Security FAQ

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Revision as of 19:06, 18 May 2013 by NT (talk | contribs) (mostly m, move monkey tail bolts)
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By Nightjar

See also Door security, which covers a lot of similar material


It is impossible to stop the determined thief who wants to break into _your_ property. The only function of security is to make the average thief decide that it is easier to steal from someone else than from you. Therefore, beyond a certain minimum level, the degree of security you need depends on what your neighbours have.

The most important thing about security is to use it. 'Walk-in' thefts, where the thief gains entry through an unlocked door are probably the most common single type of theft.

Simple ideas, like leaving a radio on while out and having internal lights (NOT the hall light) on at night, or on a timer, are surprisingly effective deterrents.

It is also a good idea to ensure that, if you are out, the thief cannot simply open a door and walk out. A thief who has to climb out through a window is not only more likely to attract attention, he (usually) will often also be able to carry less. So, fit at least one lock which needs a key to open it to all doors and don't leave the keys either in the locks or near to hand.

Door Classification

External Doors

External doors are, obviously, the doors to the outside of the property. Less obviously, perhaps, is that any door which leads to an area which cannot be made secure, such as most attached garages, must be treated as an external door, even if it is actually inside the building.

Final Exit Door

It will be impossible to lock at least one door from inside when leaving the property - usually the front door. This is known as the final exit door. In some cases it may be convenient to treat two or more doors as final exit doors.

Internal Doors

Unless made to external door specifications, internal doors should not be considered secure doors. They are best left unlocked to avoid unnecessary damage.

French Doors and Patio Doors

French Doors should be treated as glazed external doors. Patio doors should be fitted with multi-point (preferably four point) locks and anti-lift blocks.

Garage Doors

Hinged garage doors, especially if to the standards of external doors above, are the easiest to make secure. Up and over doors are much more difficult to secure. If buying new, choose a door with two, or preferably four point locking.

Locks, Bolts etc.

British Standard Locks

The strongest locks are made to British Standard 3621/80. Not all types of lock are described in the standard, so it is possible for a very good lock to be unable to carry the BS kite mark. It is usual for insurance companies to specify at least one BS lock, normally on the front door.

Buying Locks and Bolts

The best place to buy locks and bolts is a qualified locksmith. You will get expert advice and may be guided to locks of which you were not even aware. If you know broadly what you want, a 'Security Centre' is a good alternative, but many do not have adequately qualified staff and recommendations are likely to be restricted to lines which are stocked. DIY centres do carry a wide range of locks and bolts, but you do need to know exactly what you want and they only have limited stocks of such items as Euro-profile locks.

Lock Types

There are two main types of lock: the mortise lock, which fits into a slot in the edge of the door, and the rim lock, which fits on one face of the door. The mortise lock is generally more secure, but the convenience of the cylinder rim lock makes it probably the most widely used front door lock. Either is acceptable if kite marked to BS3621/80, although rim locks should not be used on outward opening doors.

Mortise locks can be supplied as either deadlocks, with a single rectangular bolt, or as sash locks, with a second spring loaded bolt. The latter can be used with ordinary door handles and will hold the door shut even when not locked. Mortise locks should always have a minimum of 5 levers or, if cylinder operated, 6 pins. They should never be fitted where there is a joint in the door i.e. at the height of a horizontal rail.

Cylinder rim locks should be capable of being deadlocked, which may be done automatically or may be done using the key. The handle inside should also be lockable if there is glass in or by the door. Deadlocking prevents the spring loaded bolt from being pushed back by inserting a flexible tool (TV crooks usually use a credit card) into the door edge. Automatic deadlocks work by having a small tongue which detects when the bolt is engaged into the striking plate. Key deadlocking usually needs an extra turn of the key.

One special type of mortise lock is the espagnolette, which, in addition to the main bolt, also has bolts near the top and bottom of the door and sometimes also in the top and bottom edges. The last is a very good bolt for French windows. These are fitted as high security locks in several new doors, but, as they are not covered by BS3621/80, may not meet insurance requirements.

Cylinders, Levers and Wards

Locks will often be described as cylinder locks, lever locks or warded locks. Simple warded locks have no security value and should be ignored.

