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Security Glazing [Hardfacts]

Introduction

One of the most common means of breaking into otherwise secure premises is breakage of glazing, followed by opening or forcing of any securing catches or locks.

This is often made easier by the fact that most glass used to glaze windows and doors is quickly and easily broken, but this does not have to be so. This Hardfacts outlines various types of glazing material available and comments on their respective strengths or weaknesses.

General Considerations

Various types of glass and glazing material are available e.g. plastics or glass. Types of glass are generally categorised by the method of manufacture, i.e.  ‘float glass', ‘toughened glass' or ‘laminated glass'; but for each type of glass different constituent materials may be used e.g. ‘Soda lime' is the most common type of glass, but for specialist applications use may be made of ‘borosilicate' or ‘alkaline earth silicate' glass.

Whatever its type, for effective security all glazing needs to have appropriately designed, and built frames/surrounds which are then maintained in good condition. To help achieve this, the services of a competent glazier should therefore always be sought when installing new or upgraded glazing, e.g. use a member of the Glass and Glazing Federation (GGF).

As a guide, when choosing glazing the following aspects also need consideration:

  • Do not consider security of glazing in isolation. Also look at other potential weak points in the building perimeter.
  • When choosing a type of glass best suited to your security requirements, do not overlook the need to comply with any requirements that may relate to fire precautions or human safety. ‘Hardfacts' 5024 Glazing in the Workplace provides further information on safety issues.
  • Don't place undue faith in double glazing using normal float glass, as it does not significantly improve security.
  • Glazing rebates should be deep enough to adequately hold any ‘improved' glazing material in place, e.g. laminated glass usually requires a minimum rebate of 15mm to ensure it remains in place under attack.
  • Glazing should ideally be internally fixed into its frame, as externally fixed glazing is inevitably more vulnerable to removal. If externally fixed be aware that: 
    - Putty or glazing compound is soft when new and requires considerable time to harden. Note. Once hardened, and provided it and the frame remain in good condition, puttied glazing is usually difficult to remove.
    - Beading strips may be inherently weak, and in any event derive their strength from the means and number of fixings.
    Note. The use of glass sealant or security glazing tape can improve the security of external beading.
  • Glazed openings should have locks and catches that are strong enough to withstand being prized open from outside or inside should the glazing be broken. ‘Hardfacts' 3017: Door & Window Security provides further advice on physical security.

Security Qualities of Types of Glazing

Various types of glass may be used to glaze windows and doors, for example:

Float or Annealed Glass
This is the most frequently used glass. It is supplied in a range of thickness from 2 to 15mm, with 4mm being the most common. Float glass above 8mm thick is often referred to as plate glass and is usually used for fixed shop-front windows. Thicker sizes provide some increase in resistance to breakage.

One good feature of float glass is that if it is broken, and any locks cannot be overcome, intruders need time to break out most of the jagged remaining pieces to allow safe entry.

For safety reasons, float glass is sometimes retro-fitted with plastic ‘Applied Safety Film' ideally of a type designed to meet the requirements of BS6026. Safety film is designed to hold the glass together in the event of accidental breakage, thus reducing the risk of personal injury; but it can also enhance security to a degree, as it hinders breakage and helps retain broken panes within a frame. .

Glass Bricks
These are made of float glass cast into blocks resembling hollow bricks. Properly installed they can create a very strong glazed area.

Wired (Georgian Wired) Glass
This glass has a fine steel mesh sandwiched between two molten layers of glass during its manufacture. It is intended as a fire resistant material, the mesh holding the glass together in the event of a fire.

Although often fitted as ‘secure glazing', such glass is only marginally more resistant to breakage than float glass. However, it is harder to break a wired glass pane completely out of its frame e.g. to gain full entry and the mesh can act as a visual deterrent to breakage.  

Toughened/Tempered (Safety) Glass
This is glass that has been subjected to a special heat treatment after being cut to size, the process being designed to improve the safety of the glass when it breaks.

Whilst more resistant to breakage than float glass when hit by large soft object, e.g. accidental impact by a person, when hit by a small sharp object it is very easily broken, which is why small pointed hammers are provided adjacent to toughened glass windows on trains and coaches to facilitate emergency exit.

Once a pane of toughened glass is broken it shatters into hundreds of tiny cubes, which can then be easily knocked out of the frame to create a smooth opening.

Laminated Glass
This is the only glass generally recognised as providing ‘security glazing'. It is made by assembling two or more pieces of float glass, one each side of an interlayer of thin plastic called PVB (Poly Vinyl Butyral). It comes in various thicknesses (grades), the thicker grades containing more layers of glass and PVB. The current manufacturing standard for laminated glass is BSEN 14449.

