Choosing a deadbolt for an older door

Start at the frame, not the lock

A deadbolt resists force by putting a steel bar across the gap between the door and the frame. Everything about how well that works depends on what the bar lands in. On a typical brick row house, the jamb is a piece of softwood nailed to the structure behind it, and the strike plate is often held by two screws three quarters of an inch long.

That is the weak point, and it is the least expensive thing on the door to fix. A box strike with three-inch screws driven through the jamb into the framing behind it changes the failure point entirely. It costs very little and takes minutes.

If you do nothing else on an older door, do that. A basic deadbolt with a properly anchored strike outperforms a top-tier lock screwed into loose trim, every time.

Grades, and what they actually promise

American hardware is graded by ANSI and BHMA on tested cycles and strength. Three levels, and they mean something specific:

On a house, the step from Grade 3 to Grade 2 is worth taking. The step from Grade 2 to Grade 1 is worth less than the same money spent on the strike and the frame, on a wooden opening. On a shop door that cycles hundreds of times a day, Grade 1 earns its price through durability rather than resistance to attack. That distinction is covered further on the commercial page.

Bolt throw

The bolt should project a full inch when thrown. Shorter bolts leave less material engaged in the frame and are easier to spread the door away from. Check the specification rather than assuming — not everything sold as a deadbolt manages it, and some do only when the door is perfectly aligned.

Related, and more common than people think: a bolt that is not fully throwing because the strike pocket is shallow or slightly out of line. It feels locked and it is barely engaged. This is worth checking on any door you have not looked at closely.

Single or double cylinder

A single-cylinder deadbolt takes a key outside and a thumbturn inside. A double-cylinder version needs a key on both sides, which is sometimes suggested for doors with glass beside them, on the theory that breaking the glass then gains nothing.

The trade is serious. A door that needs a key from the inside is a door you cannot leave through in the dark without finding one, and on any premises people work in it is generally not permissible at all. For a house, a single cylinder plus laminated glazing or a security film on the sidelight is the better answer. If you do fit a double cylinder anywhere, the key has to live within reach of the door and everyone in the household needs to know where.

The cylinder itself

Two features are worth understanding, and both address specific attacks rather than being general upgrades:

• Anti-drill. Hardened pins or plates that resist a drill bit aimed at the shear line.
• Anti-bump. Design features that make a bump key ineffective. Worth having, though bumping is far rarer in practice than the internet suggests.

A restricted keyway — where blanks are controlled so copies cannot be cut at a hardware counter — addresses a different problem again: keys spreading rather than locks being attacked.

Fitting into an older door

Three measurements decide whether a given lock fits, and all three go wrong on period doors:

• Door thickness. Standard is 1¾ inches. Older DC doors are sometimes thicker, which needs a specific model or an extension kit.
• Backset. Edge of the door to the center of the bore, usually 2⅜ or 2¾ inches. Period doors turn up other numbers.
• Bore size. Standard is 2⅛ inches. Smaller needs enlarging; larger needs a filler.

On a historic door there is a fourth consideration: whether the change is visible from the street, and therefore whether it needs review. That applies across Capitol Hill and Georgetown particularly.