This reference guide reflects CDF Distributors’ standard fabrication specifications and commercial door preparation practices used across thousands of door assemblies shipped nationwide from CDF’s Nashville, Tennessee facility. The preparation types, dimensions, and compatibility information in this guide are based on current ANSI/SDI standards and CDF’s production specifications.

For questions about door preparations or to configure a door assembly with the correct preps for your hardware, contact CDF at (855) 769-9895 or sales@cdfdoors.com. Configure your assembly online at cdfdistributors.com using ProBuilder.

About This Guide

What is the difference between a cylindrical lock prep and a mortise lock prep?

A cylindrical lock prep consists of a 2-1/8” cross bore through the door face and a 1” edge bore for the latch, positioned at a 2-3/4” backset. A mortise lock prep is a rectangular pocket machined into the door edge to accept the entire mortise lock body, plus a cylinder bore for the mortise cylinder. The two preps are fundamentally different and not interchangeable. The lock type specified in your hardware schedule determines which prep to order.

Can I change the door prep after the door has been manufactured?

Field modifications to change a door prep are not recommended, especially on fire-rated doors where unauthorized modifications void the fire label. On non-rated hollow metal doors, some modifications are possible as shop labor services, but the preferred approach is to order the correct prep from the factory. Contact CDF at (855) 769-9895 to discuss options if a door has been ordered with the wrong prep.

What minimum trim size is required to cover a cylindrical lock prep?

The lock rose or exit device trim plate must be at least 3-1/4” in diameter to fully cover the enhanced cylindrical lock prep with through-bolt notching. If your trim is smaller than 3-1/4”, the prep cutout will be visible around the trim edges. Verify your hardware’s trim dimensions against this requirement before ordering.

Do fire-rated doors require different preparations than non-rated doors?

Fire-rated doors (20-minute, 45-minute, 60-minute, 90-minute, and 180-minute/3-hour) use the same general prep types as non-rated doors, but all preparations must be applied at the factory in accordance with the door’s fire listing. Field modifications to fire-rated doors are prohibited. All hardware installed on fire-rated doors must be listed and labeled for the specific fire rating of the assembly.

How does door handing affect the door prep?

Door handing determines which edge of the door receives the hinge prep and which edge receives the lock prep. All preparations are machined based on the specified hand (LH, RH, LHR, or RHR). A door prepped for one hand cannot be flipped to serve as the opposite hand without the preps being on the wrong edges. Always verify handing before placing the order.

What is closer reinforcement and when is it needed?

Closer reinforcement is an internal steel plate welded inside the top rail of a hollow metal door during fabrication. It provides structural support for surface-mounted door closer mounting screws. Closer reinforcement is needed any time a surface-applied closer is specified in the hardware schedule. Without it, the closer screws may pull through the door face over time. The reinforcement location (pull side or push side) depends on the closer arm type.

Can a door be prepped for both an exit device and a deadbolt?

Yes, a door can receive multiple preparations for different hardware. For example, a door can include an exit device prep, a deadbolt prep, and closer reinforcement simultaneously. Each prep must be specified separately at the time of order. CDF’s ProBuilder allows you to add multiple hardware preparations during the door configuration process. Make sure to check required fire rating for your opening as that will limit hardware configurations available.

What happens if the exit device trim does not cover the door prep?

If the exit device trim plate or escutcheon does not fully cover the door prep cutout, the machined openings will be visible around the trim, creating an aesthetic issue and a potential security concern. In this situation, the door should be ordered with a blank face at the lock location and prepped specifically for the exit device. CDF can perform custom shop modifications to match non-standard exit device requirements.

Frequently Asked Questions

CDF’s ProBuilder tool allows contractors, architects, and facility managers to configure commercial door assemblies with the correct preparations for their hardware. During configuration, ProBuilder validates that the selected door prep is compatible with the specified hardware, checks handing, and confirms fire-rating compliance. ProBuilder displays compatible options and provides instant pricing at cdfdistributors.com.

