A mortice lock is a lock that fits inside a rectangular pocket cut into the edge of a door, rather than being mounted on the surface. The lock body sits hidden within the door itself, with only the keyhole, handle, and a thin faceplate visible. This design makes mortice locks harder to tamper with than surface-mounted alternatives and gives the door a cleaner appearance. They’re the standard lock type on most wooden front and back doors in the UK.
How a Mortice Lock Works
The name comes from the “mortice” (or “mortise”), which is simply the rectangular pocket chiseled into the door’s edge to house the lock body. That lock body is the core of the whole mechanism. It contains either a deadbolt, a spring latch, or both, depending on the type of lock.
When you turn the key, it lifts a set of internal levers to a precise height. Only when every lever is lifted to exactly the right position can the deadbolt slide across into a metal plate (called the strike plate) fixed to the door frame. This is what makes the door secure. The more levers inside, the more possible key combinations exist, and the harder the lock is to pick.
Other visible parts include the faceplate, which is the narrow metal strip you see on the door’s edge, and the spindle, a metal bar that passes through the door and connects the handles on each side.
Sash Locks vs. Deadlocks
Mortice locks come in two main types, and the difference is straightforward.
A mortice sashlock combines two things in one case: a spring-loaded latch and a deadbolt. The latch is the part that clicks the door shut when you push it closed, operated by the door handle. The deadbolt is the heavier, more secure bolt you throw with the key. Most front doors use a sashlock because it handles both everyday use (opening and closing with the handle) and security (locking with the key).
A mortice deadlock is simpler. It has only a deadbolt and no latch or handle. You operate it with a key alone, sometimes with a thumb turn on the inside. Deadlocks are typically added as a second lock on a door for extra security, or used on doors that don’t need a handle, like a side gate or shed door.
Lever Count and Security Ratings
The number of levers inside a mortice lock directly affects how secure it is. The two most common options are 3-lever and 5-lever locks.
A 3-lever mortice lock offers basic security and is generally suitable for internal doors within a home, such as bedroom or bathroom doors. It has fewer possible key combinations, so it’s easier to pick and less resistant to forced entry.
A 5-lever mortice lock is the standard for external doors. Insurance companies in the UK typically require a 5-lever lock that meets British Standard BS3621 on any exterior door. Locks meeting this standard include features like anti-pick lever mechanisms, hardened steel bolts that resist sawing, and a box-shaped strike plate that reinforces the frame. If your home insurance policy specifies lock requirements, this is almost certainly what they mean.
Why Mortice Locks Are More Secure Than Surface Locks
The main security advantage is in how a mortice lock is installed. Because the entire mechanism is buried inside the door, there are no exposed screws or visible components for someone to target. Only the keyhole is accessible from the outside. A surface-mounted rim lock, by contrast, sits on the inside face of the door and is fixed with screws that can potentially be forced or pried.
A properly fitted mortice lock in a solid wooden door, paired with a robust strike plate screwed deep into the frame, creates a strong connection between door and frame. High-quality 5-lever models add anti-saw bolts and anti-pick features that make forced entry significantly more difficult. This is why mortice locks remain the preferred choice for external door security on traditional wooden doors, even as other lock technologies have evolved.
How to Measure for a Replacement
If you need to replace a mortice lock, getting the right measurements is critical. A lock that’s even slightly wrong won’t fit the existing pocket in your door. You’ll need to measure several things:
- Case depth: the distance from the front edge of the lock (where the faceplate sits) straight back to the rear of the lock body.
- Backset: the distance from the faceplate to the center of the keyhole or spindle hole. This determines where your handle and keyhole sit on the door’s face.
- Body height: the full height of the lock case from top to bottom.
- Faceplate width: the width of the metal strip visible on the door’s edge.
For a sashlock, you’ll also need the distance between the center of the spindle hole and the center of the keyhole. This is sometimes called the “positioning zone” or PZ measurement. If you’re replacing a deadlock, you can skip this since there’s no spindle hole. Taking the old lock out of the door and measuring it directly is the easiest approach. Most hardware stores and locksmiths can match a replacement from these numbers.
Installation Basics
Installing a mortice lock from scratch requires cutting a precise rectangular pocket into the edge of the door. The process starts by marking out the lock body’s dimensions on the door edge using the lock itself or a template as a guide. A series of holes are then drilled into the edge to remove most of the wood, and a chisel is used to clean up the pocket to the exact depth specified by the lock manufacturer.
Once the pocket fits the lock body snugly, holes are drilled through the face of the door for the keyhole and, on a sashlock, the spindle. The lock body slides into the pocket, the faceplate is screwed flush with the door edge, and the strike plate is fitted into a matching recess in the frame. Handles, escutcheon plates (the decorative surrounds for the keyhole), and any trim are added last.
This is a job that rewards precision. A pocket cut too wide will leave the lock loose, while one cut too deep can weaken the door. For solid hardwood doors, many people prefer to have a locksmith handle the installation, especially on exterior doors where security is the priority. On softwood interior doors, it’s a manageable DIY project with a chisel, drill, and some patience.