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Scottish Advice Note: Determining the fire risk posed by external wall systems in existing multi-storey residential buildings

This Scottish Advice Note (SAN) provides advice for those responsible for fire safety in residential buildings including building owners/managers/residents groups responsible for determining the fire risk posed by external wall systems on existing multi-storey residential buildings. It will also be of interest to fire risk assessors and specialist external wall appraisal experts.


Part 2: Technical Advice

This part focuses on the process of determining fire risk. Sections 1 and 2 establish benchmarks to assist in the determination of risk and provide relevant background information. Sections 3 and 4 provide risk-based guidance on cladding systems and the UK Government Fire Test results respectively, which can inform the determination. Annex 1 contains information relevant to other external wall system features.

1. Benchmarks for Assessing Risk

Fire risk assessors often use guidance from a variety of sources as a means to inform their professional judgement in the assessment of risk. Guidance within the Technical Handbooks that support Building Regulations can be used as a benchmark to assist with the fire safety risk assessment process for existing buildings.

In April 2021, as a precautionary measure, the guidance within the Technical Handbooks removed reference to BS 8414 and BR 135 as a means of complying with the mandatory standards. Clause 2.7.0 clarifies that this is an interim measure, and confirms that a BS 8414 test / BR 135 report may still be used but that local authority verifiers are requested to notify the Scottish Government Building Standards Division of any building warrant applications made citing BS 8414 as a route to compliance.

There are two different routes that an external wall system can meet mandatory building standards and supporting guidance:

I. By being constructed of materials that achieve an appropriate European Classification standard to BS EN 13501-1 'Fire classification of construction products and building elements. Classification using data from reaction to fire tests'.

This classification system has seven classes A1, A2, B, C, D, E and F which denote fire performance: A1 being the highest performance and F being no performance. Some classes can have sub-indices s1, s2 or s3 to indicate smoke development, and sub-indices d0, d1, or d2 to indicate potential for flaming droplets. An example with sub-indices is B-s3,d2, where s3 indicates emissions with high volume smoke intensity (whereas s1 would indicate absent or very little smoke emission) and d2 indicates high/intense dripping droplets (whereas d0 would indicate no burning droplets).

From October 2019, the Technical Handbooks specify external wall systems on all hospitals and care homes of any height and other residential buildings with any storey more than 11 m above the adjoining ground to be constructed of products achieving European Classification A1 (non-combustible) or A2 (will not significantly contribute to fire load and fire growth). An external wall system for residential buildings (other than hospitals and care homes) with any storey at a height not more than 11 m may be constructed from products achieving European Classification B, C, D or E provided the building is more than 1 m from an adjacent building boundary.

II. Since 1 May 2005, an alternative to European Classification A1 and A2, has been by classification under 'BR 135 Fire Performance of external thermal insulation for walls of multistorey buildings' on the basis of the large scale fire test specified in BS 8414 'Fire Performance of External Cladding Systems'.

BS 8414 contains a large scale façade test method which allows the external fire performance of both applied and supported non-loadbearing external wall cladding systems to be determined. The BS 8414 tests are based on a post flashover compartment fire emanating from a window or door opening or from an external fire source of equivalent fire size.

BR 135 contains guidance on the principles and design methodologies related to fire-spread performance and assists with assessing the performance of the cladding system. Assessment of the test results require a competent professional to assess the extent to which the system tested is relevant to the building in question. BS 9414 'Fire performance of external cladding systems' provides additional information on the application of results from BS 8414 tests.

Although based on current standards, these benchmarks should not be treated as prescriptive or minimum standards or even recommendations. It would be unreasonable to expect existing buildings which were previously deemed compliant with Building Regulations to be upgraded to current standards, unless justified by a significant risk to life. Benchmarks should be regarded as comparators to assess how far removed existing provision is from current standards. The assessor must then decide whether deviations from the benchmarks result in unacceptable risk and, if so, what reasonable measures may be required to reduce that risk to an acceptable level. This does not mean the risk is eradicated, but it should be reduced to a level where life safety is ensured so far as is reasonably practicable.