For assistance selecting the correct door preparation for your project, 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 Door Assembly

Commercial door preparations are governed by several industry standards that define prep locations, dimensions, and tolerances. The following standards apply to the door preparations covered in this guide.

• ANSI/SDI A250.8 (SDI-100): Recommended Specifications for Standard Steel Doors and Frames. Defines standard hinge locations, lock prep locations, and reinforcement requirements.

• ANSI A115 Series: Hardware prep standards for cylindrical locks, mortise locks, and exit devices.

• ANSI/BHMA A156 Series: Hardware performance standards that inform prep requirements for specific device types.

• NFPA 80: Standard for Fire Doors and Other Opening Protectives. Governs modifications and preparations on fire-rated door assemblies.

• UL 10C: Positive Pressure Fire Tests of Door Assemblies. The listing standard under which fire-rated doors and their preparations are tested and certified.

Industry Standards for Door Preparations

Ordering the wrong door preparation is one of the most frequent errors in commercial door specification. The following mistakes result in delays, rework, or replacement orders.

• Ordering a cylindrical lock prep when the hardware schedule calls for a mortise lock. Cylindrical and mortise preps are not interchangeable. The door must be reordered.

• Specifying the wrong handing. All preps are applied based on the door hand. An incorrectly handed door has all preparations on the wrong edges and typically cannot be corrected in the field.

• Assuming exit device trim will cover a standard cylindrical lock prep. Not all exit device trim plates are large enough to cover enhanced cylindrical lock preps. Verify trim dimensions before ordering.

• Omitting closer reinforcement. If a surface-mounted closer is specified but closer reinforcement is not included in the door order, the closer screws may not hold in the hollow metal door face.

• Modifying a fire-rated door in the field. Any unauthorized cutting, drilling, or machining of a fire-rated door voids the fire label and creates a code violation. All preps must be applied at the factory.

• Forgetting the deadbolt prep. If the hardware schedule includes a deadbolt in addition to the primary lock, the deadbolt prep must be specified separately at the time of order.

Common Door Prep Ordering Mistakes

Choosing the right door preparation requires matching the prep type to the hardware specified in your project’s hardware schedule. The following decision points help determine the correct prep for common commercial hardware scenarios.

Lock Type Determines the Primary Prep

The primary lock or latch device on the door dictates the main door prep. If the hardware schedule calls for a cylindrical lockset, the door receives a cylindrical lock prep. If it calls for a mortise lock, the door receives a mortise lock prep. These two prep types are fundamentally different and cannot be substituted for each other without reordering the door.

Exit Devices May Override the Lock Prep

When an exit device (panic hardware) is specified, the exit device type determines the required prep. A rim exit device may require its own face and edge prep. If the exit device uses lever trim on the exterior side, that trim may mount into a cylindrical lock prep or a mortise lock prep depending on the trim type. Always verify that the exit device trim plate or escutcheon covers the door prep completely. If the trim does not cover the prep, the door should be ordered with a prep specific to the exit device.

Trim Rose and Escutcheon Coverage

For cylindrical lock preps with enhanced through-bolt notching, the trim rose or escutcheon must be at least 3-1/4” in diameter to fully cover the prep cutout on the door face. This applies to both standalone cylindrical locksets and exit device lever trim. If the trim component is smaller than 3-1/4”, the cutout will be visible around the trim, which is both an aesthetic and a security concern. Verify trim dimensions against the prep before ordering.

Shop Modifications for Non-Standard Requirements

If the specified hardware does not align with a standard prep option, CDF can perform custom shop modifications. Custom preps are applied at the factory as a separate fabrication step. Shop modifications ensure that the door’s fire rating (if applicable) is maintained and that the hardware fits correctly. Contact CDF’s sales team to discuss custom prep requirements for non-standard hardware configurations.

Selecting the Correct Door Prep for Your Hardware

Fire-rated doors cannot have unauthorized field modifications. Any additional holes, cutouts, or machining performed after the door leaves the factory can void the fire rating and create a code violation. All preparations must be completed at the factory and applied in accordance with the door manufacturer’s fire listing. CDF applies fire labels to every fire-rated door and frame assembly before shipping.