Cylinder locks use a cylinder which is kept from rotating by 5 or more pins. When the key pushes these pins back the correct amount, the cylinder is free to turn. Cylinder lock keys are generally flat pieces of metal, often with grooves on either side. When used as Europrofile or oval cylinders, they can be used with mortise locks and then have the advantage that 'changing the locks' only involves fitting new cylinders. BS locks will have 7 pin cylinders.

Lever locks work by the key lifting 3 or more levers a pre-set amount to allow the lock to turn. They use a traditional 'key' shaped key. The minimum number of levers for any security application should be five.

Common Keys, Master Keys and Restricted Profiles

A common key system is one in which all the locks use the same key. This is a considerable simplification in a property with a number of locks. If you want to include padlocks on the common key, it will be necessary to use a cylinder lock system. Do not confuse this with the system used for some key switches in which the same key is supplied to all buyers, also known as common key. This is obviously much less secure.

A Master key system also allows one key, the master key, to unlock all the locks. However, it also allows other keys to be made which only open some of the locks. This can be useful if you want to carry only one key yourself but to give limited access to other people. For example, the person who feeds the cat while you are on holiday could get into an outhouse or a gardener could be let into a garage or tool store without either having access to the main house or even to the area the other can enter.

Restricted profile keys are cylinder lock keys supplied by a particular locksmith. Only keys with that particular cross-section will fit your locks and their main advantage is that copies cannot easily be made.


There is a bewildering variety of bolts available, but only a few need to be covered here. To see the full variety, visit an architectural ironmonger.

Tower, slide & monkey tail bolts

The most common type is the simple tower bolt. These are generally unattractive and of little use except to hold a gate or a shed door. Similar in operation are slide bolts, which are generally more attractive and better suited for indoor use. Bolts of this type are not usually accepted by insurance companies as adequate security by themselves, since they simply pull open. They are, however, very useful on the front door for extra security at night.

Monkey tail bolts are simply tower bolts with extended handles to make them easier to reach without bending or stretching. The name comes from the shape of the handle.

Rack mortise bolt

Much more secure are rack mortise bolts. These are recessed into the edge of a door or window and need a key with a gear wheel type profile to move the bolt. In many cases insurance companies will accept these as a 'key operated lock' when used in addition to the main door lock or on a window. Like mortise locks, these can weaken the door or window if fitted into a joint.

Recessed bolt

Another very useful bolt is a recessed bolt. These are let into the edge, both top and bottom, of the first closing leaf of a double door or French window. They can only be operated when the second leaf is open and are therefore, very secure.

Lockable tower bolt

For sheds and gates there is a variety of tower bolt which has 'ears' fitted to take a padlock. These are adequate to stop the casual thief and greater security is rarely justified unless the shed is built of brick and fitted with a solidly boarded roof. Unlike a house, the door of the shed may not be its weakest point. Many sheds, including even some pre-fabricated concrete types, will yield to a good hard kick in the walls (ouch!).

Hinge bolt

The only other bolt which is of interest for security purposes is the hinge bolt. These are solid rods, usually fitted 75mm (3") below the top hinge and above the bottom hinge, in an external door. When the door is closed they enter holes in the door frame and prevent the door from being opened by removing the hinges. They are essential on outward opening doors unless special hinges are fitted.

Window Locks

The number of different types of window locks is huge. Covering each one in detail is beyond the scope of this FAQ. The best advice is to consult a locksmith or a security centre.

Recommendations for Particular Applications

External Doors

The police recommendations for the construction of external doors are:

  • They should be not less than 44mm (1 3/4") thick
  • The should be of solid construction (i.e. not hollow)
  • Any panels should be not less than 9mm (3/8") thick
  • They should be hung from three 100mm (4") steel (not cast) or brass hinges
  • Any glass in or immediately next to the door should be laminated. Outward opening doors should be fitted with hinge bolts.

Any external door to NHBC standard will meet the first two requirements. Hinges are traditionally fitted 150mm (6") from the top, at the centre height and 225mm (9") from the bottom.

Final Exit Door

The front door is usually a final exit door although there may be more than one. If you are going to fit only one BS 3621/80 lock, this is the door to fit it to. The door should have two locks - one at 1/3 the height from the bottom and the other at 1/3 the height from the top. The lower one should be a BS 3621/80 mortise deadlock. The upper is often a cylinder rim lock. If preferred, it could be a mortise sashlock and it could be as low as the centre height of the door.

