Specifying Door Locks

Mechanical door locks regularly remain even after electronic access control is added. Indeed, most are designed to work with what is already hung on the door. However, what happens when a lock needs to be replaced or changed? Understanding the basics of selecting and installing door locks is valuable for every designer, installer, or end user to know. In this note, we take a look at the basic types of locking hardware, which types of openings use them, and provide a general overview of how to install them. 

Major Types

The range of lock hardware is broad, with each type having its own 'best use' and relative strengths. The major types used in commercial buildings are shown below:

In the sections below, we discuss where each type is used and how to make the best choice depending on the application.

Door Preps Largely Decide Lock Choice

The most important aspect driving door lock selection is how the 'door is prepped', or how it has been fabricated to work with locks. Different forms of locks require different configurations of holes and pockets cut into the door, and in most cases these preps are done at the manufacturer well before they are hung.

Therefore, in many cases, lock selection is decided by the door type, and the task is condensed to finding which product can be installed without modifying or replacing the door. In the sections below, we address the major types of 'door preps' and which models of hardware they accept.

Cylindrical

This type of lock is also called a "Bored" lock, which essentially is designed to slip inside a 2 1/8" hole drilled thru the door. The locks designed to use this prep are round in shape, and typically use the hole to support the lock in the door. While the majority of doors include this prep, it is not the most common seen in Electronic Access Control, because most of the time these locks are used for interior, or low-security doors.

While these types of locks are well suited for light-duty use, they contain only one latch - the piece that slides into the frame. High security doors often include several points of latching and even when a cylindrical lock is built to withstand many cycles, it still need other seperate components (like a deadbolt) for high security applications. 

• Pros: Inexpensive ($50 - $300), easy to install
• Cons: Single latch not as secure as other types, not as durable as mortise locks
• Where Used: Interior Doors, Offices, Passageways, Low-Medium Volume Doors

Mortise

One of the oldest types of locks is also the most secure. Compared to a cylindrical lock, a mortise lock is big, heavy, and full of complex parts. However those properties make it very durable, strong, and able to withstand constant use. Mortise locks require a pocket cut into the edge of the door, which requires more craft skill that a single bored hole. However, because that pocket is larger than a cylindrical lock, multiple latches are typical features of mortise hardware.

Not only do multiple bolts slide into the frame, but mortise locks support full-size 'high security' mortise lock cylinders featuring 'bump/pick resistance', special security pinning, and other tampering protections. 

Mortise locks are commonly used in doors requiring high security and high volumes, but are generally too expensive to use on interior doors or light-duty office/ passageway openings. Doors using mortise hardware must be specified to handle both the size and weight of a mortise lock:

• Pros: Very durable, support multiple security latches
• Cons: Expensive ($400 - $2000), field cutting a door to support a mortise lock is difficult
• Where Used: Exterior Doors, High Volume Doors, High Security Doors

Surface

Also called 'Rim style, or Edge-prepped Locks', these locks typically require minimal door prep, and some types do not even occupy the core of a door at all. The most common type of hardware in this category are exit devices, a mainstay of high-volume, emergency egress openings. In the picture below, notice the latch of the lock is attached to the 'surface' of the door, hence the name:

Surface door hardware is typically secured with surface strikes (not mortise strikes) or maglocks where permitted. Exit devices are costly but are very durable and typically withstand high amounts of abuse and tampering.

• Pros: Meets Life/Safety Emergency Egress Codes, Most doors, regardless of factory prep, support Surface Hardware installation
• Cons: Expensive ($1000 - $3000), and potentially disruptive to aesthetics. Difficult to hang on glass doors.
• Where Used: Egress Doors, High Security Doors

Deadbolts

This type of lock is seldom used alone without additional separate handles, and NEVER on emergency egress doors [link no longer available] because the bolt typically requires rotation of a key or thumbturn to retract. Like cylindrical locks, deadbolts are easy to install, requiring only a hole to be drilled through the door. However, because of their limited convenience, deadbolts are primarily used to enhance the security of other locks hung on the door.

For example, when used with a cylindrical lock, a deadbolt means another independent lock must be defeated to gain illicit access. Deadbolts are typically used to increase security during 'dark hours' - used when a facility is locked up for the night or when it is unoccupied. The image below shows a typical example of how deadbolts are used, in conjunction with a leverset on a perimeter door:

• Pros: Inexpensive ($50 - $200), easy to install
• Cons: Cannot be installed on egress doors, Seperate lock must be pinned to match other locks in use
• Where Used: Generally used to increase security by provide another latching point.

Deadlatch

This type of hardware is a hybrid between a deadbolt and a mortise lockset. Deadlatches are commonly used on glass storefront doors with thin frames. Most properties using latchbolt equipped doors are unlocked during occupied hours [link no longer available], and free egress and entrance is permitted. Therefore, a latchbolt is only used to keep a door locked when occupants are gone. Like a deadbolt, most models lack handles to retract the latch, although some types feature 'exit device'-like paddles when required by code.  Unlike a deadbolt, a door frame cannot simply have a bored hole for install, and the frame must be prepped similarly to a mortise style lock. Most latchbolts are very strong, and may include multiple latches or even hook bolts that anchor firmly into the adjoining frame.

• Pros: Stronger than traditional deadbolts, the ideal architectural/security compromise for glass doors
• Cons: Handles must be installed separately, retraction functions from key only. Doors difficult to field prep to fit latchbolts.
• Where Used: Thin Frame (Glass) Storefronts

Lock Selection

Doors clearly drive the types of locks that can be used to secure them. When it comes to selecting the specific type of lock to install, these factors:

• How is the Door Prepped? The section above clearly defines how door prep influences lock hardware selection. Taking note of the prep will narrow selection criteria to a few basic types.
• How thick is the door? Doors have varying thicknesses. In the Americas, doors usually are 1.75" or 1.375". However, European models range between 30mm and 55mm thick. This measurement is critical in determining the latch position in the door, and can limit the overall thickness of the lock.
• Is this an Egress door? If the door falls in an egress or emergency egress path, certain lock types (like deadbolts) should not be used. Hardware like exit devices maybe be required, excluding selection of other types.
• How do codes affect lock selection? Many municipalities outlaw maglocks, meaning that 'electrified hardware or strikes must be used. Local code interpretations often exclude types of locks, or otherwise conditionally approve their use depending on building classification.
• How frequently will this door be used? How often the door and lock is cycled influences hardware grading. For heavy duty commercial use, ANSI/BHMI Grade 1 hardware is ideal, while an infrequently used closet or storage door is better suited to use economy-grade Grade 3 hardware.

Choosing the right lock is typically driven by the 'context' attributes of the opening, rather than selecting the lock first and sizing the opening to fit.