Physical Security – Perimeter Tips

Perimeter security

As for the exterior protection, perimeter security measures should be put in place. Perimeter protection is the first line of defense in providing physical security for a facility. This can be accomplished by installing fences or other physical barriers, outside lighting, lockable gates, intrusion detectors, or a guard force. Perimeter protection also includes walls, lockable doors and windows, bars and grills, and fire escapes.

In addition to defining the physical limits of a facility and controlling access, a perimeter barrier also creates a physical and psychological deterrent to unauthorized entry. It delays intrusion into an area, making the possibility of detection and apprehension more likely. It aids security forces in controlling access and assists in directing the flow of persons and vehicles through designated entrances.

Every vulnerable point should be protected to deter or prevent unauthorized access to the facility.  The roof, basement, and walls of a building may contain vulnerable points of potential entry. A security survey of the perimeter should address manholes and tunnels, gates leading to the basement, elevator shafts, ventilation openings, skylights.

Physical barriers

Physical barriers are normally two types, natural and structural.

  • Natural barriers: include mountains, cliffs, canyons, rivers, or other terrain difficult to traverse. If a natural barrier forms one side or any part of the perimeter, it in itself should not automatically be considered an adequate perimeter barrier, since it may be overcome by a determined intruder.
  • Structural barriers: are man-made devices such as fences, walls, floors, roofs, grills, bars, or other structures that deter penetration. Structural barriers should be provided for that portion of the perimeter, if required.


Fences are the most common perimeter barrier or control. Two types are normally used, chain link and barbed wire. The choice is dependent primarily upon the degree of permanence of the facility and local ordinances. A perimeter fence should be continuous, be kept free of plant growth, and be maintained in good condition.

  • Chain link: this fencing should be laid out in straight lines to permit unhampered observation. It should be taunt and securely fastened to rigid metal posts set in concrete. Anti-erosion measures like surface priming may be necessary. Where the fence traverses culverts, troughs, or other openings, the openings should be protected by fencing, iron grills, or other barriers to prevent passage of intruders. Chain link fencing is low in maintenance cost, a minimal safety hazard, and has openings small enough to discourage the passage of pilfered articles.
  • Barbed wire: standard barbed wire is twisted, double strand, number 12 gauge wire, with four-point barbs spaced four inches apart. Barbed wire fencing, including gates intended to prevent trespassing, should be no less than seven feet in height plus a top guard, tightly stretched, and should be firmly affixed to posts not more than six feet apart. Distances between strands should not exceed six inches.

Top guard

An overhang of barbed wire along the top of a fence, facing outward and upward at an angle of 45 degrees. Three or four strands of barbed wire spaced six inches apart are used, but the length of the supporting arms and the number of strands can be increased when required. The supporting arms should be affixed to the top of the fence posts and be of sufficient height to increase the overall height of the fence at least one foot. Where a building of less than three stories is used to form a part of the perimeter, a top guard should be used along the outside wall to deter access to the roof.


Provide a break in a perimeter fence or wall to allow entry. Gates are protected by locks, intermittent guard patrols, fixed guard posts, contact alarms, CCTV, or a combination of these. The number of gates and perimeter entrances should be limited to those absolutely necessary, but should be sufficient to accommodate the peak flow of pedestrian and vehicular traffic. Gates should be adequately lighted. They should be locked when not manned and periodically inspected by a roving guard force. Utility openings in a fence that do not serve as gates should be locked, guarded, or otherwise protected. Intrusion detection devices may be desirable when the gate is used intermittently or when a higher level of protection is desired. Alternatives to detection devices include coded card keys, push button combination locks, and CCTV.

Protective lighting

A valuable and inexpensive deterrent to crime. It improves visibility for checking badges and people at entrances, inspecting vehicles, preventing illegal entry, and detecting intruders both outside and inside buildings and grounds. Locate protective lighting where it will illuminate shadowed areas and be directed at probable routes of intrusion. Also, overlap lighting to prevent dark areas. If justified, include emergency power for lighting.