The sections which follow give an overview of the common types of external wall system available and explore further how the benchmarks are used to inform the risk assessment process for existing residential buildings. The results of large and intermediate scale fire tests are also provided in Section 4 and should be referred to when assessing external wall systems that do not meet the benchmarks.

2. Common Types of External Wall System

Metal Composite Material (MCM) panels can be defined as thin metal skins (usually not more than 0.5 mm) bonded together by a core material (usually not more than 3-5 mm) having an overall panel thickness typically of no more than 6-10 mm, which comprise a number of layers, two or more of which are made of metal, alloy or metal compound, typically aluminium, zinc and copper. They are often used as rainscreen panels which prevent significant amounts of water from penetrating into the wall construction. Thermal insulation, airtightness and structural stability are provided by the second, inner part of the wall construction.

MCM panels are popular because of their precise flatness, variety of surface finishes and colours, lightweight and formability. However, fire performance varies greatly according to the composition of the core material and the panels can melt, warp, disband or delaminate in a fire. Three cores materials are commonly available, each with a different fire performance. Table 1 shows information on different categories relative to core type used by the UK Government screening tests following the Grenfell Tower fire.

Table 1: Summary characteristics of MCM panels by core type
Category Common core composition Calorific Value MJ/Kg [1]
1 [2] (non-combustible or will not significantly contribute to fire load and fire growth) Largely of mineral composition affording a high standard of fire performance ≤ 3
2 (fire retardant - FR) A mixture of polyethylene and other, inorganic additives to enhance fire performance >3 and ≤ 35
3 (non-FR) Unmodified polyethylene (PE); > 35

[1] Screen test based on BS EN ISO 1716 'Reaction to fire tests for products. Determination of the gross heat of combustion (calorific value)'.

[2] European Classification A1 (non-combustible) or A2 (will not significantly contribute to fire load and fire growth) as defined in BS EN 13501-1: 2018

The type of cladding on Grenfell Tower was ACM Category 3 with an unmodified polyethylene core.

High Pressure Laminate (HPL) panels are usually made by taking sheets of wood or paper fibre, layering them with a resin and bonding them under heat and pressure. The fire classification can vary depending on its material properties, thickness and whether or not it incorporates fire retardant additives. HPL panels with fire retardant chemicals added are sometimes referred to as FR grade and may achieve European Classification B-s1,d0 in accordance with BS EN 13501-1. Panels manufactured without fire retardant can be European Classification C, D or even lower depending on the thickness and make-up of the panel (e.g. fibre and resin used).

Other common types of external wall system include rendered systems with stonewool, expanded polystyrene, polyurethane and phenolic foams, brick slips, timber, masonry, tile, slate, stone, metal sheeting, insulated core sandwich panels etc. The fire performance of each system will vary depending the fire properties of the cladding material and where relevant, the type and thickness of encapsulation present. Prior to 1 October 2019, guidance in the Technical Handbooks permitted the use of combustible cladding systems where buildings did not have a storey above 18m and the building was more than 1m to the boundary. It is important to establish whether the cladding system complied with the building regulations and guidance available at the time the external wall system was installed (see Annex 3).

Additional Note on Products

There is a procedure by which certain construction products can be assigned a particular fire classification without the need for testing (known as 'Classified Without Testing'). Such products have well established reaction to fire performance and have been agreed by the European Commission's Standing Committee on Construction. Products belonging to European Classification A1 (non-combustible) or A2 (will not significantly contribute to fire load and fire growth) and are listed in Commission Decision 96/603/EC of 4th October 1996. This means that the products listed do not require to be tested provided they do not contain more than 1.0% by weight or volume (whichever is the lower) of homogeneously distributed organic material. Materials listed include, concrete, natural stone and slate products, clay bricks and tiles, glass, mineral wool etc.

Many buildings in Scotland have external wall systems constructed from brick, block, stone or concrete cladding. These materials achieve a European classification A1 or A2 as per EC decision noted above therefore are not considered to be of risk.

