Security Guide
The request usually arrives the same way. There is glass next to the door, or a glass panel in it, and the caller has worked out that anyone willing to break that glass can reach through and turn the thumb turn. The obvious fix seems to be a deadbolt with a key on both sides, so there is nothing on the inside to turn. That lock exists, it is sold openly, and installing it on the wrong door in New York City is one of the more serious mistakes a well-intentioned homeowner can make.
This guide explains what a double-cylinder deadbolt is, the real problem it is trying to solve, why exit requirements make it a bad answer on most doors, and the hardware that solves the same glass problem without trapping anyone behind the door.
A standard residential deadbolt is single cylinder. There is a keyed cylinder on the outside and a thumb turn on the inside, so you need a key to get in and only your hand to get out. A double-cylinder deadbolt replaces that thumb turn with a second keyed cylinder. Both sides need a key. Nothing about the bolt itself changes; the difference is entirely in how the inside operates.
Mechanically it is a sound lock. Nobody argues it is weak. The problem is not strength, it is what happens when someone inside needs to leave in a hurry.
Double cylinders exist because of glazing. A door with a glass panel, a sidelight beside the frame, or a transom within arm's reach of the lock has a weakness no deadbolt can fix: the attacker does not have to defeat the lock at all. Breaking a pane and reaching in to turn the thumb turn takes seconds and requires no skill.
The same logic applies to a French door with glass running down to the handle, a storefront with a full-glass entry, and a brownstone vestibule door with original leaded panels. If you have one of these, the concern is real and worth addressing. Our guides to French door lock security in NYC and glass door lock repair cover the hardware side of those doors.
Building and fire codes work from a principle that is easy to state and hard to argue with: a door in a means of egress has to open from the inside without a key, a tool, or any special knowledge. Someone leaving a burning apartment at three in the morning, in smoke, possibly carrying a child, cannot be required to find a key first.
A NYC apartment entrance door is part of the way out of the building. So is the front door of a house, the rear door of a restaurant, and the back exit of a shop. On any of those, a lock that needs a key from the inside turns an exit into a trap. The failure is not theoretical. It is the specific scenario the rule was written to prevent.
It gets worse in a shared household. The person who installed the lock knows where the key hangs. A guest, a roommate's visitor, a child, a babysitter, or a firefighter searching the unit does not. A door that only opens for people who know the system is not an exit.
Whether a particular door is legally classed as a required exit is a determination for the building department or the fire official with jurisdiction, not for a homeowner and not for a locksmith. The practical rule to work from is simple: if a person could end up behind that door needing to get out, do not put a key on the inside.
New York City adds complications that do not exist in a suburban house.
Apartment entrance doors in multiple dwellings are normally self-closing fire-rated assemblies, and the rating covers the whole unit: door, frame, hinges, closer and hardware. Swapping the deadbolt for something that is not part of the listed assembly can compromise the rating independently of the egress issue. Our guide to fire escape and exit door lock requirements in NYC covers the same principle at building level.
There is also an enforcement dimension. Non-compliant hardware on an apartment door can surface in an inspection, a complaint, or a sale, and the correction falls on whoever installed it. If you rent, altering the entry door without permission is also a lease problem. The related issues are covered in HPD violations involving door locks.
And finally, the honest observation about apartment doors: most of them have no glass at all. If your door is a solid metal-clad slab in a corridor, the entire argument for a double cylinder evaporates. There is nothing to break and nothing to reach through.
Business owners ask about this more often than homeowners, usually about a rear door or a stockroom exit, and the rule is applied more strictly rather than less. Any door serving as a required exit from an occupied space must open from the inside without a key. Chains, padlocks and keyed interior cylinders on rear exits are a recurring source of violations and, periodically, of tragedy.
Where a key on both sides is sometimes acceptable is on a door that is not an exit from an occupied area at all, such as an interior storage closet or a mechanical room that nobody works in. Even then, confirm it rather than assume. Exterior roll-down gates are governed separately again, and are covered in storefront security gate lock repair. If your concern is overnight security for a glass-fronted shop, overnight storefront security is the better starting point.
Security film applied to the inside face of the glazing holds the pane together when it is struck. The glass still breaks, but it stays in the frame as a sheet instead of falling away, so reaching through quietly stops being an option. Laminated glazing achieves the same thing as part of the glass itself and is the stronger version of the idea. On a fire-rated or landmarked door, confirm what the assembly allows before changing any glazing.
Where the panel is decorative rather than useful, replacing it with a solid panel removes the problem permanently. That is often the right call on a rear or side door nobody looks out of.
Distance matters. If the lock can be relocated so the thumb turn is more than an arm's length from any glass, the attack no longer works. On some doors this is as simple as the position of a secondary lock. It is worth asking about before accepting a key on both sides as the only option.
A reinforced frame and a properly engaged bolt matter more on most NYC doors than the thumb turn question ever will, because the common forced entry is a kick at the strike side rather than a reach through glass. See strike plates and door frame reinforcement, and for secondary hardware, jimmy-proof and police locks.
Hardware exists with a thumb turn that can be pulled off with a key, the idea being that you leave it in place while you are home and remove it when the property is empty. It is offered as a compromise and it is better than a fixed double cylinder, but it carries the same hazard the moment the turn is out while anyone is inside, and acceptance is still up to the building and the code official.
If you are considering one, ask before you buy, and be clear about which door it is going on. The answer for an interior storage room is not the answer for the door you leave through.
Describe the door rather than the lock you think you want. Say whether there is glass and how close it is to the lock, whether the door is an apartment entrance in a multiple dwelling, whether it is labeled as fire rated, whether it is a required exit from a business, and whether you rent or own. Those five answers determine the right hardware faster than any catalog.
A locksmith who agrees to put a key on both sides of your apartment entrance door without asking any of that is not doing you a favor. For how to tell a careful outfit from a careless one, see how to choose a locksmith in NYC and how to verify a locksmith license.
residential locksmith service commercial locksmith service deadbolt installation in NYC best lock type for a NYC apartment contact us
Need a licensed NYC locksmith? Emergency Locksmith NY is licensed and insured, NYC DCA License #1362785-DCA, serving all five boroughs 24/7. Call (718) 412-1332.
Treat it as off limits on any door you would need to get out through. Building and fire codes require a door in a means of egress to open from the inside without a key, a tool or special knowledge, and an apartment entrance door is part of the way out of the building. Whether a specific door qualifies is a determination for the code official, not the homeowner.
Glass. If a door has a panel or a sidelight within reach of the lock, breaking it lets someone turn the thumb turn by hand. A double cylinder removes the thumb turn so there is nothing to reach for, which solves that one problem and creates a worse one.
Address the glass rather than the exit. Security film or laminated glazing keeps a broken pane in its frame so nobody can reach through, and replacing a decorative panel with a solid one removes the weakness entirely. Keep the inside of the door openable by hand.
That is the usual argument and it does not hold up. The lock stays installed, anyone else in the unit may not know where the key is kept, and the door becomes a hazard the moment a person is behind it. The risk is not limited to the hours you planned for.
Those exist and are sometimes offered as a middle ground, but the hazard returns the moment the turn is removed while anyone is inside, and whether it is accepted is up to the building and the code official. Ask before buying one, and be specific about which door it is for.
Yes, and it is enforced more strictly. Any door serving as a required exit from an occupied space has to open from the inside without a key. Rear exits, stockroom doors and back-of-house entrances are a frequent source of violations.