Moldings Skill Level: Beginner (Level 1) · Estimated Time: 20-30 Minutes Per Window

Introduction #

Installing mullion strips is a key step in finish carpentry, especially in projects where the aesthetic of divided light windows is desired. These strips give windows a classic, paneled look without the need for separate glass panes. Mullion strips are often installed in new construction or during remodeling projects to enhance architectural detail and add character to a space.

The single most important concept when installing mullion strips is precision. Accurate measurements and cuts ensure that the strips fit snugly and align perfectly with the window frame, creating a seamless look. This level of precision separates professional installations from amateur attempts, minimizing gaps and ensuring a clean, finished appearance.

Before You Begin #

Before you install mullion strips, confirm that the following prerequisites are complete.

Prerequisites #

• Windows are installed and secured - if you haven't built out the interior window frame yet, do that first.
• Drywall and painting are complete.
• Window casing is installed - see our guide on how to case a window if that step is still pending.

What You Need To Know #

When you install mullion strips, understanding these core techniques will save you time and prevent callbacks.

• Ensure mullion strips are cut to the exact width and height of the window panes.
• Use a miter saw for precise 45-degree cuts on the ends of the strips.
• Use a pin nailer to secure strips without splitting the material.

Tools #

• Miter Saw - for precise angle cuts.
• Pin Nailer - for securing strips without visible nails.
• Measuring Tape - for accurate measurements.
• Square - to ensure straight cuts.

Materials #

• Mullion Strips - typically wood or composite, available at lumber suppliers.
• Wood Glue - for additional bonding strength.
• Finishing Nails or Pins - for securing strips.

Installation Steps #

Step 1. Measure and Cut Strips #

Measure the window panes to determine the length of each mullion strip. Use a miter saw to cut the strips to the required length, ensuring 45-degree angles on the ends for a perfect fit.

• Measure twice, cut once to avoid waste.
• Ensure cuts are clean and precise.

Step 2. Dry Fit the Strips #

Place the cut strips into the window frame to check fit. Adjust as necessary to ensure all pieces align perfectly.

• Check for gaps and adjust cuts if needed.

Step 3. Apply Wood Glue #

Apply a thin line of wood glue along the edges of the strips where they will contact the window frame. This provides additional bonding strength.

• Use a small brush to spread glue evenly.

Step 4. Secure Strips with Pin Nailer #

Using a pin nailer, secure the strips in place. Ensure nails are driven into the frame and not through the glass.

• Use 23-gauge pins to minimize visibility.

Step 5. Final Adjustments and Cleanup #

Check all joints and seams for alignment. Clean any excess glue and ensure the strips are firmly attached.

• Wipe away excess glue with a damp cloth.

Pro Tips #

Common Mistakes #

• Mistake: Misaligned strips - Ensure all cuts are precise and angles are consistent.
• Mistake: Visible nail holes - Use a pin nailer and drive pins fully below the surface.

QC / Punch Checklist #

• Check for gaps between strips and window frame.
• Ensure all strips are level and aligned.
• Verify that all glue residue is cleaned up.