3. Risk-based Guidance for External Wall Systems

Fire safety risk assessment does not routinely involve opening up construction. As explained in Part 1, an intrusive appraisal may be required to identify the type and thickness of materials used or to check for suspected deficiencies in structural fire protection, such as inadequate provision of cavity barriers or fire stopping. Intrusive inspection is usually a one-off exercise which requires a contractor to open up construction and make good after the inspection.

When determining the risk, a range of risk factors and related issues must be considered. The following guidance is provided to assist and is split into 3 sections:

  • All residential buildings regardless of height;
  • Buildings with any storey at a height more than 11m; and
  • Buildings with any storey at a height not more than 11m.

(a) All residential buildings regardless of height

(i) Category 3 MCM cladding

Extensively clad - In light of the Grenfell Tower tragedy and evidence from subsequent UK Government fire tests, any building extensively clad with an external wall system incorporating Category 3 MCM should have the MCM removed without delay. Interim measures may need to be put in place until this is done.

Partially clad - Buildings that are partially clad in Category 3 MCM would also be expected to require remediation. Any decision not to remove Category 3 MCM from a partially clad building must be robustly justified in the fire safety risk assessment and any accompanying appraisal report.

(ii) Risk factors and other considerations for all external wall systems

The fire risk assessor will need to consider a range of risk factors to fully determine the risk posed by external wall systems, such as:

  • The ability of occupants to recognise and respond to a fire or a warning of fire without assistance;
  • The type of evacuation strategy in use, whether 'stay put', delayed or simultaneous and the anticipated evacuation time, should evacuation become necessary;
  • Staffing levels, where required for the type of evacuation method employed;
  • The type, position and extent of the cladding system and its potential for fire spread;
  • The risk of ignition of the cladding system both from external sources and from inside the building e.g. via any unprotected window reveals;
  • The quality of construction or presence of building defects;
  • The combustibility of other aspects of the building construction;
  • The height, use and complexity of the building;
  • Fire safety measures within the building such as fire separation, compartmentation, automatic fire suppression, automatic fire detection and warning (although internal measures may not by themselves prevent an fire involving the envelope having consequences);
  • The number, location and arrangement of escape routes;
  • General management and maintenance arrangements e.g. golden thread of information including a current fire safety risk assessment and any external wall appraisal report, condition of building structure and fixtures and fittings;
  • The suitability of facilities for firefighting, including site access and water supplies and other operational considerations e.g. dry or wet risers, smoke ventilation systems and the shielding effect of cladding on the effectiveness of water jets on fire within the cavity.

Further considerations for combustible external wall systems on both extensively and partially clad buildings:

  • An internal fire can break out of the building envelope via unprotected openings such as windows or doors. Cavity barriers should be provided around the openings to inhibit fire spread directly into the cavity. Other apertures in the building envelope may also pose a risk, including areas around balconies and other features such as solar shading and shutters, as well as unprotected service penetrations which may be present such as vents, pipes, ducts etc. Cladding should be removed if a fire could spread and compromise external features provided for life safety, such as automatic opening vents.
  • Cladding which crosses any vertical or horizontal fire separation or compartmentation lines or cavity barriers may enable a fire to spread beyond those lines. A vertically aligned partial band of cladding creates a greater risk of rapid fire spread vertically up the building than a partial horizontal band. Where panels are not directly located adjacent to each other, an appraisal specialist should be able to calculate the likely radiant heat flux, size of any flame, as well as how it might behave in differing weather conditions, and whether or not it could ignite another panel.
  • Cladding around entrances, exits and fire escapes may impede escape and access for firefighting due to the potential for falling debris.
  • Cladding located at or near to ground level may be vulnerable to accidental or deliberate exposure to fire e.g. parked cars and bin stores. Cladding may require to be removed or protected against ignition if it could be vulnerable to either accidental or deliberate ignition which could compromise safe access to and exit from the building.
  • The potential for fire spread to/from a neighbouring building should be considered. Cladding may need to be removed if presenting an impingement/radiative hazard. If thought to be a concern, hand calculations or computer modelling of the radiant heat flux on the façade may be required.
  • The potential for falling debris or burning droplets to contribute to downward fire spread should be considered. This could involve other cladding panels or combustible materials in cavities or stored on balconies.
  • Where there are concerns about the quality of construction and installation, including method of fixing or regarding the presence or specification of components such as cavity barriers, an appraisal of the external wall system will be necessary.

