How To Trim Windows With Metal Siding for Proper Insulation and Performance

Siding and trim are the blanket of a building, protecting it from the harsh elements while simultaneously reflecting a home’s style, character and charisma. When dealing with residential steel siding, you need to ensure you are trimming your windows correctly around the metal lap siding.

Below are some tips, tricks and how-tos for contractors trimming windows while installing steel siding. By the end, you’ll be able to perfectly trim your clients’ windows to allow their steel lap siding to shine.

Know Your Window Trim Codes

Depending on your geographical region, building codes can vary and may affect how you trim out your windows. Before you begin working on a particular project, you should check the area’s codes and make sure you are following the standards set for local policies and climates. 

How To Trim Windows

(If you are already an expert at installing window trim and want to know the specifics on installing Klauer window trim, click here.)

1. Measure Windows Accurately

Once you have accurate measurements and dimensions, you can now trim out the perimeter of the windows. 

2. Cut Trim

You should use a piece of J‑channel for the trim. Start with the side pieces. It’s smart to use the factory cut ends for the side pieces because the top of them is finished for a clean final look. 

For marking reveals, you should use a combination square. You could also use a compass to scribe the marks, or simply measure them yourself and mark the reveals. 

3. Bottom Trim

Clean up the ends and square them around. You are going to do something similar to the bottom trim as you did your side trim, but it includes a little metal trick. 

4. Install the Trim

Make sure to use some silicone caulk around the perimeter of the window, starting with the bottom. Push your bottom trim up into your bead of caulk to create a tight seal. 

Repeat the caulking on the side, and run a bead overlapping the bottom trim and going outward to ensure water will run out and not into the window. 

5. Install the Steel Siding

You’ve installed your steel siding up to the window, now it’s time to measure it up to the J‑channel from the sides, bottom, and to the top. To be tight to the top of the window, add a little extra siding.

Align your siding to the window frame and nail it in on both sides. Now it’s time to add the top J‑channel.

6. Top Trim

Now that you’ve cut and installed several other pieces of trim, double check your measurements to ensure the top trim is the correct size. And be sure to add caulk underneath the protruding trim to make sure no water leaks in. 

Now you have a perfectly trimmed window.