How To Assemble Exterior Window Trim

Adding exterior window trim completes the look of your home and protects it from the elements. Without that trim, water can seep in around the window. Air can also leak in or out, making it harder to keep your home comfortable. In this guide, we’ll explain how to assemble exterior window trim and add decorative designs that reflect your style. We also included the video above, featuring lead carpenter Adam Spink with an efficient way to get flawless window trim.

Materials and Tools for Assembling Exterior Window Trim

Before you get started, gather all the tools and materials to keep your project running smoothly:

• 1 1/2-inch exterior grade coarse thread Kreg screws
• 23-gauge stainless steel nails (1 3/16-inch)
• Clamps (including Collins clamps for miters)
• Pre-treated, rot-resistant pine trim boards
• Exterior-grade wood glue
• Kreg pocket screw system
• Miter saw
• Multi-tool
• Nail guns (brad nailer and 23-gauge pin nailer)
• Palm sander
• Stainless steel brad nails (2-inch and 1-inch)
• Table saw

Preparing the Window Trim Pieces

There’s some prep work required if you want a successful trim installation. These steps include cutting the trim and beveling the leg casings.

Cutting the Trim

Following Spink’s advice in the video, start by creating a cut list based on your window’s dimensions. Measure and cut the following pieces:

• Cap
• Crown molding
• Head casing
• Leg casings (left and right)
• Return pieces
• Sill

Beveling the Leg Casings

Add a bevel cut to the back of the leg casings to fit the window flange and butyl tape. This will give you a snug fit against the house and allow your casing to sit flat against the wall. Here are the steps to bevel the leg casings:

1. Set the table saw to a 3-degree angle.
2. Rip a bevel along the back edge of each leg casing.
3. Cut an additional small rabbet to allow for the window gasket.

Assembling the Main Window Frame

With your pieces prepped, you can assemble the main frame of your window trim. This forms the backbone of the trim and gets it ready for decorative elements.

Attaching the Head Casing to the Legs

A sturdy framework keeps everything aligned and supported. Here’s how to attach the head casing to the leg casings:

1. Apply exterior-grade wood glue to the joints.
2. Use the Kreg pocket screw system to join the head casing to the leg casings.
3. Clamp the pieces together for a tight fit while you screw them together.

Adding Backer Braces

Take the steps below to add extra support and stability:

1. Cut small backer braces from scrap trim material.
2. Glue and nail these braces to the back of the frame and make sure the joints overlap.
3. Allow the braces to extend about an inch beyond the front of the frame to help catch the cap piece later.

Installing the Sill

The sill helps deflect water and blends the trim with your existing siding. Since the sill plays an important role in your window’s exterior trim, follow the steps below carefully:

1. Use a 7/8-inch spacer to set the sill at the correct distance from the house.
2. Apply glue to the bottom of the leg casings.
3. Attach the sill using Kreg screws, extending 1 inch beyond each side of the frame.

Adding Decorative Elements to the Window Trim

For a more polished finish, you can add decorative trim elements to your trim to complement its structure. Here are some ideas to boost your window trim’s look:

Attaching the Cap

You or your contractor will place the cap on top, which creates a cohesive look and adds some protection to the structure. Here are the steps you’ll take to attach the cap: 

1. Mark 45-degree angles on either side of the head casing to align the cap.
2. Apply glue to the top of the head casing.
3. Nail the cap in place using 2-inch stainless steel brad nails.

Marking the angles will help you properly align the cap, and fastening it will make it sturdier.  

Installing the Crown Molding

Crown molding can give your window trim an elegant and sophisticated look. Here are the basic steps to add it: 

1. Cut the crown molding to fit, with 45-degree miters at each end.
2. Apply glue to the back of the crown molding.
3. Use 1-inch, 18-gauge brad nails to tack the crown in place.
4. Secure the mitered corners with Collins clamps so you get a tight fit while the glue dries.
5. Use 23-gauge pin nails for additional security if needed.

Pay close attention to your cuts for precise angles and a perfect fit.

Preparing for Window Siding Installation

Before you install the siding, you’ll need to cut the returns and make room for the rain screen.

Cutting the Returns

Spink recommends leaving the crown molding returns long so you can make precise adjustments. This method will help you get an accurate installation:

1. Use a multi-tool to make a rough cut, removing excess material.
2. Make a second, more precise cut using a shingle as a guide.
3. Test fit the shingle for a tight, flush fit.

Accommodating the Rain Screen

The rain screen system helps protect the house against moisture. To accommodate it, leave extra space behind the trim for the rain screen material. Make sure the sill extends far enough to allow shingles to tuck in properly.

Finishing Touches to Your New Window Trim

Now that most of the work is complete, it’s time to put on the final touches. This helps make your trim look polished and professional:

1. Sand any slight imperfections in the joints with a palm sander.
2. Prime and paint the trim to match the house exterior.
3. Install the trim on the house and make any adjustments so it fits.  

Additional Pro Tips for Installing Window Trim

Here are a few tips we recommend you take to give your window trim installation a more professional finish: 

• Use a level to check the trim’s alignment and correct any deviations.
• Keep a consistent workflow to help all pieces fit together seamlessly.
• Pre-drill holes for screws in thinner trim pieces to prevent cracking.

Our Conclusion

Exterior window trim helps to give your home a seamless look and protects it against the elements. By following Spink’s method, you’ll create a durable window trim that will last for many years. We recommend you contact a professional contractor for help if you run into any issues, and remember to practice safety precautions.