Other police recommendations are that, if there is no side window, a door viewer should be fitted and, in any case, a door chain should also be fitted. Make sure that the chain is really substantial - many are of little value. If possible, also try to fit it so that any attempt to force the door does not simply pull the screws straight out.

Fire Exit Door

At least one door should be viewed as a fire exit door, which can be opened from inside without a key. This should have one slide bolt at the bottom and a second not more than 1200mm (4ft) high. These are normally fitted to a final exit door with a cylinder rim lock as the upper lock, although some mortise locks can be fitted with turn handles to open them from inside.

Other External Doors

External doors which can only be locked from inside usually need no more than a single mortise lock at or near the centre height with a rack mortise bolt at each top and bottom. If money allows, use a BS 3621/80 lock.

French Doors and Patio Doors

French doors and hinged patio doors can be treated as double external doors. Fit recessed bolts into the leaf which closes first. The second closing leaf can have either a good espagnolette lock or a mortise sashlock and two mortise rack bolts. In this case the rack mortise bolts should be fitted into the top and bottom of the leaf and engage into the outer frame.

Modern sliding patio doors should be fitted with multi-point locks and anti-lift blocks as standard. Older ones may only have a single point lock. Special patio door locks are available for these and two should be fitted - one each at top and bottom.

Garage Doors

Hinged garage doors can be treated as for French doors. As garages cannot be considered to be particularly secure, tower bolts or monkey tail bolts may be used on the inside of the doors instead of the recessed bolts and rack mortise bolts.

Up and over doors are particularly vulnerable, especially those with only a single point lock. Special garage door locks are sold which can be fitted on either side at the bottom of the door. As an alternative, a padlock bolt or hasp can be fitted either side at the bottom and the door can then be padlocked shut.

If electric power is available to the garage, a good DIY alternative is to fit electromagnetic locks to either side at the bottom. They are particularly useful where an automatic door opener is fitted as they can be switched off by the operating signal. A disadvantage is that they need a standby power supply to work during a power cut (they fail to OFF). Magnetic locks are remarkably expensive but electromagnets sold as holding magnets are considerably cheaper and can easily be adapted. A 6.2W 50mm diameter holding magnet will give up to 720N of holding force, about 72kg.


All accessible windows should be fitted with a key operated lock. Large windows may need two locks. This includes not only ground floor windows but also windows opening onto a flat roof and windows adjacent to the soil pipe leading from a WC. Soil pipes provide a fairly safe climbing route and are usually near a window.


Lighting is the cheapest form of security. A bulkhead light with two 9W energy saving bulbs, fitted with a photocell, can light up the whole side of a house from dusk to dawn and only use about 90 units of electricity in a year - particularly good value for those on off-peak tariffs. Alternatively a light with a movement detector (PIR) can be used which involves a higher initial cost but may produce lower overall running costs.

If fitting a single light, put it over the main entrance. Other outside doors will, however, also benefit from a light above.

Planting for Security

Thorny plants are a good deterrent and may be used either as hedges or at specific risk points. A climbing rose will not only cover an unsightly soil pipe, it will also reduce the risk of a thief climbing it. The best plants to use depend on the soil and your area, but many police crime prevention units can give advice for your locality.


There are two schools of thought about alarms. Some people believe that an alarm will deter the casual thief, while others think that it will suggest that you have something worth stealing. This is an area where it is worth looking at what the neighbours have. It is probably a bad idea to be either the first or the last in the area to have an alarm.

It certainly is a waste to fit an alarm unless you already have a secure property. Good locks are your first line of defence.

Fitting an alarm system is easily tackled by a competent DIY-er. Some insurers give a discount for a NACOSS maintained alarm system. Most NACOSS installers will approve and maintain a DIY fitted system provided that it meets with their installation guidelines, which generally rules out the cheaper DIY systems. As a (very) rough guide, you are unlikely to have problems with a system which can replay the last 100 events.

Alarm system technology is constantly changing. Because of the rate of change, the best advice has to be to visit a local alarm system specialist to see what is currently available. You can, for example, now get systems which do not need to be hard wired into separate protection areas but are fully programmable from the keypad.