(b) Buildings with any storey at a height more than 11m

The Technical Handbooks specify new building work involving external wall systems on residential buildings with any storey at a height of more than 11 m above the adjoining ground to be constructed of products achieving European Classification A1 (non-combustible) or A2 (will not significantly contribute to fire load and fire growth). This is to ensure that external walls in taller buildings do not contribute to the development of fire or to vertical fire spread up the facade of the building.

The 11 m storey height threshold is based on the reach capability of a fire and rescue service ground mounted water jet where there is sufficient pressure and flow in the water main. In circumstances where there is insufficient pressure and flow in the water main, additional SFRS resources may be required and assistance sought from Scottish Water. In addition, external rescue by the fire and rescue service above this height would depend on the availability of specialist height appliances and adequate site access around the perimeter of the building. Cavity barriers and / or fire stopping between flats / compartments are intended to prevent unrestricted fire spread behind the external wall cladding.

Failure to comply with current building regulation guidance does not automatically make an existing building unsafe, so there needs to be flexibility when using current benchmarks and each risk assessment or appraisal must be building specific. It is possible that some higher risk buildings under 11 m may require remediation to reduce risk; equally, it is possible for lower risk buildings above 11 m to fall short of the benchmark, without it posing an unacceptable risk to life. At all times, the key question must be whether the presence of the external wall system poses a risk to life and the full range of risk factors and considerations set out in the preceding section should be taken into account.

Due to changing requirements over time, some external wall systems in existing residential buildings will not achieve the benchmark and may be European Classification B-s3,d2 or lower (or may have been considered in terms of surface spread of flame and fire propagation i.e. the now obsolete British Standard Class 0). In such cases, the combustibility of the material beyond the direct surface of the product should be considered in the risk assessment and appraisal, as this can contribute to fire spread over the external walls of buildings. Subject to supporting test evidence, it may be appropriate to retain cladding panels achieving European Classification B-s3,d2 if any core material within the products and any insulation material achieves European Classification A2-s3,d2 or better.

Where BS 8414 and BR135 have been used to achieve compliance under Building Regulations, reference may require to be made to BS 9414 'Fire performance of external cladding systems' to ensure that the results of those tests have been applied appropriately. Assessments carried out before the publication of BS 9414 may be acceptable but should be considered on a case by case basis.

(c) Buildings with any storey at a height not more than 11m

For buildings with any storey at a height of not more than 11 m above the adjoining ground, the fire safety risk assessment must also take into account the full range of risk factors in part 3(a) above. It is recognised that there may not always be the same degree of risk to life in a building which has a storey height not more than 11m, compared to taller buildings with more occupants, longer escape routes, larger fire loading, impact of building height on firefighting operations etc.

As a result, European Classification B, C, D or E is generally allowed under building regulation guidance for external wall systems if the building is more than 1 m from the building boundary and any storey is at a height of not more than 11 m. In many cases, this will not pose an unacceptable risk to life. That said, it should be remembered that fire service vehicular access and mains water pressure may be inadequate in existing buildings and any other changes to the use and layout of the building may have a detrimental impact on risk. To guard against this, it should be a guiding principle of the fire safety risk assessment process that occupants can escape safely without an over-reliance on fire service intervention.

Some buildings are considered as being higher risk due to the vulnerability of occupants and other factors. For example, building regulation guidance requires that, in general, all new hospitals and care homes regardless of height should achieve European Classification A1 (non-combustible) or A2 (will not significantly contribute to fire load and fire growth) or satisfy BR135 when tested in accordance with BS 8414 as an alternative means of compliance. For existing buildings not more than 11m, remediation, if required at all, may only be necessary for those which are higher risk e.g. premises with vulnerable occupants and delayed evacuation.

