Why Frame Prep Is a Mortise-Specific Problem
This article covers what has to happen at the frame before a mortise lockset can be installed correctly -- a step that gets less attention than door-side prep but causes just as many callbacks. It is aimed at commercial subs coordinating with hollow metal frame suppliers, facility managers overseeing renovation work, and project managers who need to catch frame errors before the lock ships.
Mortise locks are case-installed in the door edge, but they depend entirely on a precisely prepared strike pocket and frame face to function. Get the frame wrong and no amount of careful door prep will save you.
What Is Frame Prep for a Mortise Lock?
Frame prep refers to the set of factory or field-applied cutouts, taps, reinforcements, and clearance provisions built into the hollow metal frame to accept the strike assembly and associated trim for a mortise lockset. Unlike a cylindrical lock, which uses a simple strike plate screwed to the face of the frame stop, a mortise strike requires a precisely located pocket -- sometimes called a strike box or strike cutout -- recessed into the frame face with tapped screw holes to secure the strike faceplate.
Because hollow metal frames are typically ordered before the hardware schedule is finalized, mismatches between the frame prep and the specified lock are one of the most common -- and most avoidable -- field problems in commercial door hardware.
The Four Things the Frame Has to Get Right
1. Strike Pocket Location and Depth
The strike pocket centerline must align with the lock centerline on the door. That centerline is driven by the lock backset -- typically 2-3/4 inches for standard commercial mortise locks, though some applications use other dimensions. The frame strike is located at half the door thickness plus any clearance for silencers or gasketing from the face of the frame stop.
Template documents from manufacturers like Accentra (formerly Yale) make this explicit: Dimension A = 1/2 door thickness plus clearance for silencers or gasketing when used. Skipping that silencer clearance is a classic mistake -- the lock and strike end up misaligned and the strike requires field modification, which is time-consuming and can compromise the fire rating on a labeled opening.
2. Tapped Screw Holes -- Size and Pattern
Mortise strikes are secured with machine screws into tapped holes in the frame. The two most common thread sizes in commercial hollow metal work are 12-24 and 1/4-20, and they are not interchangeable. The correct tap size depends on the lock series and the strike model specified.
- Many standard ANSI A115.1-compatible strikes use 12-24 taps (2 or 4 places depending on strike size).
- Heavier-duty or wider strikes may call for 1/4-20 taps.
- Some frames are prepped with sex-nut (SNB) options for through-bolt attachment on wood-stile or composite door assemblies paired with hollow metal frames.
Always confirm the tap requirement from the lock manufacturer's template before the frame is ordered. Retapping in the field on an already-painted frame is messy and risks weakening the connection.
3. Plaster Guard Clearance
Between the back of the strike faceplate and the inner frame cavity sits the plaster guard -- a formed piece supplied by the frame manufacturer to prevent grout, plaster, or drywall compound from fouling the strike pocket. Frame templates for mortise hardware consistently show a minimum of 1 inch (25 mm) from the plaster guard to the door face, with a 3/32-inch tolerance on strike centerline location.
In masonry or grouted frame installations, this clearance is especially critical. Grouting a frame without a properly installed plaster guard can permanently block the strike pocket, requiring frame replacement in the worst cases.
4. Reinforcement at the Strike Pocket
Hollow metal frames typically include a reinforcement plate at the strike location -- a heavier gauge steel insert welded or formed into the frame profile. Standard cutout dimensions in the reinforcement must match the strike case depth. Most ANSI-standard mortise strikes require a minimum cutout of 1-1/4 inches (32 mm) in the reinforcement.
For high-traffic applications -- school corridors, hospital egress doors, industrial exit routes -- a heavier reinforcement spec is worth the upcharge at the time of frame order. Retrofit reinforcement after installation is rarely practical.
Where Frame Prep Goes Wrong: Common Field Scenarios
- Frame ordered to a generic prep, lock spec changed later. The hardware schedule gets revised after frames ship. The new mortise lock has a different strike profile and the tap pattern does not match. Field retapping or strike modification adds cost and delays.
- No silencer hole in the frame stop. Silencers maintain the required 1/16-inch air space between the door face and frame stop -- a gap that hardware is designed around. Missing silencer holes mean the door rests metal-on-metal against the stop, throwing off the lock-to-strike alignment.
- Fire-rated opening, wrong strike depth. On labeled fire door assemblies, the strike prep must match the listed hardware exactly. Modifications to strike pockets on fire-rated frames require careful review; field modifications that are not covered by the listing can create a compliance issue at inspection.
- Exit device mortise case, frame prepped for lockset only. Mortise exit devices -- used in schools, healthcare facilities, and high-occupancy commercial buildings -- use a longer case than a standard mortise lockset. The strike pocket dimensions, tap locations, and case depth requirements are different. Frame templates for mortise exit devices must be sourced separately.
Specifying Frame Prep Correctly on the Hardware Schedule
The hardware schedule should call out the required frame prep by referencing the lock manufacturer's template number. Manufacturers including Accentra, Corbin Russwin, Sargent, and Hager publish separate template documents for hollow metal door and frame prep, wood door and hollow metal frame combinations, and specialty applications such as lead-lined doors used in healthcare imaging suites.
Key information to confirm before frames are ordered:
- Lock backset (standard 2-3/4 inch or project-specific)
- Strike model and ANSI strike standard (A115.1 is most common for commercial mortise)
- Tap size and pattern (12-24 or 1/4-20; 2 or 4 places)
- SNB (sex nut bolt) option required or not
- Door thickness (minimum 1-3/4 inch for standard commercial mortise)
- Silencer provision in frame stop
- Plaster guard requirement for wall type
Putting It Together: Who Catches Frame Prep Errors
On a well-run project, the hardware distributor issues templates to the frame supplier as part of the submittal package. The general contractor or construction manager confirms that frame submittals reflect the correct prep before approving fabrication. When that loop does not close -- hardware schedule finalized late, templates not issued, frame shop working from a generic spec -- field problems follow.
If you are sourcing mortise locksets, exit devices with mortise cases, or electrified mortise hardware and are not sure which frame prep template applies, DoorwaysPlus can help you identify the right template and coordinate with your frame supplier before the order ships. Preferred lines including Accentra, Corbin Russwin, Sargent, and Hager all publish detailed frame prep documentation, and our team can pull the relevant template for your opening.
Getting the frame right the first time is always cheaper than fixing it after the wall is closed.