What Secondary Door Locks Actually Do
A primary deadbolt is your first line of defense. Secondary locks and viewers give you a second chance to screen visitors or slow forced entry.
Common add-ons include door chains, flip latches, security bars (sometimes called door jammer bars or floor braces), and peepholes or digital viewers. Each serves a different purpose: chains and latches let you crack the door open while maintaining a barrier; bars resist forced entry from kicks; viewers let you see who's outside without opening the door.
None of these replaces a quality deadbolt. Think of them as supplements that address specific situations—screening strangers, delaying an intruder, or confirming identity before you unlock the main lock.
Door Chain Locks: Limited but Useful
A door chain lock mounts to the door and frame, letting you open the door a few inches while the chain holds it. They're standard in many apartments and hotels because they let you talk to a delivery driver or verify credentials without fully unlocking.
The downside: most chains attach with short screws into hollow-core doors or thin frames. A hard shoulder push will rip the mounting plate off the wood. If serious forced-entry resistance is your goal, a chain won't deliver it.
Where chains do help: screening unknown visitors, accepting a package hand-off, or giving elderly residents a way to check identity before opening. Upgrade to a flip guard or swing bar if you want slightly better screw engagement, but understand the limits—these devices buy seconds, not minutes.
Security Bars and Floor Braces: Real Resistance
A door security bar wedges between the door and the floor (or mounts into a floor bracket) at an angle, using physics to transfer kick force into the ground. When installed correctly, bars can withstand hundreds of pounds of impact—far more than a chain.
Bars work on inward-swinging doors. They're popular for sliding glass doors and front entries where the door opens into the home. Some models telescope to fit different door heights; others include alarms that sound if the bar is jarred.
Limitations: you must be inside to set the bar, so it's purely a nighttime or stay-at-home solution. It won't help if you're away. And it does nothing for outward-swinging doors. Still, for apartment renters who can't modify the frame or anyone looking for portable, no-drill security, a quality bar is one of the better options.
Make sure the rubber foot grips your floor type—carpet, tile, and hardwood each need different traction. A bar that slips is useless.
Peephole Installation: See Before You Open
Peephole installation is straightforward: drill a centered hole at eye height (usually 58–60 inches), insert the viewer barrel, and thread the eyepiece on from the inside. Wide-angle viewers (180° or 200°) let you see someone standing off to the side.
For solid-core or metal doors, use a step bit to avoid splintering. Hollow-core doors need a backing plate or reinforcement tube so the viewer doesn't punch through. Most peepholes cost under $15 and install in under ten minutes.
Digital peepholes—small cameras with an interior screen—solve the height problem for kids, wheelchair users, or anyone who can't reach a standard viewer. Battery or hardwired models are available. Some integrate with smart-home hubs and record clips when motion is detected.
If you're renting, check your lease. Many landlords allow peephole installation; some even require it under local codes. A small hole is easy to patch if you move, and the security payoff is immediate.
Flip Guards, Swing Bars, and U-Bar Locks
Flip guards and swing bars are cousins of the door chain. Instead of a chain, a solid metal arm pivots into a catch, letting the door open a few inches. The arm typically mounts with longer screws than a chain, so it's marginally harder to kick through—but still not as strong as a reinforced deadbolt strike.
U-bar locks (also called London bars or Birmingham bars) slide horizontally across the inside of the door into a bracket on the frame. They offer better screw engagement than chains and work well on solid doors. Installation requires precise alignment, and they're bulkier, but they're a step up in actual strength.
These devices shine in high-traffic rental units where you frequently answer the door for deliveries or service calls. Pair any secondary latch with a peephole so you can screen before unlocking the deadbolt.
When to Upgrade Your Primary Hardware Instead
If your deadbolt is builder-grade with half-inch screws in a flimsy strike plate, adding a chain or bar is like putting a bike lock on a cardboard box. Reinforce the primary lock first.
Upgrade the strike plate to a heavy-duty box strike secured with 3-inch screws into the wall stud. Replace a brass deadbolt with a hardened-steel model rated ANSI/BHMA Grade 1. Check that your door is solid-core or metal-clad; hollow-core doors can be kicked through even if the lock holds.
Once your deadbolt and frame are solid, secondary devices add meaningful value. Before that, they're mostly theater. A professional locksmith can assess your door in minutes and recommend the most cost-effective upgrades. For more on our range of residential and commercial locksmith services, visit our services page.
Installation Tips and Common Mistakes
Mount any chain, latch, or swing bar as high as comfortably reachable—around 60 inches—so kids can't unlatch it from inside while you're distracted. Use the longest screws your hardware and door thickness allow, and drill pilot holes to prevent splitting.
For security bars, measure your door height accurately. A bar that's too short won't wedge firmly; too long and it won't fit. Test the angle and foot grip on your actual floor surface before trusting it overnight.
When installing a peephole, clean both sides of the hole and use a drop of thread-lock adhesive so vibration from door slams doesn't loosen the viewer over time. Mark your drill bit with tape to avoid punching through too far on hollow doors.
Common mistake: stacking multiple weak devices instead of fixing the root problem. Two chains don't equal one good deadbolt. If you're unsure what your door needs, reach out—our team can walk you through options or handle the install on the spot. Check out our blog for more security tips and how-tos.