Door Latches: Latch Bolts, Deadlatches, and How a Door Stays Shut

The Latch Bolt

ANSI/BHMA A156.2, the standard for bored and preassembled locks and latches, defines a latch bolt as a lock part with a beveled end. The bolt sticks out from the front of the lock when extended, but end pressure can push it back into the case, and the lock mechanism can also draw it back.

When the door closes, the latch bolt projects into a hole in the strike plate and holds the door shut. The LIST dictionary adds the key behavior: the bolt is spring-actuated and "when aligned with the strike, engages it automatically." The bevel is what makes this work. As the door swings shut, the angled face rides over the lip of the strike, the spring compresses, and the bolt pops into the strike opening.

The LIST dictionary defines throw as the distance a bolt or latch sticks out from the lock's faceplate or case when fully extended. Throw varies by product and application. For example, Schlage's ND Series catalog states that all ND Series latches have a 1/2-inch throw, except the anti-friction fire door latch, which has a 3/4-inch throw and is offered for pairs of doors.

Spring Latch vs. Deadlatch

The simplest latch is a spring latch: in the LIST dictionary's words, "a latch operated under spring pressure, having no deadlatching feature." Since end pressure on its beveled face can retract it, a spring latch holds a door closed but does little to keep it locked. The loiding article covers why that matters, at a conceptual level.

A deadlatch adds a second part. A156.2 defines the dead locking latch bolt, "also called dead latch," as a latch bolt with a plunger that, when pressed in, automatically locks the extended latch bolt so end pressure cannot push it back.

The plunger is a separate piece. A156.2 calls it the auxiliary dead latch: "a plunger which, when actuated, automatically locks a projected latch bolt against return by end pressure." The LIST dictionary uses guard bolt for a spring-loaded bolt that, when activated, deadlocks a latch bolt.

The plunger only works when it is held in by the strike. Dormakaba's installation bulletin for one of its locks says:

• the deadlocking plunger must touch the strike plate when the door is closed
• the plunger must not enter the strike opening
• a gap of more than 1/8 inch between the latchbolt and strike "may reduce security and/or cause improper latch operation"

If the plunger slips into the strike hole, it never depresses, and the latch behaves like a plain spring latch.

Product catalogs show the difference in the function list. Schlage's ND Series catalog, for example, lists some functions with a "springlatch" and many others with a "deadlatch." The BHMA function descriptions in A156.2-2022 likewise name a "dead locking latch bolt" for functions such as the F80 communicating lock. See lock functions.

Night Latches

The LIST dictionary defines a night latch as "a rim mounted auxiliary latch bolt." It is a latch mounted on the surface of the door, usually added as a second fastener rather than serving as the main lockset. Rim locks and rim cylinders are covered in rim and mortise cylinders.

Latches vs. Deadbolts

The LIST dictionary defines a deadbolt as a bolt that "requires a deliberate action to extend" and resists end pressure in the unlocking direction once fully extended. That is the basic contrast:

• A latch extends on its own when the door closes.
• A deadbolt has to be thrown with a key, a thumbturn, or a motor.

Many residential entry doors use both: a latching lockset for everyday closing and a separate deadbolt for locking. Commercial locksets may combine a latch and deadbolt in one body, as in many mortise locks.

For durability, BHMA grades the two product types under different standards. ANSI/BHMA A156.2-2022 sets the Grade 1 bored lock figure at 1,000,000 cycles. ANSI/BHMA A156.36 sets the Grade 1 deadbolt figure at 250,000 cycles. See cylindrical locks and ANSI/BHMA lock grades.

Latching on Fire Doors

On a fire-rated opening, the latch is part of the fire protection. As door-hardware consultant Lori Greene explains on iDigHardware, each fire door must have an active latchbolt so the door stays latched under the pressure a fire creates.

• Throw is set by the listing. Recent editions of NFPA 80 no longer state a required latch throw. The minimum throw comes from the door manufacturer's listings and appears on the fire door label. A typical minimum is between 1/2 inch and 3/4 inch, depending on the listing.
• Listed hardware. Parts used in a fire door assembly must be listed and labeled for that use.
• Pairs of doors. Positive latching extends to the inactive leaf of most fire-rated pairs. It can be provided by fire exit hardware, two-point latches, or automatic flush bolts.

At the same time, most doors in a means of egress must unlatch with one releasing motion. Exact requirements depend on the adopted code and the authority having jurisdiction (AHJ). This article does not replace either. See fire-rated door hardware and panic hardware.

Electrified Latching

A latch can also be controlled electrically. Options include:

• an electric strike, which releases the keeper in the frame while the latch stays put
• electric latch retraction, which pulls the latch bolt back inside the device
• an electrified lockset, which locks or frees the lever

On fire doors, iDigHardware notes that NFPA 80 allows devices that do not latch in normal mode, as long as the door becomes latched when the fire alarm activates.

When a Latch Misbehaves

Most latch complaints are about alignment. Dormakaba's bulletin tells installers to mortise the jamb so the strike box and strike plate fit properly, and to line up the strike and latchbolt centers accurately. A door that has to be slammed, or that bounces open, often points to the strike or the door's hang before it points to the lock. A latch that closes without engaging its deadlocking plunger is a quiet security problem, because the door looks locked. Strike adjustment is covered in strike plates and door hinges.

Related Topics

• Strike plates: the latch's other half
• Lock functions: springlatch and deadlatch functions
• Deadbolts: the deliberate-action bolt
• Cylindrical locks and mortise locks: common latch housings
• Loiding: why deadlatching exists (conceptual)
• Fire-rated door hardware: positive latching
• Panic hardware: latching exit devices
• Electric strikes and electric latch retraction
• Door hinges
• Rim locks

Sources