What is the difference between an astragal and a mullion on a double door?

An astragal mounts to one door leaf and overlaps the adjacent leaf to seal the center gap. A mullion is a separate vertical member anchored to the frame head and floor that provides a structural mounting point for rim exit devices. Astragals are used when one leaf is inactive with flush bolts. Mullions are used when both leaves need independent rim exit devices.

When is a mullion required instead of an astragal?

A mullion is required when both door leaves must operate independently with rim exit devices and there is no inactive leaf to carry flush bolts. Mullions are also specified when the opening needs structural reinforcement at the center point, in retrofit applications where concealed hardware is not feasible, or when fire code or life safety code requires a structural member for proper panic hardware function.

Can a removable mullion be used in a fire-rated opening?

Yes. Removable mullions can be used in fire-rated openings provided the mullion is listed and labeled as part of the fire-rated assembly. Fire-rated removable mullions are available at 20-minute, 45-minute, 60-minute, 90-minute, and 3-hour (180-minute) ratings. The complete assembly, including the frame, doors, hardware, and mullion, must be covered under the same fire test listing. Verify requirements with the AHJ.

What is a Z astragal?

A Z astragal is a metal overlapping astragal with a Z-shaped cross section. It is installed on the inactive door leaf in a double door pair. The Z profile wraps around the door edge and overlaps the active leaf, providing edge protection, a mounting surface for strike plates and flush bolt preparations, and a seal against light, sound, and drafts.

When are vinyl astragals used instead of metal astragals?

Vinyl astragals are used on push/pull double doors where both leaves carry surface-mounted vertical rod exit devices. Because both doors are active and swing independently, the flexible vinyl material accommodates door movement. The two vinyl astragal halves overlap at the center meeting point to maintain the gap seal.

Do astragals and mullions affect the clear opening width of a double door?

A Z astragal adds minimal width to the inactive leaf edge and does not significantly reduce clear opening width when both leaves are open. A fixed mullion permanently reduces the clear opening width by the width of the mullion member. A removable mullion can be taken out to provide full clear-width opening when needed, then reinstalled for everyday use with hardware support.

What fire ratings are available for astragals and mullions?

Both astragals and mullions are available in 20-minute, 45-minute, 60-minute, 90-minute, and 3-hour (180-minute) fire-rated configurations. Depending on the type of astragal your opening requires, fire rating will vary. Please contact one of our Door Experts to ensure you are getting exactly what you need. All components in the fire-rated assembly must carry matching fire labels. CDF labels all fire-rated astragals, mullions, doors, frames, and hardware before shipping.

How do I choose between a removable and fixed mullion?

Choose a removable mullion when the opening occasionally needs full clear width for moving large items such as equipment, furniture, or gurneys. Choose a fixed mullion when full clear width is never required and maximum structural reinforcement and forced-entry resistance at the center point are the priority. Both types support rim exit devices on both leaves and are available in all five fire rating levels (20-minute, 45-minute, 60-minute, 90-minute, and 3-hour/180-minute).

Frequently Asked Questions

CDF’s ProBuilder tool allows contractors and facility managers to configure double door assemblies online, including astragal and mullion selection. During configuration, ProBuilder displays compatible hardware options based on the door pair setup and provides instant pricing for the complete assembly.

For assistance selecting the correct astragal or mullion for your double door opening, call (855) 769-9895 or email sales@cdfdoors.com. CDF’s sales team is available Monday through Friday, 7:30 AM to 4:30 PM CT.

Configure Your Double Door Assembly

Astragals and mullions interact with several building code requirements for commercial door openings. The following points are relevant to code compliance decisions.

• Life safety codes require that panic hardware functions properly on all exit doors. Mullions ensure that rim exit devices have a proper strike surface, which is necessary for positive latching as required by code.

• Fire codes require that fire-rated assemblies include only listed and labeled components. Fire-rated astragals and mullions (20-minute, 45-minute, 60-minute, 90-minute, and 3-hour/180-minute) must be part of the tested assembly listing.

• Accessibility codes may govern the maximum force required to open a door and the minimum clear opening width. Removable mullions affect clear width calculations and should be evaluated against accessibility requirements.

• Always verify astragal and mullion selections with the project architect, fire protection engineer, and the Authority Having Jurisdiction (AHJ).

Code Compliance Considerations

Mullions are used in retrofit scenarios where the existing frame or door configuration does not accommodate concealed hardware, or where it is impractical to modify one door to act as the inactive leaf with flush bolts. Adding a removable mullion to an existing double door frame allows rim exit devices to be installed on both leaves without modifying the doors themselves. This approach is common in building upgrades where exit hardware must be brought into current code compliance without replacing the entire door assembly.

Retrofit Applications

Both astragals and mullions contribute to the alignment, environmental sealing, and security of double door openings. The specific benefits depend on the component type and configuration.

• Alignment: Mullions help maintain consistent alignment of the two door leaves, which is critical for proper weatherstripping performance, energy efficiency, and acoustic control. Astragals maintain alignment at the meeting point by overlapping the adjacent leaf.

• Environmental sealing: Both components seal the center gap against light, sound, drafts, and smoke infiltration. In exterior openings subject to wind or pressure changes, mullions provide additional structural resistance that helps maintain closing pressure and door balance.

• Forced-entry resistance: Mullions reinforce the area where the doors meet, preventing prying or spreading the doors apart. Z astragals provide moderate resistance through the overlapping profile, while mullions provide greater structural resistance as a frame-anchored member.

Alignment, Sealing, and Security Functions

Fire-rated mullions must be tested and listed as part of the complete door assembly. Removable mullions in fire-rated openings must meet the same listing requirements as fixed mullions. The Authority Having Jurisdiction (AHJ) may require documentation showing that the mullion, astragal, and all associated hardware are included in the fire-rated assembly listing. CDF labels all fire-rated components before shipping.

