Why Door Prep Is the Decision That Happens First -- and Hurts Last
This article is for commercial subcontractors, door and hardware suppliers, and facility project managers who specify or install exit devices on hollow metal doors. The core problem: exit device prep must be locked in before the door is fabricated, but device selection often lags behind. When those two timelines collide, the result is field modifications, voided fire labels, and delays. Understanding how each device type drives the cutout pattern -- and what the hollow metal door itself must provide -- prevents the most common (and most expensive) mistakes on the job.
What Is Exit Device Prep?
Exit device prep refers to the factory-cut holes, mortised pockets, rod channels, and internal reinforcing plates built into a hollow metal door to accept a specific category of panic hardware. Because hollow metal doors are fabricated from cold-rolled steel sheets formed and welded before they leave the plant, the cutout pattern must match the device type. Field drilling and cutting on a labeled fire door is tightly restricted -- and in many cases voids the label entirely.
The governing reference for hardware preparation in steel doors is ANSI/BHMA A156.115, which coordinates with ANSI/SDI A250.6 (recommended practice for hardware reinforcing on standard steel doors and frames). These standards define where reinforcing is required and what dimensions must be held.
The Four Device Types and What They Each Demand from the Door
Rim Exit Devices
The most common type. The lock case mounts on the surface of the door face at the lock stile; the latchbolt projects into a surface-applied strike on the frame face or mullion. Prep requirements are relatively straightforward:
- A cutout in the door face at the lock stile for the case mounting and cylinder
- A reinforcing plate behind the case mounting holes (critical on lighter-gauge doors -- do not assume 18-gauge is always adequate without checking the device manufacturer template)
- The frame strike location must match the NAAMM or SDI standard height -- 38 inches from the bottom of the frame to the centerline of the touch bar
Rim devices are non-handed in many configurations, which gives you some flexibility, but cylinder trim on the outside is handed -- confirm handing before the door ships.
Mortise Exit Devices
The lock case is mortised into the edge of the door stile, similar to a mortise lockset. This is the most secure mounting method and is common in schools, healthcare corridors, and high-traffic institutional openings.
- Requires a full-depth mortise pocket in the lock stile -- this must be factory cut; field mortising a labeled hollow metal door is not permissible under NFPA 80
- The pocket depth and width must match the specific device case exactly -- mortise exit devices are not interchangeable across manufacturers without verifying case dimensions
- Reinforcing steel is required around the mortise pocket -- standard SDI reinforcing may not be sufficient for heavy-duty devices; confirm with the door manufacturer
- The frame strike is a mortised strike in the jamb, not a surface-applied unit -- frame prep must be coordinated simultaneously
Mortise prep doors cost more and take longer to fabricate. If the hardware schedule changes from a rim to a mortise device after the door is ordered, you are typically looking at a new door.
Surface Vertical Rod (SVR) Devices
Rods run on the face of the door stile from the lock case to a top strike at the header and a bottom strike at the threshold or floor. Used on pairs of doors, particularly where a mullion is not present.
- Top and bottom strikes require frame prep -- the header must have a strike box; the floor or threshold must have a corresponding strike or floor socket
- The bottom rod travels close to the floor -- verify that threshold height does not impede rod travel; even a standard saddle threshold can conflict with a bottom latch rod if clearances are not coordinated
- SVR devices are generally not used on fire-rated pairs where an overlapping astragal is required on both leaves -- coordinate with the door manufacturer
- Case mounting requires reinforcing at the lock stile; door face cutouts for the rod guides must also be factory prepped
Concealed Vertical Rod (CVR) Devices
Rods run inside the door stile, concealed from view. Cleaner appearance, higher cost, and the most demanding prep of all four types.
