What is a Mullion?

A mullion is a vertical bar that divides a door or window frame into separate sections. Invented during the Middle Ages, they were a common design element in Renaissance, Victorian, and Gothic architecture. Mullions lend structural support to large windows and double doors. While Medieval and Gothic Cathedrals used stone mullions, modern mullions are generally made from wood, steel, or aluminum. Wood mullions are typically found in residential settings, while steel and aluminum mullions are normally installed in government and commercial buildings, due to their enhanced durability. There are several types of door mullions.

• Fixed. Permanently separate two door panels. They not only provide stability, but strength. A steel or aluminum mullion allows doors to resist winds and impact far better than they would on their own.
• Removable. A useful feature in high-traffic areas, removable mullions can be taken out in order to widen the doorway, allowing you to accommodate oversized objects and large groups of people.
• Decorative. Some buildings, such as hotels, use mullions to improve the aesthetic appeal of their property. Adding a mullion to French doors or glass-panel double doors can lend your building a traditional or cottage atmosphere.

Mullions are often confused with muntins and transoms. A muntin is a small strip of wood or metal that separates panes of glass into a grid or grille, while mullions divide whole window and door units. Modern muntins are often used decoratively, but in older architectural styles, they ensured the window’s weight was distributed evenly across the frame. Transoms, on the other hand, are horizontal beams that sit above doors, usually to support a glass window over the doorway.

Role of Mullions in Double Doors

Mullions provide several key benefits for commercial and industrial doors. Depending on their location, they may provide:

• Stability. Mullions create a solid frame that double doors can lock into. This reduces pressure on the hinges and prevents doors from warping under their own weight, thereby extending their lifespan.
• Protection. A secure anchor makes it far more difficult for intruders to force their way through a double door. It’s an additional barrier they have to overcome before they can access your property.
• Weatherproofing. Installing a mullion in a set of exterior doors creates a tighter seal that blocks moisture and reduces energy loss, making your building more efficient.

Mullions perform a critical role in terms of both security and function. By strengthening doors, they reinforce security while reducing weaknesses that could lead to greater long-term costs.

Commercial Door Maintenance and Repair

Vortex service teams are trained to identify issues in all aspects of your doors, including mullions. We can detect when they’re misaligned, warped, corroded, or simply too worn to provide reliable support. We tighten screws, treat rust, and replace degraded components to ensure your doors continue to work as intended. Keep your business secure. Contact us today to learn more about our repair and maintenance programs.