Deadlock vs Deadlatch: Which Lock Does Your Door Need?

Deadlock and deadlatch sound almost the same, and they are often confused, even in hardware shops. They are different locks that do different jobs. Choosing the wrong one can leave a door less secure than you think, or make it easy to lock yourself out.

This guide explains how each lock works, where each belongs, the pros and cons of each, and why many homes in Surry Hills and the inner city end up with both on the front door.

The short answer

• A deadlatch locks automatically when the door closes. You need a key or a turn-handle to open it. It is about convenience.
• A deadlock only locks when you lock it with a key or snib. When it is locked, the bolt cannot be pushed back. It is about strength.

Most front doors benefit from both: the deadlatch for everyday convenience, and the deadlock for when you leave the house or go to bed.

How a deadlatch works

A deadlatch, sometimes called a nightlatch, has a spring-loaded bolt with a sloped face. When you close the door, the sloped face rides over the strike plate in the frame and the bolt springs out into the hole. The door is now latched, and you did not need to do anything.

A plain latch can be pushed back by sliding something thin between the door and the frame. A deadlatch adds a small extra plunger, called the deadlocking pin or trigger bolt. When the door is properly closed, the plunger is pressed in against the strike plate, and that locks the main bolt so it cannot be pushed back. That is the "dead" part of deadlatch.

Many deadlatches also have:

• A hold-back button, which keeps the bolt retracted so the door does not lock behind you. Useful when you are carrying things in and out.
• A key-locking inside handle, so the inside turn-handle cannot be used without a key. This stops someone breaking a glass panel and reaching through to open the door.

Strengths of a deadlatch

• It locks every time the door closes, so you never forget.
• It is quick to use, which suits busy households.
• It is often already fitted on older terrace front doors.

Weaknesses of a deadlatch

• The bolt is usually shorter than a deadlock bolt and less resistant to force.
• It only deadlocks properly if the door is fully closed and aligned. If the door has dropped, the plunger may not engage, and the latch can be slipped.
• It is the main cause of lockouts, because it locks behind you.

How a deadlock works

A deadlock has a solid, square-ended bolt with no spring. It only moves when you turn a key, or a snib on the inside. Once it is thrown, it cannot be pushed back by force on the bolt.

Deadlocks come in a few forms:

• Single-cylinder deadlocks have a key outside and a turn-snib inside.
• Double-cylinder deadlocks need a key on both sides.
• Mortice deadlocks sit inside the thickness of the door, which makes them very strong on solid timber doors.

Strengths of a deadlock

• A longer, stronger bolt that resists forcing.
• It cannot be slipped with a card or thin tool.
• A keyed inside cylinder stops someone reaching through a broken glass panel.

Weaknesses of a deadlock

• It only protects you if you remember to lock it.
• A double-cylinder deadlock can slow your exit in an emergency if the key is not nearby.
• Fitting one well into an older door takes care.

A word on safety and keyed inside cylinders

Double-cylinder deadlocks, which need a key on both sides, are excellent for stopping someone who breaks a glass panel from reaching in. They also carry a real risk: in a fire, you need to find the key to get out.

If you choose keyed inside cylinders, keep a key near the door, out of reach from any glass, and make sure everyone in the house knows where it is. Never deadlock yourself inside overnight with the key somewhere else. When we fit these locks, we talk this through with you, because getting out safely matters more than anything else.

Which one does your door need?

It depends on the door, how you use it and what you want to protect.

Terrace front doors

Many Surry Hills, Darlinghurst and Paddington terraces still rely on a single deadlatch on the front door. It works, but it is often the weakest point on the house. Adding a deadlock above or below the deadlatch is one of the most useful upgrades. You use the deadlatch day to day and throw the deadlock when you go out or go to bed.

On heritage doors, placement matters. A new lock needs to sit in solid timber and away from joints in the door. We agree placement with you before cutting anything.

Back doors onto laneways

Back doors are out of sight from the street, which makes them attractive to intruders. A deadlock is usually the better choice here, often with a strong strike plate fixed into solid frame timber. If you go in and out the back door a lot, a deadlatch plus a deadlock gives you both convenience and strength.

Apartment doors

Apartment lot doors are often fire-rated and closed by a closer. Many have a lever set that locks automatically. Any extra lock must suit a fire-rated door and must not stop it closing and latching. A deadlatch designed for apartment doors, sometimes combined with a deadlock, is common.

Security screen doors

Most security screens use a triple-lock or deadlock mechanism. The same principle applies: convenience versus strength, and making sure you can get out quickly.

Commercial doors

Shops and offices often use mortice deadlocks on solid doors, sometimes with a deadlatch function for staff. The right choice depends on how many people come and go and whether the door is a required exit. Our commercial locksmith page has more detail.

The door matters more than the lock

Whatever you choose, a lock is only as strong as what it is fixed into. The most common reason doors are forced is not the lock failing, but the frame splitting around the strike plate.

A good installation includes:

• A strike plate fixed with long screws into the solid timber of the frame, not just the thin trim.
• A door that hangs correctly, so the bolt meets the strike cleanly.
• A lock set at the right height and depth.

Our door lock installation page explains how we approach this.

Common mistakes

These are the mistakes we see most often.

1. Relying on a deadlatch alone on a vulnerable door. Fine for convenience, not enough for a laneway back door.
2. Fitting a deadlock and never using it. If you will not lock it, it does nothing.
3. Leaving the door misaligned. A deadlatch on a dropped door may never deadlock.
4. Short screws in the strike plate. The lock holds, the frame gives way.
5. Keyed inside cylinders with no key nearby. A safety risk in an emergency.

Looking after each type of lock

Both locks last far longer with a little care, and most of it takes seconds.

Deadlatches

• Check the plunger. Close the door gently and try pushing the bolt back with a fingernail through the gap. If it moves, the deadlocking plunger is not engaging, usually because the door has dropped or the strike has shifted.
• Do not leave the hold-back button on. It is easy to forget, and a held-back latch is no lock at all.
• Watch for slamming. A deadlatch that only locks when the door is slammed is telling you the door is out of line.

Deadlocks

• Throw the bolt fully. A half-turned deadlock bolt is much weaker than a fully extended one. If the key will not turn all the way, the bolt is hitting the strike, and the door needs adjusting.
• Use the right lubricant. A small amount of dry graphite or a lock-specific product once or twice a year keeps the cylinder smooth. General household oils collect dust and make things worse over time.
• Do not force a stiff key. Forcing it is how keys snap inside cylinders.

Upgrading without replacing the door

If your front door is solid but its locks are weak, you rarely need a new door. The usual upgrade path is:

1. Keep the existing deadlatch if it works, or replace it with a better one of the same size so no new holes are needed.
2. Add a deadlock in solid timber above or below it.
3. Fit a reinforced strike plate with long screws into the frame.
4. Key both locks alike so you still carry one key.

For a terrace front door, that combination gives you everyday convenience, real strength when you go out, and no extra key on your ring.

How much does it cost?

The cost depends on the lock quality, the door and how much work is needed to fit it. Adding a deadlock to an existing door costs more than swapping one deadlatch for another, because new holes have to be cut. If you are also rekeying, you can usually have the new lock keyed to the same key as the others. See our guide to locksmith costs for what affects the price.

The bottom line

A deadlatch locks automatically and is about convenience. A deadlock locks only when you lock it and is about strength. For most front doors, especially on terraces, having both is the best answer: the deadlatch for everyday use, the deadlock when you leave or go to bed.

If you would like advice on your own door, call Surry Hills Locksmith Service on 02 3820 5414, open 7 days, 7:00am to 12:00am, or request a quote and we will call you back.