Verify all fire rating requirements with project specifications and the AHJ before ordering.

Fire-rated double door assemblies require that every component in the opening carry a matching fire label. This includes the frame, both door leaves, all hardware, and the astragal or mullion. The following table summarizes fire rating levels and their application to astragals and mullions.

Fire Rating Requirements for Astragals and Mullions

The choice between an astragal and a mullion is driven primarily by the exit device configuration on the double door pair. The type of panic hardware determines which center component is required and how the doors latch and secure.

Astragal with Flush Bolts (Inactive Leaf Configuration)

In the most common double door configuration, one leaf is designated as inactive. The inactive leaf carries a Z astragal and is secured at the top and bottom by flush bolts that engage the frame head and floor strike. The active leaf carries the primary exit device (rim, mortise, or vertical rod) and latches into the strike plate on the Z astragal. During normal operation, only the active leaf swings. The inactive leaf remains bolted in place unless manually unbolted for wide opening.

Mullion with Rim Exit Devices (Both Leaves Active)

When both door leaves must be independently operable with rim exit devices, a mullion is required. The mullion provides a strike surface for each rim device. Both leaves latch into the mullion, and either leaf can be opened independently by pushing the exit device bar. This configuration eliminates the need for one leaf to be designated as inactive and removes the need for flush bolts or concealed vertical rods. Removable mullions add the option of full clear-width opening when needed.

Vinyl Astragal with Surface Vertical Rod Devices

When both leaves carry surface-mounted vertical rod exit devices, vinyl astragals are used instead of a mullion. Each leaf has its own vertical rod device with latching points at the frame head and floor. The vinyl astragals overlap at the center to maintain the gap seal. This configuration is used when rim devices and a mullion are not desired but both leaves must be independently active.

How Astragals and Mullions Interact with Exit Devices

Astragals and mullions both address the center gap in double door openings, but they serve different functions and are selected based on the hardware configuration. The following table compares the two components across key selection criteria.

Astragal vs. Mullion: When to Use Each

Fixed mullions are specified when the opening does not need to provide full clear width and when maximum structural integrity at the center point is the priority. They are common in high-security applications and in openings where code requires a permanent structural member.

Removable mullions allow each door to latch into the mullion using rim exit devices, providing positive locking on both leaves without relying on one door being designated as inactive with concealed vertical rods or flush bolts. When both doors need to open fully for wide loads, the mullion can be removed by key or tool, then reinstalled afterward.

Fixed Mullions

Fixed mullions are permanently installed and cannot be removed without tools and modification to the frame assembly. The following table summarizes fixed mullion characteristics.

A mullion is a vertical structural member positioned between two door leaves in a double door frame. Unlike an astragal, which mounts to a door leaf, a mullion is anchored to the frame head at the top and to the floor or threshold at the bottom. Mullions provide a fixed point for mounting rim exit devices and locking hardware, and they reinforce the center of the opening against forced entry. Mullions are available in fixed and removable configurations.

Removable Mullions

Removable mullions provide hardware mounting support during normal operation while allowing full clear-width opening when the mullion is removed. The following table covers removable mullion characteristics.

What Is a Mullion?

Vinyl astragals are used on double door configurations where both leaves are active and equipped with surface vertical rod exit devices. Because both doors swing independently, the vinyl material flexes during operation. The two astragal halves must overlap at the meeting point to maintain an effective seal. This configuration is common on push/pull pairs in commercial corridors and exit pathways.

The Z-shaped profile wraps around the edge of the inactive door, creating an overlapping seal against the active leaf. This overlap prevents the doors from being pried apart at the center and provides a surface for strike plates used with flush bolts or other latching hardware. Z astragals are standard on pairs where one leaf is designated as the inactive door, secured by top and bottom flush bolts.

Vinyl Astragals

Vinyl astragals serve a different application than metal Z astragals. The following table outlines when vinyl astragals are used.

An astragal, also called a meeting stile, is a vertical component that covers the gap between two door leaves in a double door system. Astragals are mounted to the edge of one door leaf and overlap the adjacent leaf when the doors are closed. They seal the center gap to block light, sound, drafts, smoke, and fire from passing between the doors. Astragals are made from metal or vinyl depending on the door hardware configuration and application requirements.

Z-Type Astragals

Z astragals are the most common type used in commercial double door applications. The following table summarizes their characteristics.

What Is an Astragal?

This reference guide is published by CDF Distributors, a commercial door and frame supplier that fabricates hollow metal assemblies in-house and ships nationwide. The information reflects CDF’s standard product specifications and the technical knowledge applied across thousands of double door assemblies. This guide covers astragal types, mullion types, their interaction with exit devices and flush bolts, fire rating considerations, and selection criteria for commercial double door openings.

About This Guide

Astragals and mullions are vertical components installed between the two leaves of a commercial double door opening. An astragal is a metal or vinyl strip mounted to one door leaf that covers the gap where the two doors meet, sealing against light, sound, drafts, smoke, and fire. A mullion is a fixed or removable vertical member anchored to the frame head and floor, providing a structural mounting point for rim exit devices and locking hardware. Both components serve double door openings, but they function differently and are selected based on the hardware configuration, fire rating requirements, and whether full clear-width opening is needed.

CDF Distributors supplies astragals and mullions as part of complete commercial door assemblies fabricated at its Nashville, Tennessee facility. You can configure double door assemblies with the correct astragal or mullion using CDF’s ProBuilder tool at cdfdistributors.com. For assistance selecting the right component for your double door opening, call (855) 769-9895 or email sales@cdfdoors.com.