When specifying hardware for fire-rated openings, verify that the hardware is listed for the specific fire rating of the assembly. For example, a closer may be listed for use on 90-minute doors but not on 180-minute (3-hour) doors. The Authority Having Jurisdiction (AHJ) has final authority over fire-rated assembly compliance.

Fire-rated doors have additional requirements and restrictions on preparations. Every preparation made to a fire-rated door must maintain the integrity of the fire rating. Hardware installed on fire-rated doors must be listed and labeled for use on fire-rated assemblies. The following table summarizes fire rating levels and their impact on door preparations.

Fire-Rated Door Preparations

When ordering a prepped door, the handing must be specified correctly because the factory machines all preparations based on the hand. A door prepped for right hand cannot be flipped to serve as a left hand door without the hinge and lock preps being on the wrong edges. Incorrect handing is one of the most common ordering errors and typically requires a replacement door rather than field correction.

CDF’s ProBuilder validates handing against the selected hardware during the configuration process, helping prevent mismatches before the order is placed.

Door handing and door preparations are closely linked. The handing of a door (left hand, right hand, left hand reverse, or right hand reverse) determines which edge of the door receives the hinge prep and which edge receives the lock prep. It also determines the orientation of asymmetrical preparations such as exit device reinforcement and closer reinforcement.

How Door Preparations Relate to Door Handing

Deadbolt preps are specified separately from the primary lock prep. When ordering a door that requires both a cylindrical lockset and a deadbolt, both preparations must be included in the door order. CDF’s ProBuilder allows you to add deadbolt preparations during the door configuration process.

The closer mounting location (pull side vs. push side) depends on the closer arm type specified for the project. Regular arm closers mount on the pull side of the door, while parallel arm closers mount on the push side. The reinforcement must be on the correct side to support the closer. CDF includes closer reinforcement as part of the door assembly when a closer is specified in the hardware schedule.

Deadbolt Prep

A deadbolt prep is a separate bore and edge cutout positioned above or below the primary lock prep. Commercial deadbolts provide an additional locking point independent of the primary lockset. The deadbolt prep includes a face bore for the thumb turn or cylinder and an edge bore for the deadbolt.

Hinge prep locations are standardized according to the Steel Door Institute (SDI) and the Door and Hardware Institute (DHI). The top hinge is located with its top edge 7” from the top of the door, the bottom hinge is located with its bottom edge 11” from the bottom of the door, and the middle hinge is centered between the two. CDF pre-punches hinge locations on both doors and frames during fabrication.

When a continuous (piano) hinge is specified instead of butt hinges, the door receives a continuous hinge prep along the full length of the hinge edge rather than individual mortise cutouts. Continuous hinges are common in high-traffic and high-abuse applications.

Closer Reinforcement

Door closer preparations consist of internal reinforcement plates welded inside the top rail of a hollow metal door. This reinforcement provides the structural support needed to mount a surface-applied door closer. Without closer reinforcement, the closer mounting screws may pull through the door face over time, especially on high-traffic doors.

Not all exit device heads will cover every cylindrical lock prep pattern. When using an exit device with lever trim on a door that has a cylindrical lock prep, verify that the trim plate or escutcheon fully covers the prep cutout. If the exit device trim does not cover the prep, the door should be ordered with a blank face at the lock location and prepped specifically for the exit device being installed.

Hinge Prep

Hinge preparations are the factory-machined mortises on the hinge edge of the door that accept butt hinges. Standard commercial doors receive three hinge preps: one near the top of the door, one near the bottom, and one centered between the two. Heavy doors or doors taller than 7’0” may require a fourth hinge. The hinge prep on the door must match the corresponding hinge prep on the frame for proper alignment.

Mortise lock preps and cylindrical lock preps are not interchangeable. A door ordered with a cylindrical lock prep cannot accept a mortise lock without significant field modification, which may compromise the door’s fire rating and structural integrity. Always confirm the lock type before ordering the door.

Exit Device Prep

Exit device preparations accommodate panic hardware (also called crash bars or push bars) that are required on egress doors by building codes. The prep type varies depending on whether the exit device is rim-mounted, surface vertical rod, or concealed vertical rod.

