Select Committee on Environment, Transport and Regional Affairs Minutes of Evidence


Memorandum by the Department of the Environment, Transport and the Regions (ROF 31)

  The following memorandum is intended to address the issues that the Committee wish to examine.

1.  WHETHER A RISK IS POSED BY SUCH CLADDING

  The following points under this section are intended to set out the risks associated with external cladding. Some background information is also provided which it is hoped will help explain the philosophy behind the Building Regulations (section 3) that were developed to minimise the risk.

  Schedule 1 of the Regulations contain functional requirements which, where relevant, must be complied with. Part B of schedule 1 deals with fire safety and Requirement B4 (1), which has particular relevance to cladding systems, states:

  "The external walls of the building shall resist the spread of fire over the walls and from one building to another, having regard to the height, use and position of the building."

1.1  External fire spread

  External walls or the cladding attached to external walls can contribute to the fire spread both internally and externally if adequate precautions are not taken. The main function of an external wall in the context of external fire spread, is for it to be able to confine the fire to the building. This is intended to restrict the fire from hazarding a nearby building and can also aid fire-fighting. The origins of this requirement lie in the great fire of London. However the extent to which external fire spread needs to be considered is largely dependent on the amount of space that there is around the building. An external wall is considered to be an element of structure if it has a loadbearing function and it should then not collapse prematurely in fire. To achieve this and to help prevent external fire spread it may need to have fire resistance. In tall blocks of flats the loadbearing element is usually the structural frame of the building and the infill walls will only need to have fire resistance if they are located sufficiently close to a boundary. The standard of fire resistance needed depends on the use and height of the building. If the side of a building is sufficiently removed from the boundary then it need not have any fire resistance. Conversely, where the wall is on or very close to the boundary, then most or all of the wall will need to be fire-resisting.

1.2  Flammability at external wall surfaces

  In addition to fire resistance, it is necessary to consider the outside face of the wall in terms of its susceptibility to ignition and subsequent flame spread over its surface. Typically, sources of ignition could be flames issuing out of windows or other openings caused by a fire within the building or alternatively from an external fire source. External fire spread to the cladding can be caused by fire radiation from another building or from a source immediately adjacent to the cladding such as the ignition of refuse caused by arson. The standard of fire precautions that are necessary is affected by:

    (a)  the distance to the boundary;

    (b)  the height of the building; and

    (c)  the use of the building.

  Where external fire fighting might be difficult, high standards of performance against fire propagation and spread of flame are needed. Therefore where the external wall of a building is on or very close to a boundary these standards apply. Because of this difficulty in fighting external fire spread in the upper parts of high buildings it is necessary to apply higher standards of fire performance to the upper parts of such buildings regardless of the distance to the boundary. Where a low building is not close to a boundary, there is no restriction on the flammability of external wall claddings. Also a lesser standard of performance is acceptable for the lower parts of a high building unless it is on or close to the boundary.

1.3  Materials of limited combustibility.

  In high buildings the risk from fire spread is such that the combustibility of materials used in the construction of external walls, including thermal insulation materials, needs to be limited. The exception to this is where both leaves of the cladding are of masonry construction, such as brick or block, in which case the insulating material need not be of limited combustibility. A material of limited combustibility is a material with a performance specification: this includes non-combustible materials or materials that are defined by reference to a method of test. Typically, plasterboard would be considered as a material of limited combustibility.

1.4  Cavities

  Hidden voids in construction can provide a route for fire spread throughout or around the building and this can be particularly relevant in the context of external cladding systems. Any void between the new cladding and the existing building should be closed at regular intervals or at the line of compartmentation. Typically the floor of each flat will form the line of compartmentation, an issue covered in paragraph 3.1.

1.5  Surface Flame Spread

  Construction materials and their behaviour in relation to fire are classified using a number of standard tests such that the performance of particular elements of buildings can be specified without reference to specific materials.

  The provisions necessary to reduce the spread of flame over the surface of a material are based on the comparison of the results of small scale fire tests with larger scale fire research and experience of real fires. Any guidance that is given must be sufficient to provide a satisfactory level of safety whilst being practical in its application.

  The surfaces of materials (including cladding systems) are classified by reference to two British Standard test methods. These are the spread of flame test which measures the distance a flame will spread across the surface of a sample and the fire propagation test which assesses the contribution that the sample makes to fire development. The spread of flame test has four classes. These are class one to class four, with one being the highest performance rating. Class "O" is a further class, defined for the purposes of the Building Regulations, that is used for critical situations where a higher standard of performance than that of Class one is appropriate. The Building Regulation issues relating to flame spread are covered in paragraph 3.1.

  Whilst non-combustible materials are inherently of the Class "O" referred to above, many materials that are by definition combustible will also achieve this classification. The intent of this methodology is to identify materials that will have a low risk of fire spread.

2.  THE EXTENT OF THE USE OF EXTERNAL CLADDING SYSTEMS

  2.1  The Department does not collect statistics on the use of cladding systems but it is believed that external cladding systems are widely used.

  2.2  The Department has a call-off contract with the Fire Research Station to investigate real fires and this highlights any areas of concern that affect Building Regulations. The following are fires notified to the Department that involved external fire spread but were not necessarily attributed to the cladding system:

  Knowsley Heights, Liverpool, 1991. Deliberate fire spread up and behind rainscreen cladding, extended over 11 floors. Building Regulations were changed as a result of this.