- The door must have a hollow stile cavity large enough to accept the rod assembly -- not all door constructions accommodate CVR without specifying it at time of order
- Top and bottom latch case cutouts must be factory drilled per the manufacturer template -- do not attempt to locate these by eye in the field
- The door must be placed horizontal during installation so rods can be threaded through the stile cavity correctly; attempting installation on a hung door is a common field mistake
- A recessed bottom channel (used on some doors to allow field trimming) can interfere with the bottom latch case -- confirm bottom channel construction with the door manufacturer before specifying CVR
- CVR devices are handed -- this must be confirmed on the order, not assumed
Reinforcing: The Detail That Disappears on the Submittal
Every exit device type requires steel reinforcing plates behind the mounting holes. SDI and ANSI/SDI A250.6 define minimum reinforcing for standard hardware locations, but exit devices -- especially heavy-duty or electrified models -- may require reinforcing beyond the standard. When a door is built to standard SDI hardware locations with standard reinforcing, then the hardware schedule switches to a heavier device or an electrified device with a motor latch retraction, the reinforcing behind the case may be undersized. Verify reinforcing requirements from the device manufacturer's template against what the door specification calls out -- this is a shop drawing review catch, not a field fix.
Fire-Rated Openings: The Rules Tighten
On labeled fire door assemblies, NFPA 80 significantly restricts field modification. Key rules that affect exit device prep on hollow metal doors:
- Fire exit hardware must only be installed on doors labeled for use with fire exit hardware -- a door prepped for a lockset will not typically carry adequate reinforcing for a panic device case
- Field-cut holes in labeled hollow metal fire doors are limited; modifications beyond the label's allowance require re-labeling or a new door
- Dogging (holding the latch retracted with a hex key) is not permitted on fire-rated openings -- verify that the device specified does not include a mechanical dogging feature, or that the feature is disabled
- Vertical rod devices on 180-minute fire-rated pairs have specific astragal and sequencing requirements -- a coordinator is required when an astragal is present
Frame Coordination: The Other Half of the Prep Problem
Exit device prep is not only a door issue -- the frame must be ordered to match. Strike locations, strike box depths, soffit width, and stop height all interact with the device type:
- Soffit width: Frames with soffits narrower than 2 inches may require auxiliary strike components for SVR and rim devices -- flag this early
- Stop height: Standard stop height is 5/8 inch. Some SVR latch assemblies must be repositioned if the stop height differs from what the template assumes
- NAAMM vs. SDI hardware locations: Touch bar centerline at 38 inches (NAAMM) vs. strike centerline at 40-5/16 inches (SDI) -- these are not the same dimension; confirm which standard the door and frame are built to before positioning the strike
Before You Order: A Pre-Fabrication Checklist
- Device type confirmed: rim, mortise, SVR, or CVR
- Handing confirmed (CVR and most mortise devices are handed)
- Fire rating of the opening -- does the device need a fire exit hardware listing?
- Dogging feature compatible with occupancy and fire rating
- Reinforcing spec matches the device manufacturer template
- Bottom channel construction confirmed (relevant for CVR and bottom rod travel)
- Frame soffit width, stop height, and strike location coordinated with device type
- Threshold height confirmed -- no conflict with bottom latch rod on SVR or CVR
- Astragal and coordinator requirements reviewed for pairs
Preferred Device Lines Worth Specifying
When building a hardware schedule around hollow metal door prep, device lines with stable case dimensions and consistent template libraries simplify coordination across a multi-opening project. Sargent, Corbin Russwin, and Hager all offer rim, mortise, SVR, and CVR configurations with documented templates that align with standard NAAMM and SDI prep locations. PDQ and Accentra (formerly Yale) are also solid options for institutional and light commercial applications where long-term parts availability matters. DoorwaysPlus carries these lines and can help match the device to the prep before the door order is placed -- not after the doors arrive on the loading dock.
Questions about how to spec exit device prep for a specific opening? Contact the team at DoorwaysPlus.com -- we review hardware schedules, coordinate with door manufacturers, and stock preferred device lines ready to ship.