For most doors, choose a single-cylinder deadbolt: key outside, thumb-turn inside, so anyone indoors can leave without hunting for a key. Consider a double-cylinder deadbolt, keyed on both sides, only when glass or a thin panel within arm's reach of the lock could be broken to reach the thumb-turn. The trade is exit speed: anyone inside then needs a key to get out.
What the choice actually hinges on
A deadbolt is a solid bolt that slides into the door frame and stays there until someone turns it back. A spring latch is different because it snaps shut on its own. Both deadbolt types have a key cylinder on the outside. The difference is the inside face. A single-cylinder deadbolt has a thumb-turn, a small knob you twist by hand. A double-cylinder deadbolt has a second keyhole.
So the decision rests on what surrounds the lock. Glass in the door, a sidelight (the narrow window beside some doors), or a thin panel close enough to break and reach through leaves the thumb-turn exposed. A solid door with nothing breakable near the lock does not have that problem.
Single-cylinder: quick to leave, exposed near glass
With a single-cylinder deadbolt, you use a key outside and the thumb-turn inside. Nobody indoors needs a key to leave. That matters in the dark, in a hurry, or when the person leaving is a child or a guest. There is also no interior key to misplace.
The weakness is that the thumb-turn works for any hand that reaches it. You can check your own door. Rest your hand on the thumb-turn and look at the nearest pane of glass. If an arm pushed through that pane could land where your hand is, someone outside can reach the thumb-turn.
Double-cylinder: a key on both sides, and what that costs inside
A double-cylinder deadbolt needs a key to move the bolt from either side. Breaking a nearby pane no longer opens the door, because there is no thumb-turn to grab. That is the real gain, and it only counts where glass or a weak panel sits near the lock.
The cost falls on the people inside. Anyone leaving through that door has to find the key first. In smoke or darkness, a key that is not where it belongs turns a quick exit into a search. Leaving the key in the inside cylinder fixes that, but it brings the glass problem right back.
Some doors need extra thought. If the door is the main way out of a home, a shared building, or a business where people inside do not carry keys, get a professional opinion on whether a keyed interior suits that particular door before you buy.
Questions to answer with your door in front of you
Each of these questions leans toward one type or the other.
- Is there glass or a thin panel within arm's reach of the lock? Yes leans double-cylinder. No leans single-cylinder.
- Is this the main exit, or the only way out of the unit? A door people must get through quickly leans single-cylinder.
- Who is inside, such as young children, older relatives, overnight guests, staff, or tenants? The more people who would need to find a key, the stronger the case for a thumb-turn.
- Is there one fixed spot by the door where an interior key would actually stay? If not, a double cylinder works against you.
- Is it a private home door, a shared building entry, or a storefront? Shared entries and storefronts have people inside who hold no key, and that raises exit questions worth a locksmith's view.
Sometimes the answers conflict, as when glass sits right beside the lock on the main exit. Then the door itself is usually the issue. Changing the glass or the door setup can remove the reason for a key on the inside, so weigh that before settling on a cylinder.
Getting a locksmith's opinion before you buy
A locksmith's opinion helps when your answers are mixed, when the door is a shared entry or a storefront, or when you are swapping one type for the other. Have a few details ready before you call: what the door is made of, whether there is glass in it or beside it, who uses it daily, and whether you are replacing a deadbolt or adding one where none exists.
Gramercy Locksmith & Security is a mobile locksmith working in Gramercy, Union Square, East Village, Stuyvesant Town, Kips Bay, and Murray Hill, and changes deadbolts on home doors, shared building entries, and storefronts. To talk through which type fits your door, call (914) 444-0959.
Frequently asked questions
Can I switch from a single-cylinder to a double-cylinder deadbolt later?
Switching from a single-cylinder to a double-cylinder deadbolt is usually a full deadbolt replacement, not an add-on part. Many deadbolts fit a similar door preparation, but older or unusual doors can need adjustment first. Have a locksmith confirm the fit before buying the new deadbolt.
Where should the interior key for a double-cylinder deadbolt be kept?
The interior key for a double-cylinder deadbolt belongs in one fixed spot beside the door, such as a hook, low enough for everyone in the household to reach and far enough from any glass that an arm reaching through cannot get it. Everyone who stays there, guests included, should know where it is. A key that drifts into a drawer or a coat pocket is slow to find in an emergency.
Does the cylinder type change how many keys I need to manage in a rental or shared property?
A double-cylinder deadbolt usually takes the same key on both sides, so it does not add a new key type. It does add copies, because anyone who locks that door from inside needs a key kept there too. In a rental or shared property, rekeying or a master key system helps keep those copies under control when occupants change.
Is a double-cylinder deadbolt appropriate for a storefront door?
A double-cylinder deadbolt is often considered for a glass storefront door, because a thumb-turn behind glass is easy to reach once the glass breaks. The tradeoff shows up during open hours, when staff and customers inside may need to leave without a key. A storefront door locked only after closing, with nobody inside, raises different questions than one locked while people are working. Have a locksmith look at how that door is used before choosing.