY trim by painting it!

Once the caulk dried, I patched all of my nail holes and got to work painting the trim. And I painted the inner drywall frame between the trim and the windows.

I was originally not going to paint that area the same bright semigloss white, and I have no idea why not…because it looks AMAZING! So here’s my finished trim. What do you think?!

I love how light and open it makes everything look. But I sure am glad this project is over. It definitely was more work than I thought it’d be, but I am extremely pleased with how it turned out! What do you think?

Pin my post about adding DIY window trim to builder-grade windows!