Rim Locks: Surface-Mounted Locks and Night Latches

How a Rim Lock Is Built

Wikipedia describes the traditional rim lock as "of a basic design using (usually) a single lever and a sliding bolt." Wards, fixed obstructions inside the case that a key must clear, "can be used for additional security." Wikipedia notes that such locks "are not used where high security is required." See warded locks and lever tumbler locks for those mechanisms.

Older rim locks could be large. Wikipedia says "most older locks were large, some as big as 40 by 25 centimetres".

Because the case is on the surface, a rim lock needs no deep pocket in the door. Wikipedia notes that some rim lock and latch sets "have a shallower backset than does a modern bored cylindrical lock or mortise lock, allowing their use on doors with narrow rails." See cylindrical locks.

Where Rim Locks Are Used

Britain and Ireland. Wikipedia calls the rim lock "the oldest type of lock used in the United Kingdom and Ireland". It adds that "most rimlocks used today on exterior doors in the British Isles are night latches."

United States. According to Wikipedia, "rim locks and rim latches are often used on wooden screen doors," and a rim lock "may also be seen on an antique pantry, pie safe, and other cabinets."

The Night Latch

A night latch is the modern descendant of the rim lock. Wikipedia defines it as "a lock that is fitted on the surface of a door; it is operated from the exterior side of the door by a key and from the interior … side of the door by a knob." Night latches "are usually rim locks." They are made in two versions:

• Deadlocking. The latch "automatically locks once shut."
• Non-deadlocking. The latch can be pushed back when the door is closed.

Wikipedia explains that "historically, such locks were intended for use at night-time, hence the name." It also describes a tradeoff. The keyless exit a night latch gives "is a valuable fire safety measure, but may be a security risk" if an intruder can break nearby glass and reach the inside knob.

Night latches have a long patent history. The term was in use in the United States by 1870, when Randolph S. Foster of Madison, New Jersey, was granted U.S. Patent 99,076 for an "Improvement in Night-Latches." Earlier still, the German Polytechnisches Journal reported an 1830 English patent to James Carpenter and John Young, locksmiths of Willenhall near Wolverhampton. Their door lock had double tumblers and a latch that could be lifted by a key from one side of the door and by a handle from the other. The report does not say whether the lock was rim-mounted, so this article does not call it a rim lock.

Rim Cylinders

The keyed cylinder used with surface-mounted locks is called a rim cylinder. Schlage's key systems glossary defines a rim cylinder as "a cylinder with a tailpiece to actuate the lock mechanism typically used with surface applied locks and attached to the lock or door with a mounting plate and machine screws." The same glossary defines the tailpiece as "an actuator attached to the rear of a cylinder, parallel to the plug, used on rim, key-in-knob or other special cylinders."

High-security cylinders are offered in this format too. Abloy, for example, describes its PROTEC² CY402T as a round ANSI cylinder "used with rim locks and mortice lock cases." For more on cylinder formats, see rim and mortise cylinders and high-security locks.

Standards: BS 3621 Nightlatches

In the UK, higher-security night latches are often sold against BS 3621. Manufacturers' product pages show what that looks like in practice:

• ERA. Its BS 3621:2007 nightlatch "automatically deadlocks on closing the door and the internal handle can be locked for increased security." Its datasheet says the bolt can be held back "with the use of the snib feature" and that the lock "meets the tough criteria laid down by insurance companies and the Police."
• Yale. Its BS1 nightlatch is "kitemarked to the insurance required BS3621:2007" and "complies with BS EN 12209 & BS EN 1303:2005." See EN 1303.

Those product pages cite the 2007 edition. BSI lists the current edition as BS 3621:2017+A1:2024, published 26 September 2024. Its scope covers lock assemblies that "can be locked from both sides with a key thereby only allowing key egress." Locks that always allow keyless exit from inside are covered by a different standard, BS 8621. Locks that offer keyless exit plus a secure no-exit mode fall under BS 10621. Whether a given door should use key-egress or keyless-egress hardware is a fire-safety question.

Related Topics

• Rim and mortise cylinders
• Mortise locks
• Latches
• Deadbolts
• Warded locks
• Lever tumbler locks
• Cylindrical locks
• EN 1303
• Abloy
• High-security locks

Sources