--- title: How to Install a Mullion Strip description: "On this page[Introduction](#)[Before You Begin](#)[Prerequisites](#)[What You Need To Know](#)[Tools](#)[Materials](#)[Installation Steps](#)[Step 1. Measure and Cut Strips](#)[Step 2. Dry Fit the Strips](#)[Step 3. Apply Wood Glue](#)[Step 4. Secure Strips with Pin Nailer](#)[Step 5. Final Adjustments and Cleanup](#)[Pro Tips](#)[Common Mistakes](#)[QC / Punch Checklist](#)[Related…" url: "https://letsbuildstuff.co/learn/install-mullion-strip/" updated: "2026-09-26" category: Moldings tags: - Level 1 - Supplemental Guide --- # How to Install a Mullion Strip On this page[Introduction](#)[Before You Begin](#)[Prerequisites](#)[What You Need To Know](#)[Tools](#)[Materials](#)[Installation Steps](#)[Step 1. Measure and Cut Strips](#)[Step 2. Dry Fit the Strips](#)[Step 3. Apply Wood Glue](#)[Step 4. Secure Strips with Pin Nailer](#)[Step 5. Final Adjustments and Cleanup](#)[Pro Tips](#)[Common Mistakes](#)[QC / Punch Checklist](#)[Related Guides](#)[Overview](#lbs-field-overview)[Steps](#lbs-field-steps)[Checks](#lbs-field-checks)[Fix a problem](#lbs-field-fix) [Moldings](/learn-category/moldings/) Skill Level: [Beginner (Level 1)](/learn/difficulty-levels/#level-1) · Estimated Time: 20-30 Minutes Per Window ## Introduction Installing mullion strips is a key step in [finish carpentry](/learn/trim-a-house/), 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](/learn/interior-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](/learn/interior-window-frame/) yet, do that first. - Drywall and painting are complete. - [Window casing](/glossary/moldings/#casing) is installed - see our guide on how to [case a window](/learn/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](/glossary/tools/#miter-saw-12in-sliding-compound) 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](/glossary/layout-and-shop-talk/#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](/glossary/materials/#pva-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](/recommends/dewalt-dwpn23100-amazon) 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 **Pro Tip:** Use a clamp to hold strips in place while the glue sets for a more secure bond. **Pro Tip:** Pre-drill holes for pins in hardwood strips to prevent splitting. ## 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. ## Related Guides - [Interior Window Frame for Medium-Tall Openings (90″ to 119″)](https://letsbuildstuff.co/learn/tall-interior-window-frame-medium/) - [Interior Window Frame for Tall Openings (120″ and Above)](https://letsbuildstuff.co/learn/install-interior-frame-120-above/) - [How to Install a Transom Window Above a Door](https://letsbuildstuff.co/learn/install-transom-window-above-door/) --- ## Reference Photos The following photos show examples of this type of work completed in the field. These serve as visual references for quality standards, proper installation techniques, and finished results. Showing 3 of 3 photos [![Unfinished wood mullion strip installed vertically between two adjoining multi-lite windows.](https://letsbuildstuff.co/website_ad59d41f/wp-content/uploads/2026/03/20241122_145218-1-225x300.jpg)](https://letsbuildstuff.co/website_ad59d41f/wp-content/uploads/2026/03/20241122_145218-1-scaled.jpg) Unfinished wood mullion strip installed vertically between two adjoining multi-lite windows. [![Several stained wood strips staged on a subfloor near an aluminum track, work in progress](https://letsbuildstuff.co/website_ad59d41f/wp-content/uploads/2026/03/20240528_134133-225x300.jpg)](https://letsbuildstuff.co/website_ad59d41f/wp-content/uploads/2026/03/20240528_134133.jpg) Several stained wood strips staged on a subfloor near an aluminum track, work in progress [![Mitered white trim corner joint with drilled screw holes, natural wood rail behind, mid install](https://letsbuildstuff.co/website_ad59d41f/wp-content/uploads/2026/03/20250807_074722-1-225x300.jpg)](https://letsbuildstuff.co/website_ad59d41f/wp-content/uploads/2026/03/20250807_074722-1-scaled.jpg) Mitered white trim corner joint with drilled screw holes, natural wood rail behind, mid install × ‹ › Written by **David Hospodka**, Sun’s Up Construction David Hospodka learned finish carpentry from his father, alongside his brother Joel, and worked his way through college trimming houses. He then spent about ten years as a railroad bridge engineer, handling design, construction, maintenance and project management. He came back to finish carpentry full time in 2017. [About Let’s Build Stuff](https://letsbuildstuff.co/about/)