Rim Exit Device Prep

A rim exit device mounts on the interior face of the door and engages a strike mounted on the frame. The door prep includes reinforcement at the device mounting locations and a cutout or through-hole for the latch mechanism on the lock edge. Rim exit devices are the most common type and are used on single doors and the active leaf of pairs.

Surface Vertical Rod Exit Device Prep

Surface vertical rod devices have rods that run vertically on the door face from the push bar to latching points at the top and bottom of the door. The prep includes reinforcement at the device body location and through-holes or cutouts at the top and bottom rails for the rod bolts. These devices are used on the active leaf of a door pair where no vertical mullion is present.

Concealed Vertical Rod Exit Device Prep

Concealed vertical rod devices route the rods inside the door rather than on the surface. This prep requires internal channels, top rail cutouts, and bottom rail cutouts machined into the door. The prep is more complex than a surface vertical rod prep and must be specified at the time of order. Concealed vertical rod preps are common on pairs of doors in architectural applications where a clean door face is desired.

CDF’s standard cylindrical lock prep includes additional through-bolt notching around the cross bore. This enhanced prep pattern allows for faster hardware installation because through-bolts can be inserted without additional field drilling. When specifying cylindrical lock hardware with this prep, the lock rose or exit device trim plate must be at least 3-1/4” in diameter to fully cover the prep cutout on the door face.

The 2-3/4” backset is compatible with cylindrical locksets from all major commercial hardware manufacturers. If your project requires a 2-3/8” backset (common in residential applications but rare in commercial work), confirm compatibility before ordering.

Mortise Lock Prep

A mortise lock prep is a rectangular pocket machined into the lock edge of the door to accept a mortise lock body. The mortise pocket is traditional 8 inches long. Mortise locks are typically specified on high-traffic commercial applications, institutional facilities, and projects requiring heavy-duty locksets with integrated deadbolts. The mortise pocket is significantly larger than a cylindrical bore and requires precise machining to ensure the lock body seats correctly.

Cylindrical Lock Prep

The cylindrical lock prep is the standard preparation for commercial bored locksets. It consists of a cross bore (a hole through the door face) and an edge bore (a hole through the door edge) that intersect inside the door. The cross bore allows the lock chassis and trim to mount on each face of the door, while the edge bore accepts the latch assembly.

Commercial steel and wood doors can receive several categories of factory preparations. The following table summarizes the primary prep types used in commercial construction, along with the hardware each prep accommodates and its typical location on the door.

Types of Commercial Door Preparations

A door preparation is a set of holes, cutouts, reinforcements, and mortises machined into a door at the factory to accept specific types of hardware. Preparations are made on both the door face and door edge, and their precise dimensions, locations, and patterns vary by hardware type. The term "prep" refers to the complete set of machining operations required for a given hardware category—for example, a cylindrical lock prep includes both the face bore and the edge bore positioned at a specific backset.

Door preparations are critical because commercial hardware is not interchangeable across prep types. A door prepped for a cylindrical lock cannot accept a mortise lock without additional machining, and a door prepped for surface-mounted exit devices will have different reinforcement locations than one prepped for concealed vertical rod devices. Ordering the correct prep at the time of purchase ensures clean installation and avoids costly field rework.

What Is a Door Preparation?

A door preparation (commonly called a "door prep") is a factory-machined cutout in a commercial steel or wood door that allows specific hardware to be installed. Every commercial door—except unprepped wood slab doors—ships with one or more preparations that determine which locks, exit devices, hinges, closers, and other hardware can be mounted to it. Selecting the correct door prep at the time of order eliminates field modifications, reduces installation time, and ensures the hardware fits and functions as designed.

CDF Distributors fabricates commercial door assemblies with factory-applied preparations at its Nashville, Tennessee facility. You can configure doors with the correct preparations using CDF’s ProBuilder tool at cdfdistributors.com. For assistance selecting the right prep for your hardware, call (855) 769-9895 or email sales@cdfdoors.com.