  Mercantile credit building, Basingstoke, 1991. Fire on 8th floor spread up the building behind glass curtain walling.

  Three storey block in Milton Keynes, 1995. Roof destroyed.

  Alpha House Coventry, 1997. Flames travelled up the outside of the block from 13th to 17th floor. No fire penetration of the flats.

  Butler House, Grays, Essex, 1997. Fire in top flat of 14 storey block caused uPVC window frames to melt and drip, which in turn caused some damage to cladding.

3.  THE ADEQUACY OF THE REGULATIONS PERTAINING TO THEIR USE

  3.1  In England and Wales (Scotland has a different set of Building Regulations) where new buildings are erected, or existing buildings materially altered, or in certain cases where there is a material change of use, then the work is required to comply with the Building Regulations 1991. As far as fire is concerned, the purpose of the Regulations is to secure reasonable standards of health and safety for persons in or about buildings (and any others who may be affected by buildings, or matters connected with buildings). Schedule 1 of the Regulations contains the functional requirements and Requirement B4(1), which has particular relevance to cladding systems, is given in paragraph 1.

  Guidance on how to comply with the functional Requirements of Part B is given in Approved Document B (fire safety). Approved Document B includes several provisions to restrict the materials used in external walls and cladding by reference to the surface spread of flame rating. These provisions are as follows:

  The external surfaces of walls of any building closer than one metre to its boundary (and therefore closer to other buildings) should be class "O" in order to reduce the risk of external fire spread from one building to another.

  Where a building is 20 metres or more in height, the external surfaces of walls more than 20 metres from ground level should achieve a class "O" surface spread of flame rating. Below this height timber cladding at least 9mm thick, or some other materials that are less restrictive than class "O" materials, could be used. This is to reduce the risk of fire spread over the walls of tall buildings whilst allowing certain commonly used materials to be retained in positions where fire fighting operations from the ground could be effective.

  In the case of the outer cladding of a wall of "rainscreen construction", which has a drained and ventilated cavity, the surface of the outer cladding which faces the cavity should also satisfy the provisions detailed above. This is to take account of the specific problems associated with this type of construction.

  Approved Document B states that the external envelope of the building should not provide a medium for fire spread if it is likely to be a risk to health or safety. The Document also points out that the use of combustible materials for a cladding framework, or of combustible thermal insulation, may present a risk in tall buildings. Therefore in a building with a storey at more than 20 metre above ground level any insulation material used in the external wall construction should be a material of limited combustibility.

  With regard to fire stopping Approved Document B suggests that a cavity barrier should be provided at the junctions between an external cavity wall that is not of masonry construction and every compartment floor. The BRE guidance on avoiding risks with thermal insulation, which is referenced in the Approved Document that deals with energy efficiency, recommends that to prevent fire spread, cavity barriers should be provided at every floor level. We have asked the Fire Research Station to review and update their report on the fire performance of external thermal insulation for walls of multi-storey buildings, referred to in Approved Document B.

  The current Approved Document B is being reviewed but there are no changes proposed that will affect cladding systems other than the 20 metre height mentioned above is being reduced to 18 metre to fall in line with other height dimensions relating to fire fighting. In general it is considered that the risk of serioius fire spread via external cladding will be minimal if the guidance given in Approved Document B is followed.

  The Fire Precautions (Places of Work) Regulations may have a bearing on cladding issues but these Regulations are the responsibility of the Home Office.

4.  WHAT ACTION MAY BE NECESSARY TO COUNTER ANY RISKS POSED IN EXISTING BUILDINGS AND TO AVOID ANY RISKS IN NEW BUILDINGS OR ALTERATIONS TO EXISTING BUILDINGS

  4.1  The Building Regulations in England and Wales only apply to new building work, and thus cannot be used to require any alterations to existing buildings although the Department is reviewing this in respect of the conservation of energy. The provisions of the Building Regulations as set out in the preceding section do, however, apply when new buildings are erected and thus such work is covered.

  The Building Regulations also apply to building work that is classified as a material alteration. An alteration is material if, at any stage of the work, it would result in the building not complying with certain requirements of the Regulations where it previously did. The most pertinent requirement with regard to cladding is external fire spread, but structural requirements could also be an issue.

  Thus with regard to the alteration (or replacement) of cladding, if this was the only work being carried out, and if it at no time made the external fire spread or structure any worse that it was already, the work would not be controlled by the Building Regulations. There is therefore the possibility that external cladding installed some time ago, and thus not complying with the current Building Regulations, could be replaced without being controlled by the Regulations as long as the building was not made any worse with regard to these particular requirements in the process. This is a possible problem area and one that the Department may need to review.

  The Building Regulations would need amendment to ensure that all such work was covered. It is possible that the Building Act might also need to be extended to support this. Any such amendment would need careful drafting to ensure that an undue burden was not inadvertently imposed on replacement and repair work. However a balance needs to be struck between construction costs and safety.

5.  OTHER MATTERS WHICH MAY ARISE IN THE COURSE OF QUESTIONING

  5.1  The Department has funded the Fire Research Station (BRE) to produce a method of test for "Assessing the fire performance of external cladding systems". This report will be referenced in the revised Approved Document B and it is proposed that it will become a British Standard.

  5.2  Review of Building Research Establishment Report 135—1998 "Fire performance of external thermal insulation for walls of multi-storey buildings". This review is required to give better design guidance for cladding systems, particularly with regard to cavity barriers.

July 1999


 
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