How Each Type Works

Single cylinder deadbolt. The exterior face accepts a key. The interior has a thumb turn — a small knob you rotate by hand to extend or retract the bolt. You lock and unlock the door from inside without needing a key, which means anyone inside can exit immediately.

Double cylinder deadbolt. Both the exterior and interior faces accept a key. There is no thumb turn. Locking and unlocking from either side requires the key. The interior side can't be operated by hand — only by the correct key.

The mechanical difference between them is entirely in the interior component. The cylinder, the bolt mechanism, and the lock body are otherwise identical. What changes is whether you've replaced the thumb turn with a second keyed cylinder.

The reason comes down to glass.

Many Australian homes — particularly period properties, older brick homes, and those with decorative front entries — have glass panels adjacent to or within the door itself. Sidelights, transom windows, glass insets in the door, or standard single-hung windows positioned close to the door handle are all common configurations.

If a burglar breaks that glass, they can reach through and operate a thumb turn. With a single cylinder deadbolt, that takes seconds. With a double cylinder, they can't — there's no thumb turn to reach. They'd need to pass a key through the broken glass, which they won't have.



That's the legitimate security argument for a double cylinder, and in the right context it's a real one.

The Safety Concern You Can't Ignore

Fire safety is the counterargument, and it's serious enough that some Australian states and local councils have restrictions or guidance on double cylinder deadbolts in residential properties.

In a fire, speed of exit matters. A child who wakes to smoke and tries to get out the front door can operate a thumb turn immediately. They cannot operate a keyed cylinder without a key — and in a disorienting, dark, smoke-filled hallway, finding and using a key is not a reliable process.

Adults face the same problem. The key isn't always in the lock. It's on a hook, in a bag, in a pocket. In an emergency exit, that delay is measured in seconds that matter.



This concern applies to anyone in the home who might be alone during a fire or emergency. Children, elderly occupants, anyone with limited mobility or vision — a double cylinder creates a real exit barrier for all of them.

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The right way to approach this decision is to ask two separate questions and weigh the answers against each other.



First: Is there a glass vulnerability? Look at your front door and assess honestly whether there is glass within arm's reach of the interior thumb turn. Measure it if you're unsure — can someone reaching through a broken panel get to the handle? If the answer is yes and the door is in a location that presents real exposure, the security argument for a double cylinder has merit.

Second: Who lives in the home and what does an emergency exit look like? If children, elderly relatives, or anyone who might be alone in an emergency live there, factor the exit delay into the decision. A double cylinder on a front door is a barrier that exists for everyone — including the people you most need to be able to get out quickly.

If the glass vulnerability is real and the occupants are adults who are physically capable and could reliably locate and use a key in an emergency, a double cylinder with a key kept in the interior cylinder or on a nearby hook within reach (but not reach-through-glass reach) is a workable arrangement. If children or vulnerable occupants are in the picture, the safety risk of a double cylinder outweighs the security benefit in most situations.

Alternatives That Address the Glass Problem Without the Safety Risk

If you have a glass panel adjacent to your front door and want to address the vulnerability without installing a double cylinder, there are practical alternatives.

Laminated or security glass. Replacing standard glass panels with laminated glass makes them significantly harder to break quickly. A standard pane breaks with one strike. Laminated glass takes sustained effort and time — enough to deter an opportunistic entry attempt.

Security film. Applied to existing glass, security film holds shattered glass in place and dramatically increases the force and time required to create a usable opening. It's less effective than laminated glass but substantially better than untreated standard glass and considerably cheaper.

Relocating the lock. On some door configurations, a locksmith can install the deadbolt at a height or position on the door that places the thumb turn outside the reach zone of any adjacent glass. This depends on the specific door and frame setup but is worth exploring before defaulting to a double cylinder.

Neither type is universally better. A single cylinder deadbolt on a solid door with no adjacent glass is a straightforward and appropriate choice for most Australian homes. A double cylinder may be warranted on a door with a specific glass vulnerability, provided the occupants and the key arrangement make emergency exit workable.



True Locksmith assesses door configurations and advises on appropriate lock choices across Brighton, Sandringham, Hampton, Cheltenham, Moorabbin, Bentleigh, Beaumaris, and Melbourne's Bayside and southeast suburbs. If you're not sure which type suits your front door, call 0421-767-767 and we'll look at the specific setup and give you an honest recommendation.