A door is a vulnerable point of the security of any building. A door should be installed so the hinges are on the inside to preclude removal of the screws or the use of chisels or cutting devices. Pins in exterior hinges should be welded, flanged, or otherwise secured, or hinge dowels should be used to preclude the door’s removal. The door should be metal or solid wood. Remember that locks, doors, doorframes, and accessory builder’s hardware are inseparable when evaluating barrier value. Do not put a sturdy lock on a weak door. The best door is of little value if there are exposed removable hinge pins, breakable vision panels, or other weaknesses that would allow entry. Transoms should be sealed permanently or locked from the inside with a sturdy sliding bolt lock or other similar device or equipped with bars or grills. Overhead roll doors not controlled or locked by electric power should be protected by slide bolts on the bottom bar. Chain link doors should be provided with an iron keeper and pin for securing the hand chain. The shaft on a crank operated door should be secured. A solid overhead, swinging, sliding, or accordion type garage door should be secured with a cylinder lock or padlock. Also, a metal slide bar, bolt, or crossbar should be provided on the inside. Metal accordion grate or grill-type doors should have a secured metal guide track at the top and bottom and be secured with a cylinder lock or padlock.


Windows are another vulnerable point for gaining illegal access to a building. Windows should be secured on the inside using a lock, locking bolt, slide bar, or crossbar with a padlock. The window frame must be securely fastened to the building so that it cannot be pried loose. As with glass panels in a door, window glass can be broken or cut so the intruder can reach inside and release the lock. Bars and steel grills can be used to protect a window. They should be at least one half inch in diameter, round, and spaced apart six inches on center. If a grill is used, the material should be number nine gauge two-inch square mesh. Outside hinges on a window should have non-removable pins. The hinge pins should be welded, flanged, or otherwise secured so they cannot be removed. Bars and grills must be securely fastened to the window frame so they cannot be pried loose.

Manholes, grates, and storm drains

Many facilities have manholes and tunnels providing service entrance into buildings. Other manholes may provide entrance to tunnels containing pipes for heat, gas, water, and telephone. If a tunnel penetrates the interior of a building, the manhole cover should be secured. A chain or padlock can be used to secure a manhole. Steel grates and doors flush with the ground level may provide convenient access. These openings may be designed into the facility as they may provide light and ventilation to the basement levels. If the frame is properly secured, the grates or doors can be welded into place or they can be secured with a chain and padlock. Sewers or storm drains that might provide an entrance should be secured.

Roof openings

Openings in elevators, penthouses, hatchways, or doors to the roof are often overlooked because of infrequent use. Access to a building’s roof can allow ingress to the building and access to air intakes and building Heating, Ventilating, and Air-Conditioning (HVAC) equipment (e.g., self-contained HVAC units, laboratory or bathroom exhausts) located on the roof. From a physical security perspective, roofs are like other entrances to the building and should be secured appropriately. Roofs with HVAC equipment should be treated like mechanical areas. Fencing or other barriers should restrict access from adjacent roofs. Access to roofs should be strictly controlled through keyed locks, keycards, or similar measures. Skylights are another source of entry from the roof. These openings can be protected like windows – with bars or mesh. Such protection should be installed inside the openings to make it more difficult to remove.

Mechanical areas

Prevent public sccess to mechanical areas. Mechanical areas may exist at one or more locations within a building. Some mechanical areas have access from the perimeter, other mechanical areas may only have access from the interior of a facility. These areas provide access to centralized mechanical systems (HVAC, elevator, water, etc.) including filters, air handling units, and exhaust systems. Such equipment is susceptible to tampering and may subsequently be used in a chemical, biological, or radiological attack. Keyed locks, keycards, or similar security measures should strictly control access to mechanical areas. Additional controls for access to keys, keycards, and key codes should be strictly maintained. Restrict access to building operation systems by outside maintenance personnel. To deter tampering by outside maintenance personnel, a building staff member should escort these individuals throughout their service visit and should visually inspect their work before final acceptance of the service. Alternatively, building owners and managers can ensure the reliability of pre-screened service personnel from a trusted contractor.

Building HVAC systems

Ventilation shafts, vents, or ducts, and openings in the building to accommodate ventilating fans or the air conditioning system can be used to introduce chemical, biological, and radiological (CBR) agents into a facility. Decisions concerning protective measures should be implemented based on the perceived risk associated with the facility and its tenants, engineering and architectural feasibility, and cost. Specific physical security measures to consider for the protection of the building HVAC system are cited below.