When attacked, laminated glass resists being broken  but more signifcantly the PVB interlayer helps prevent an opening being made.

The more common grades are as follows:

  • 6.4 - 6.8mm. Intended  mainly as basic ‘Safety Glass' it can nonetheless provide reasonable resistance to forced entry being stronger than float or toughened glass
  • 7.5 - 13.5mm. Intended as burglary resistant glass, this can provide considerable resistance to forced entry, according to the risk level of BSEN 356 used, e.g. P1A - P5A (impact) &/or P6B - P8B (axe attack). Note. Such glass needs to be installed within a related security glazing framework.
  • 30mm upwards. Intended as ballistic resistant glass, this is a specialist area for which suitable professional advice should be sought. A risk level should be selected according to those described in BSEN 1063 & 1522 & 1523

Other benefits of laminated glass are:

  • Design - It can be coloured or patterned
  • Ease of Use - It can be cut to size
  • UV Protection - The PVB interlayer filters ultra violet radiation from sunshine and thus helps reduce colour fading of goods on display
  • Contamination Protection - broken glass is better held in place, and as such may not fall and damage/contaminate any goods on display.

For safety critical applications, laminated glass can be made from toughened glass. When this form  is required, a poured resin material is used as the interlayer instead of PVB - the glass sometimes being referred to as resin compound glass.

Plastic Glazing
Various types of plastic sheet are potentially suitable for glazing e.g. cast or extruded acrylic, polystyrene or polycarbonate sheet. Some of these can be supplied with Glass Reinforced Fibre (GRP) or metal mesh reinforcement to hinder breakage.

Whilst some plastics have better resistance to breakage than float glass, there are some disadvantages to using plastic materials for glazing  e.g. poor optical clarity (polycarbonate), or poor light transmission (GRP or metal mesh reinforced plastic). All plastics are also vulnerable to scratching or being attacked by saw or flame. Another downside is potential loss of strength, or cloudiness, due to the effects of Ultra Violet radiation from exposure to sunshine.

Polycarbonate is probably the most useful of these plastics, as it has good impact resistance, e.g. it is used in riot shields, has reasonable optical clarity and is available in sheets of varying thicknesses. However, it flexes under sustained impact or attack and, if it is to remain securely in place, careful fixing is required.

For the reasons of appearance, plastic glazing is usually only used at industrial premises.

Identifying Installed Glass

Over the years different standards have been in force/applied to glass, some of which have been mentioned above.  Sometimes the manufacturer/installer will have voluntarily applied an idelible mark or label to their glass, but legal requirements to mark glass only exist when the glass is installed in particular safety "Critical Locations" - as defined in relevant Health and Safety and/or Building Regulations.

Identifying installed glass and any related legal safety requirements, is not straightforward, so to reiterate previous advice, always use a competent glazier when reviewing glazing matters.

However, some general information on this topic can be found in "Hardfacts" 5024 Glazing in the Workplace and on page 13 of the RISCAuthority guidance document. Physical Security for Homes - Guidance for Occupiers, which can be obtained (as a free download) from their website: http://www.riscauthority.co.uk/free-document-library/RISCAuthority-Library_detail.s24-physical-security-for-homes-guidance-for-occupiers.html    

Key Action Points

  • Check your glazing provides an appropriate level of security and, where necessary, also meets any fire or human safety criteria

  • Ensure glazing frames/surrounds and fixings are appropriately designed and installed, then maintained in good condition

  • Ensure glazed openings have adequate physical security devices

  • Consider improved glazing as part of a set of balanced security measures

  • Take professional advice and use competent installers, e.g. a member of the Glass and Glazing Federation (GGF), when reviewing or installing new/upgraded glazing

Sources of Further Information

Other ‘Hardfacts' in the Property Protection - Security series, which are available in our ‘Knowledge Store' at www.aviva.co.uk/risksolutions

The Glass and Glazing Federation Tel: 020 7939 9101 or see www.ggf.org.uk

British Standards Institute (BSI) Tel 0208 996 9000 or see www.bsi-global.com

Your local authority's Building Control (Planning) Dept

Next Steps:

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Please Note
This document contains general information and guidance and is not and should not be relied on as specific advice. The document may not cover every risk, exposure or hazard that may arise and Aviva recommend that you obtain specific advice relevant to the circumstances. AVIVA accepts no responsibility or liability towards any person who may rely upon this document.

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