The Priority Order: Not All Entry Points Are Equal
Most break-ins happen through a small number of entry points. FBI crime data consistently shows that 34% of burglars enter through the front door, 22% through a first-floor window, and 9% through the back door. The rear door, garage entry door, and basement windows account for most of the rest.
This hierarchy should drive sensor placement decisions — not the desire to cover every possible entry point first. A well-designed system covers the high-probability entries comprehensively before addressing secondary risks.
Doors: Every Exterior Door Gets a Sensor
There is no good reason to leave any exterior door unprotected. Door sensors are inexpensive, easy to install, and have near-zero false alarm rate. The sensor body mounts on the door frame; the magnet mounts on the door itself. When the door opens, the gap between body and magnet breaks the reed switch circuit and triggers the zone.
Front Door
The front door should be on a zone with an entry delay (30–45 seconds) to allow you to disarm when arriving home. The sensor goes on the hinge side of the frame, approximately 6 inches from the top — this position is least affected by door warping and maximizes the gap distance on open.
Rear and Side Doors
Back and side doors should be configured as instant zones with no entry delay. There is no legitimate reason to enter through the rear door when the alarm is armed — so any opening should trigger immediately. This is one of the most commonly missed configurations in DIY installations.
Garage Entry Door
The door between the garage and living space is a frequent blind spot. Most garages can be entered via overhead door attacks, and the interior door is the last barrier. Treat this door as a high-priority zone — instant trigger, no entry delay.
Sliding glass doors need a recessed floor or frame sensor rather than a standard surface-mount contact. The sensor should trigger on both side-slide and lift-off attacks — some sliders can be lifted off their tracks without triggering a standard contact.
Windows: Strategic Coverage, Not Complete Coverage
A typical home has 15–20 windows. Putting individual sensors on every window is possible but often unnecessary when combined with motion detection inside. The strategic approach: sensor every accessible window, use motion detectors to catch anything you miss.
Which Windows to Sensor
- All ground floor windows — every window a person can reach from grade without a ladder
- Basement windows — these are frequently targeted for access to attached garages and utility areas; also prone to being left unsecured
- Second-floor windows accessible from a roof, porch, or tree — survey your exterior and identify which second-floor windows an intruder could reach with minimal effort
- Windows near or above attached garage roofs — these are consistently the highest-risk second-floor entry points
Window Sensor Placement
For double-hung windows (the most common type in St. Louis homes), the sensor body goes on the lower window sash and the magnet goes on the upper sash immediately above it. This configuration detects the most common attack: lifting the lower sash. It does not detect the upper sash being lowered from outside — if this is a concern, add a second sensor or use a recessed contact.
For casement windows (crank-style), the sensor goes at the locking stile — the vertical edge opposite the hinge. When the window cranks open, this edge swings away from the frame and separates the sensor pair.
Glass Break Detectors: A Complement, Not a Replacement
Glass break detectors use acoustic sensors to detect the specific sound frequency signature of breaking glass. They can protect multiple windows in a room from a single device — one detector in a living room with a picture window and four adjacent windows saves four individual sensors.
Important limitations: glass break detectors have a maximum effective range (typically 15–20 feet from the glass) and must have line-of-sight to the glass they're protecting. They detect breaking — not opening. A window that is carefully cut and removed will not trigger a glass break detector but will trigger a contact sensor. The most comprehensive approach combines both.
Motion detectors are interior protection — they only trigger after someone has already entered. A perimeter of door and window sensors provides earlier detection, before an intruder is inside your home.
The Complete Perimeter Checklist
- Front door — entry delay zone
- Back door — instant zone
- Garage service door — instant zone
- All sliding doors — recessed sensors, instant zone
- All ground floor windows
- All basement windows
- Second-floor windows accessible from garage roof, porch roof, or adjacent tree
- Glass break detectors in rooms with multiple windows where individual sensors aren't cost-effective
Interior motion detectors then form a secondary layer — catching anything that gets through the perimeter without triggering a sensor. This layered approach is how professional installations are designed: perimeter first, interior second.