  • Prevent access to outdoor air intakes: one of the most important steps in protecting a building’s indoor environment is the security of the outdoor air intakes. Outdoor air enters the building through these intakes and is distributed throughout the building by the HVAC system. Introducing CBR agents into the outdoor air intakes allows a terrorist to use the HVAC system as a means of dispersing the agent throughout a building. Publicly accessible outdoor air intakes located at or below ground level are at most risk – due partly to their accessibility (which also makes visual or audible identification easier) and partly because most CBR agent releases near a building will be close to the ground and may remain there. Securing the outdoor air intakes is a critical line of defense in limiting an external CBR attack on a building.
  • Relocate outdoor air intake vents: relocating accessible air intakes to a publicly inaccessible location is preferable. Ideally, the intake should be located on a secure roof or high sidewall. The lowest edge of the outdoor air intakes should be placed at the highest feasible level above the ground or above any nearby accessible level (i.e. adjacent retaining walls, loading docks, and handrail). These measures are also beneficial in limiting the inadvertent introduction of other types of contaminants, such as landscaping chemicals, into the building.
  • Extend outdoor air intakes: If relocation of outdoor air intakes is not feasible, intake extensions can be constructed without creating adverse effects on HVAC performance. Depending upon budget, time, or the perceived threat, the intake extensions may be temporary or constructed in a permanent, architecturally compatible design. The goal is to minimize public accessibility. In general, this means the higher the extension, the better – as long as other design constraints (excessive pressure loss, dynamic and static loads on structure) are appropriately considered. An extension height of 12 feet(3.7 m) will place the intake out of reach of individuals without some assistance. Also, the entrance to the intake should be covered with a sloped metal mesh to reduce the threat of objects being tossed into the intake. A minimum slope of 45 degrees is generally adequate. Extension height should be increased where existing platforms or building features (i.e., loading docks, retaining walls) might provide access to the outdoor air intakes.
  • Establish a security zone around outdoor air intakes: Physically inaccessible outdoor air intakes are the preferred protection strategy. When outdoor air intakes are publicly accessible and relocation or physical extensions are not viable options, perimeter barriers that prevent public access to outdoor air intake areas may be an effective alternative. Iron fencing or similar see-through barriers that will not obscure visual detection of terrorist activities or a deposited CBR source are preferred. The restricted area should also include an open buffer zone between the public areas and the intake louvers. Thus, individuals attempting to enter these protective areas will be more conspicuous to security personnel and the public. Monitoring the buffer zone by physical security, CCTV, security lighting, or intrusion detection sensors will enhance this protective approach.
  • Secure return air grilles: similar to the outdoor-air intake, HVAC return-air grilles that are publicly accessible and not easily observed by security may be vulnerable to targeting for CBR contaminants. Public access facilities may be the most vulnerable to this type of CBR attack. A building-security assessment can help determine, which, if any, protective measures to employ to secure return-air grilles. Take caution that a selected measure does not adversely affect the performance of the building HVAC system. Some return-air grille protective measures include:
    1. relocating return-air grilles to inaccessible, yet observable locations.
    2. increasing security presence (human or CCTV) near vulnerable return-air grilles.
    3. directing public access away from return-air grilles.
    4. removing furniture and visual obstructions from areas near return-air grilles.

Implement security measures

Such as guards, alarms, and cameras to protect air intakes or other vulnerable areas. Difficult-to-reach out-door air intakes and mechanical rooms alone may not stop a sufficiently determined person. Security personnel, barriers that deter loitering, intrusion detection sensors, and observation cameras can further increase protection by quickly alerting personnel to security breaches near the outdoor air intakes or other vulnerable locations.

Restrict access to building information

Information on building operations – including mechanical, electrical, vertical transport, fire and life safety, security system plans and schematics, and emergency operations procedures – should be strictly controlled.

Fire escapes and building walls

Normally, outside fire escapes do not provide an entrance directly into the building. However, they can provide easy access to the roof or openings high above the ground level. Windows or other openings off the fire escape should be capable of being opened only from the inside. The exterior fire escape should not extend all the way to the ground. Walls are not normally considered possible points of entry because of their usual solid construction. However, they cannot be disregarded because intruders may be able to break through them to gain entrance. Reinforcement at critical points may be necessary to deter forced entry.

Facilities in remote locations

Large facilities located in sparsely inhabited areas have an inherent form of protection by virtue of their isolation. Constructing a fence around the perimeter usually will provide an adequate deterrent to entry. Occasional observation by a roving guard force may be necessary depending on the sensitivity of the facility. Warning signs or notices should be posed to deter trespassing on government property. CCTV systems also can be especially helpful if guard forces are available to monitor them.