4: UK Government Fire Test Results

This section contains information on the UK Government's fire test results which should also be considered by fire risk assessors and appraisal experts when assessing combustible external wall systems that do not meet the European Classification A1 or A2 benchmark, particularly for buildings with a storey at a height of more than 11m and higher risk buildings with vulnerable occupants.

(a) UK Government Large Scale Tests

(i) Aluminium Composite Material (ACM) tests

In 2017, the UK government commissioned seven BS 8414 tests on three different types of ACM cladding in combination with different types of insulation. The full test result reports and advice for building owners are summarised in Table 2 below.

Table 2: Summary of 2017 BS 8414 test results for ACM systems

ACM filler Insulation - PIR Foam Insulation - Phenolic Foam Insulation - Stone Wool

Unmodified polyethylene

(Category 3)

fail not tested fail

Fire retardant polyethylene

(Category 2)

fail fail pass

Non-combustible (A1) or will not significantly contribute to fire load and fire growth (A2)

(Category 1)

pass not tested pass

A pass indicates that the external wall system resisted the spread of fire in line with the criteria set out in BR 135. These results show ways in which compliance could be achieved and offer an indication of how remedial works could be specified for those buildings that have been found to have problems.

(ii) High Pressure Laminate (HPL) tests

HPL (FR) panels (European Classification B-s1,d0) with stone wool insulation, in the specific configuration tested, also successfully met the performance criteria set out in BR135. These panels used in combination with combustible insulation may require remediation in residential buildings with any storey at a height of more than 11m or higher risk buildings of any height with vulnerable occupants, unless fully justified in the fire safety risk assessment and any available BS8414/BR135 test evidence.

Residential buildings with any storey at a height more than 11m or higher risk buildings of any height with vulnerable occupants with European Classification C or D HPL panels are likely to require remediation unless fully justified in the fire safety risk assessment and any available BS8414/BR135 test evidence.

(b) UK Government Intermediate Scale Tests

In April 2020, the UK Government published the test and analysis report 'Fire performance of cladding materials research'. The aim was to improve understanding of fire behaviour of cladding. The research showed that none of the materials tested (including HPL), had the same fire performance characteristics of fire growth, fire spread or potential fire breakthrough as the ACM (PE) cladding on Grenfell Tower.

(c) Other Considerations

There are many possible combinations of cladding and insulation and it is possible that products from different manufacturers will perform differently in a fire. In order to carry out an assessment of the fire risk posed by external wall systems, it is important that the detailed fire tests results are made available to fire risk assessors or appraisal specialists. Confidentiality agreements may need to be entered into with the test sponsor in order to obtain the detailed information required.

Test results are highly sensitive to variations in design, construction and the parameters used in the test including the location of cavity barriers / fire stopping. The external wall system installed must accurately reflect the system as tested however, variations may be acceptable provided the variations are within direct and extended field of application rules as defined in BS EN 13501-1. For example, BS 9414 provides defined rules for variations in the application of BS 8414 test results (See Annex 4). Assessment of test results should be carried out by those with appropriate skills, knowledge and experience in fire testing e.g Chartered/ Incorporated Engineers or UKAS accredited fire test houses.

After the Grenfell Tower fire, a small number of existing BS 8414 test reports have been withdrawn due to errors and omissions in the undertaking and reporting of these tests. This includes the associated Assessments-in-Lieu of Tests (AILOT) carried out by fire test houses. Fire risk assessors or appraisal specialists should satisfy themselves that the system installed accurately reflects the system as tested (including products used in those tests/assessments).

BRE provide a list of some external wall cladding systems which achieved the BR 135 criteria when tested to BS 8414. The list is not comprehensive and does not provide any detailed fire test information. Cladding systems not on the list, but tested to BS 8414, may satisfy the BR 135 performance criteria. Reports from other UKAS accredited fire test laboratories or members of the European Group of Organisations for Fire Testing, Inspection and Certification with specific accreditation for BS 8414 testing may be also considered.

Contact

Email: Buildingstandards@gov.scot

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