Barriers against Break-Ins: Windows That can't Be Forced Open

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Most windows pose little deterrent to intruders, since inserting a knife blade between the sashes is generally all it takes to throw a latch. and unless the panes are made of special plastic, burglars can cut or break the glass and reach inside. To guard against such attacks, install one of the locks shown here on all windows that are accessible from outside.

For safety’s sake, lock any window that might become an emergency exit in a fire in such a way that it can be opened quickly from inside. For convenience, make sure one key will open all locked windows, and store the key where it's handy from inside but can't be fished for from outside.

Double-Hung Windows: One way to secure the common double-hung window—two wooden sashes that slide up and down in a frame—is to replace the standard thumb latch with a keyed unit. But two weak nesses make the keyed latch ineffective: First, like all locks anchored with common screws, it can be unscrewed. and even the remedy for this shortcoming—defacing the heads of the screws that come with the lock or fastening it with one-way screws (below) —will not keep an intruder from loosening the latch with a pry bar.

More resistant locks rely on a metal shaft that pierces both sashes and holds them tightly together. Mounted near the side of the window rather than in the middle, such locks are relatively pry resistant, and they can secure a window in two positions—closed or partly open for ventilation. A wedge-type lock, another strong barrier against break-ins, shares this feature. It comes with strike plates for both the open and shut positions.

Sliding or Casement Windows: More difficult to lock than double-hung windows, horizontally sliding glass windows are best secured by the methods used for sliding doors. Casement windows also present security challenges. If they open with a crank, either remove the crank handle from the shaft, putting it near the window but out of a burglar’s sight and reach, or replace the standard latch with one that must be locked with a key.


  • Electric drill with bits (3/16”, ¼”, 5/8”)
  • Conical grindstone bit
  • Wire cutters
  • Awl
  • Screwdriver


  • Common nails (3”)
  • Non-retractable sheet-metal screws

SAFETY TIPS: Wear goggles to protect your eyes when drilling.

Putting in screws for keeps.

A special head on non-retractable screws makes them impossible to remove without destroying screw or framing. Before tightening such screws, be certain that the lock is positioned correctly.

If non-retractable screws are not readily available, erase the slot in the head of conventional screws using a conical grindstone in an electric drill. Grind only along the sides of the slot; excessive grinding can weaken the screw, making it vulnerable to prying.

Locks for Double-Hung Windows

Locking a window with a nail.

• Drill a 3/16-inch hole through the top rail of the bottom sash and at least ½-inch into the bottom rail of the top sash. Angle the hole slightly down ward so that the nail can't fall out if the window is rattled.

• Trim the head from a 3-inch common nail with wire cutters so that the nail is just out of reach when slipped into the hole.

• Keep a magnet near the window to retrieve the nail and unlock the window.

Fitting a rod lock.

• Holding the body of the lock against the top rail of the bottom sash, locate a rod hole that misses the glass in both sashes. Mark holes for the rod and mounting screws with an awl.

• Drill holes for the mounting screws, then tape a 1/4-inch bit for a hole about 2 3/8 inches deep and drill the hole for the rod.

• Screw the lock body to the sash.

• To allow ventilation, open the bottom sash no more than 4 inches, insert the rod in the lock to mark the top sash for a second rod hole, and drill a 1/4-inch hole about ½-inch deep.

A lag bolt lock.

• Drill a 1/4-inch hole through the top rail of the bottom sash and about halfway into the bottom rail of the top sash. Position the hole about ½-inch from both the window frame and the top of the bottom sash to miss the glass.

• Enlarge the first inch of the hole with a 5/8-inch bit for the metal shield at the head of the lag bolt.

• Slip the shield onto the bolt and screw it in place with the wrench provided by the manufacturer.

A wedge lock.

• When the window is closed, place the lock on top of the lower sash and mark the line where the top of the lock meets the stile of the top sash.

• Position the strike plate on the top sash so that its top barely covers the line, and mark the locking and mounting holes.

• Using a 5/8-inch bit, drill 3/8-inch holes for the locking posts; drill smaller holes for the mounting screws provided by the manufacturer.

• Mount the second strike plate 4 inches above the first to allow for ventilation.

A Keyed Latch in a Casement Window

Installing a locking latch.

• Open the old latch and unscrew it from the window frame.

• Fit a keyed latch into the slot in the frame and fasten it with non-retractable sheet-metal screws.

Locking latches are made to fit most metal casement windows; when purchasing the latch, however, specify whether it's to fit on the left or the right side of the window.

Previous: Putting On a Better Lock

Next: Panes of Shatter-Resistant Plastic

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